PART II (Contd.)

10. MARKETING PRACTICES AND PRICES

10.1 Marketing practices

Output in Laguna Lake pens was mostly marketed (99 percent). Majority (76 percent) of the fishpen owners sold their crop by direct wholesale. Direct retailing, consigning and contractual selling were also practices but only to a limited extent. In contractual selling, the buyer is chosen while the fish is still being reared in the pens. Furthermore, the buying price is agreed upon before actual harvesting. The price for a kilo of bangos was highest when sold by contract. Surprisingly, the lowest price was received by directly retailing the fish in the market. In Rizal, almost an equal proportion of the operators have their harvest picked up by the buyer or delivered to them. In Laguna, however, majority preferred to have their crop picked up from the pen by the buyers.

10.2 Market outlets

Most popular (22 percent) outlet among Laguna Lake operators was the Malabon Fish Terminal in Malabon, Rizal. Farmer's Market in Cubao and Divisoria Market in Manila were also preferred outlets particularly by Rizal operators. Fishpens located in Binangonan, Cardona and Pililia, cited Cardona as the nearest common market. Aside from the local market, pens located in Laguna considered also the Calamba and San Pablo markets.

Traditional price behaviour theory states that everything else equal and for undifferentiated products, an increase in the amount of product offered for sale will lead to a lower price. Thus, the apprehension expressed by established marketers of milkfish in Malabon regarding the depressing effect on prices of the added fish catch from Laguna Lake appears warranted and worthy of study.

A study¹ conducted from October 29, 1973 to January 31, 1974 covering five major landing points in Rizal analyzed the impact of milkfish landings from these areas on milkfish prices in Malabon. Some 1 043 tons of bangos were landed at the five major stations along Laguna Lake in Rizal. Quezon City (mostly Farmer's Market and Rizal were the principal destinations absorbing more than 370 tons and 212 tons, respectively.

¹ Guzman, R. D., R. D. Torres and L. B. Darrah, “The Impact of Bangos Landings from Laguna Lake (Rizal points) on Bangos Prices in Malabon”. Special Studies Division, NFAC, Diliman, Quezon City.

The combined shipments to Malabon and Navotas amounted to 198 tons or about 19 percent only of all milkfish landed. Contrary to initial indication it appeared that these two markets did not absorb the bulk of the bangos landed from Laguna Lake. However, Malabon's share of the markets in Greater Manila such as Farmer's Market correspondingly shrunk with the entry of Laguna Lake milkfish.

The study concluded that milkfish price in Malabon/Navotas responded to factors other than quantity of milkfish shipped from the Rizal side of Laguna Lake. The claim that milkfish landed from Laguna Lake has caused a decline in milkfish prices in Malabon/Navotas was not warranted. On the contrary, prices observed at the landing points were higher than Malabon/Navotas prices on 83 percent of the days included in this study.

11. CREDIT PRACTICES

Most (89 percent) of the fishpen owners in Laguna Lake did not experience any shortage of operating capital. Of those who did, three reported using mainly borrowed capital to operate their fishpens while 9 percent augmented their own with some borrowed capital the proportion of which was 47 percent owned and 53 percent borrowed.

Six out of the 15 Laguna Lake borrowers chose the rural bank as their source of credit. Another six borrowed from relatives or friends while five others borrowed from other banking institutions. The amount of loan averaged 55 200. Thirteen out of 15 operators reported that their loans were used to finance their operations. Two others acquired the loan for the development of the pen structure.

Fishpen operation is a capital intensive undertaking thus it has become a monopoly of people with access to capital. Unless the small operators are given priority development support and additional external assistance and services, their participation in the pen culture development will be a disadvantage as they cannot compete with large scale operators due to limited funds and lesser access to available services. The “Biyayang Dagat” programme of the government is a credit scheme designed specifically for small and medium-scale fisheries projects including operations of a fishpen. A fishpen operator with a maximum area of three ha culturing milkfish can avail of a production loan of 10 200 per hectare payable in one year.

12. PROBLEMS IN THE INDUSTRY

Adverse weather conditions posed as the biggest problem in Laguna Lake as reported by 52 percent of the operators. This however, should be treated more as a risk than a problem. Precautionary measures should therefore be employed to lessen the risk in the business. Other problems cited were: poaching, insufficient technical support from the government, irregular supply of stock, and where supply was available dishonesty in counting was prevalent, exorbitant prices of stock in some places, and unavailability of credit for pen construction. In Laguna about three-fourths of the sample operators were visited by extension workers compared with about one-third in Rizal. Visits of extension workers were concerned mostly with damages after typhoon. Other assistance included the acquisition of a DBP loan and stocks from BFAR nurseries.

Correspondingly, it was suggested that the government, in line with its intensive food production programme could lend support to the industry by implementing a price support programme for the primary inputs of the industry and extending technical support. Likewise, a government credit scheme for fishpen was suggested by some operators.

¹ Original data in 1974 was adjusted to approximate 1980 price levels

¹ Original data in 1974 were adjusted to approximate 1980 pricelevels by using the percentage change in the Price Index for domestic products.

¹ Original data in 1974 were adjusted to approximate 1980 pricelevels by using changes in milkfish prices for receipts and change in wholesale price index for inputs for expenses.

¹ Original data in 1974 were adjusted to approximate 1980 price levelsby using the change in milkfish prices for receipts and change in wholesale price index for domestic products for expenses.

¹ Assumed a 12 percent discount rate

Fig. 1 Production and revenue functions

SCS/PCC/WP-10
SELECTION OF SUITABLE SITES FOR CAGE CULTURE
by
T. K. Mok¹

1. INTRODUCTION

One of the prerequisites for the success of a cage culture system lies in the selection of a suitable site. Prior to the establishment of a cage culture system, an extensive knowledge of the environment of the site is required, and a number of physical, hydrographical and biological factors should be taken into consideration to evaluate the suitability of the possible culture site. The site can be assessed initially through study from topographical maps, navigational charts and any available literature, and further evaluated by field investigations should the preliminary deskstudy prove promising.

2. FACTORS AFFECTING THE SELECTION OF SITES

2.1 Physical factors

2.1.1 Shelter

The degree of protection of the site from wind and wave actions should be assessed. Records from the meteorological office for the site or its vicinity may provide an indication of the extreme conditions that are likely to prevail. Areas with waves greater than about 0.5 m in height will generally be unsatisfactory. The more protection provided by the site for cage culture, the less will be the costs for anchorage and construction of extra strong rafts and cages, provided there is adequate circulation for water exchange. In general, bays, coves and inlets along an indented coastline may meet this requirement.

2.1.2 Land base

An area of flat land adjacent to the site should be available to provide a base for operation, accommodation of working staff and storage of gear and equipment.

¹ Fisheries Research Division, Agriculture and Fisheries Department, Hong Kong.

2.1.3 Topography

Water depth is a critical factor for cage culture. Sites with shallow sills and deep pits should prove to be unsuitable on account of the lack of water exchange with the sea over the sill. In general, a steep slope to shore leading to a flat bottom in 3–4 m at the lowest tide is suitable for cage culture.

2.1.4 Bottom condition

It is possible to set up cage culture on almost any type of bottom. However, on rocky bottoms it is difficult to set up anchorages, whereas a very soft muddy bottom generally indicates low water exchange rate which is not suitable for high density culture. A firm substrate of fine gravel, sand, mud or any combination of these would provide optimum conditions.

2.2 Hydrographical factors

2.2.1 Water quality

Various species differ in their requirements for suitable water quality in terms of temperature, salinity, acidity (pH), dissolved oxygen, turbidity and pollution. Data on these parameters at selected sites at varying depths should be obtained for assessment. Variations of these parameters should be well within the natural tolerance of the species to be formed.

2.2.2 Tidal currents

The rise and fall of the tide produce currents which enable water to flow through the biomass of fish under cultivation. This will bring fresh oxygenated water to and remove waste materials from the cage. A greater tidal range generally indicates a better water exchange rate which is more suitable for stocking a high density of fish. Assessment of tidal currents also assists in determining the siting of cage, land drainage and sewage outfall.

2.2.3 River/stream run-off

Sites should be situated away from points of fresh water run-off which normally affect surface salinity, turbidity and increase the possibility of presence of heavy metals leached from the catchment areas.

2.2.4 Sewage and industrial pollution

Cage culture sites should be selected where there are limited habitation and industrial activity. Drainage and sewage discharge from land into the sea should be away from the site, and account should be taken of tidal streams and prevailing winds.

2.3 Biological factors

2.3.1 Disease/predation

Areas of heavy natural disease occurrences on the species to be farmed should be avoided, while the presence of colonies of possible predators of the cultured species such as wild fowl, sea birds and puffer fish should be noted.

2.3.2 Excessive fouling

The types of fouling organisms at the site should be assessed to ensure that the water of the site will not pose serious fouling problems. Sites with excessive fouling should be avoided. Detailed study of the seasonal pattern and effect of fouling on the cage culture would involve actual testing with the raft and cage materials for a year.

2.4 Other requirements

2.4.1 Accessibility

The land base is to be situated as close as possible to public access roads and to the site at sea. Boat access to the land base is advantageous, but not a critical factor, if there is good road access with a jetty to connect the culture site.

2.4.2 Sea and land regulations and rights

The existence of regulations and rights should be noted and information on these aspects gathered from the local authorities concerned so as to avoid legal problems in the acquisition of land and sea for culture purposes.

2.4.3 Navigation interference

It is necessary that the placement of rafts and cages should not interfere with navigation of the passage of vessels and the site should be located as far away as possible from navigation fairways in order to avoid the effect of ships' wash on the rafts and cages.

2.4.4 Fresh water, electricity supply, telephone and postal services

The availability of piped fresh water and electricity supply to the land base is to be ascertained. Telephone and postal services are desirable.

2.4.5 Proximity to markets and ports

In remote areas, transport may be a problem both physically and economically. Proximity to ports should be considered in order to reduce transport costs of capital items and supply of feeds such as trash fish. Also proximity to markets is another important consideration in the reduction of distribution costs.

2.4.6 Surveillance

The nature of the site should preferably allow the rafts and cages to be kept under surveillance to ensure their continued security against vandalism or poaching. These factors should be assessed according to local conditions.

2.4.7 Local labour availability

The proximity to the local population and the nearby towns and the likelihood of attracting local labour should be noted.

3. SURVEY AND EVALUATION

In assessing the hydrographical factors and some of the physical factors, much variation may be found between different parts of the same site, and between different times of the year. A thorough site assessment should be made with frequent surveys to cover the seasonal variations within a year. The more time and effort spent on field investigations, the more complete would be the assessment of the site. If this is impracticable, observations should be taken at least during extremes of dry and wet weather, spring and neap tides, and calms and storms, since it is these extremes which may pose hazards to the cage culture systems.

All the factors mentioned above in the selection of a suitable site are closely interrelated and a decision on any one of them may have strong bearings on the others. It is often not possible to find optimum levels for all these factors together in the same site, and it is then necessary to compromise on at least a few of them. But, from the commercial point of view, every condition which is less than optimum would make it that much more difficult to set up an economical and competitive cage culture system. It should be realized that a careful study should be carried out covering every pertinent factor prior to starting the cage culture so as to avoid severe problems or failure in the end.

4. REFERENCES

Joint SCSP/SEAFDEC 1977 Workshop on Aquaculture Engineering, Iloilo, Philippines. SCS/GEN/77/15, Vol. 2, 453p.

Milne, P.H. 1972 Fish and shellfish farming in coastal waters. Fishing News (Books), Ltd., London. 208p.

Milne, P.H. 1976 Selection of sites and design of cages, fishpens and net enclosures for aquaculture. FAO Technical Conference on Aquaculture, Kyoto, Japan. FIR: AQ/Conf/76/R. 26. 15p.

SCS/PCC/WP-11
DESIGN AND CONSTRUCTION OF FLOATING CAGES AND RAFTS
by
T. K. Mok¹

1. INTRODUCTION

Cage culture, one of the recent advances in aquaculture, has seen considerable development during the last decade. Of the various cage designs, the floating type of cage culture is by far the most extensively practised on account of its ease of operation and management.

The basic structure of a floating cage system consists of an enclosure in the water made out of netting material, a surface frame with floats to which the netting is attached and a mooring device for keeping the structure in position. Various materials are being used in making cages, i.e. the enclosure; these include polyethylene, nylon and metal wire-mesh. Knotless netting is generally better in reducing clogging and eliminating abrasion to fish. Common materials used for construction of rafts, i.e. the supporting frame, are galvanized steel pipe, bamboo, wood, logs and plywood. Galvanized steel is stronger and has a longer service life than the others. The common types of floats in use include bamboo (in bundles of poles), plastic containers, plastic drums, iron drums, styrofoam floats, fiberglass floats and aluminum cylinders; and their average lives in water range from less than one year to over ten years. For anchoring the floating cages, a number of mooring devices are being widely used, including wooden pegs or stakes, bags of sand or pebbles, concrete slabs, concrete blocks and iron anchors. The use of wooden pegs and concave-type concrete slabs is limited to muddy bottoms.

¹ Fisheries Research Division, Agriculture and Fisheries Department, Hong Kong.

2. TYPES OF FLOATING CAGES IN USE

The design and construction of floating cage systems vary in different countries. An outline of some floating cages of considerable interest is in the following:

(a) The floating cages for farming yellowtail in Japan are made of synthetic nets, 10 m² and 3–5 m deep, which are suspended from floating collars constructed of bamboo or galvanized steel, and kept afloat by oil drums or styrofoam cylinders. The bottom corners of the nets are weighted to keep in shape, and four to ten of these bag nets are often joined together.

(b) Recently, circular cages made of flexible chain-link galvanized wire mesh are also used in Japan. These cages, measuring 8 m dm × 6 m deep or 14 m dm × 7 m deep, are supported by galvanized steel collars and polydrums covered in polyethylene bags. The bottom of each cage is formed by a circular galvanized steel collar. Such cages do not need cleansing but considerable wear occurs between the interlinking meshes due to waves and tidal currents, and the nets require changing once a year.

(c) The type of floating cage for Atlantic salmon farming in Scotland is constructed with rigid mesh sides to a size up to 6 m³. It has an upper framework of galvanized scaffolding tubes supported by polystyrene floats with catwalks for access. This cage is very robust and durable and is suitable for more exposed locations. It also has the advantage of easy site erection and modification.

(d) A type of very large cage for Pacific salmon farming is used in the United States, each cage measuring 50 m × 12 m × 3 m. The supporting platform is constructed from polystyrene covered by plywood in modules of 1.2 m × 2.4 m × 0.6 m. The modules are connected together by steel cables of 12.5 mm dm, and the bag net is made of nylon mesh. This type of cage provides excellent floatation and can withstand wave and wind actions in very stormy weather.

(e) A new type of cage, the globe cage or rotating cage, has recently been developed in Europe. The cage is constructed of a framework of aluminum tubes in the form of a globe (4 m in diameter) covered with nylon netting. The horizontal axis is submerged below the surface of water so that two-thirds of the globe is under water. The cage is mounted into a square raft with a circular opening in the center. The elements of the raft are made of aluminum profiles filled with polystyrene foam. To harvest fish from the cage, a net is first stretched out inside the globe through an opening, and with a simple turn of the globe, the fish will be caught inside the net. The cage can resists strong waves, swift currents and turbulence, and when it is rotated 180° every few days, the fouling organisms on the net can be left to dry and die off.

3. DESIGN AND CONSTRUCTION OF CAGES IN HONG KONG

Floating cages currently used in Hong Kong for marine fish culture differ considerably in design, size and material. Rafts are essentially constructed of wood together with various types of floats ranging from iron drums and styrofoam blocks to plastic containers of different sizes. They are either square or rectangular in shape with a size varying according to the number and size of the cages to be accommodated. The cages are mostly cubical with edges ranging from 2–4.5 m, and are made of polyethylene netting, either knotted or knotless, with a square bottom frame of galvanized pipe.

A typical 4-cage floating raft, 7 m × 8 m, is constructed from six 7m and three 8m hardwood beams, each of 15 cm × 7.5 cm in cross-section. These beams are fastened together with bolts and nuts in a frame as shown in Fig. 1. The frame is kept afloat by 15 iron drums (200 1 capacity) or styrofoam blocks fastened to the underside of the paired beams. Each of the four divisions of the raft can accommodate a polyethylene net cage of 3 m × 3 m × 3 m which is suspended from the surface corners by means of polyethylene ropes. A square frame of galvanized iron pipe (3 m²) is fixed at the bottom of the cage to weigh down and distend the cage.

A simpler type of the 4-cage raft of similar size (7 m × 7 m) is constructed with six hardwood beams (15 cm × 7.5 cm × 7.5 m) and 72–78 plastic containers (18 1 capacity) as floats. The beams are fastened together with bolts and nuts and the frame is kept afloat with the plastic containers fixed to the underside of the frame by means of nylon twine (Fig. 2).

The raft is constructed on the beach at low tide. The whole structure is floated off the beach at high tide and towed to the mooring position at the site. It is anchored in position at each corner by an iron anchor of about 30 kg with polyethylene rope. The anchoring ropes should not be too tight so as to allow for tidal difference and to provide a flexible hold without over-straining the raft at high tide. In many cases, cement blocks are used in place of anchors and this type of mooring device is more economical on account of its long life but it is heavy and difficult to work with. In some cases, it may be possible to have one or both ends of a raft or series of rafts moored to the shore if they are not too far away from it and if there is an adequate depth of water at low tide.

4. RIGGING CAGES

The method of rigging cages in Hong Kong is by means of two sets of vertical polyethylene lines which connect the corners of the bottom square frame of galvanized iron pipe to the surface corners of the raft division. One set is used for hoisting up or lowering down the cage while the other set acts as vertical rib lines which thread through the meshes of the netting.

Other countries in Southeast Asia have different methods of rigging. One of these makes use of four vertical galvanized iron pipes, each with its upper and fixed to a corner of the floating raft and its lower end welded to a metal ring. A rope fixed to a bottom corner of the net cage threads through the nearest ring and is then fastened to the surface corner. The cage will be distended to its cubical shape when each of the four ropes is tightened, and it can be pulled up easily when the ropes are slackened.

A similar method of rigging works on the same principle by using four weights or concrete blocks with imbedded rings in place of the four galvanized metal pipes. These weights are suspended by ropes at each surface corner of the raft division.

Another method of rigging applies to net cages with the bottoms slightly smaller than the tops. A weight, such as a used bottle filled with sand, is suspended from each of the four surface corners and is placed at each of the four bottom corners of the cage. The weights cause the net cage to extend horizontally into shape and when they are pulled up the net cage can be hoisted easily.

5. FACTORS AFFECTING CAGE DESIGN

The proper design of a cage is important for successful cage culture. As the cage is continuously immersed in water, its materials are subjected to the corrosive effects of sea water. The materials therefore should have high corrosion resistance and durability. The strength of the cage and its materials should be sufficient to withstand the forces exerted when it is lifted out of the water with a full load of fish at harvest. At the same time, it should be light in weight and designed for easy handling in order to keep the cost of labour at a minimum. The cage netting should preferably possess an adequate combination of softness and strength so that it will not cause abrasion to the fish when they rub against it. The floatation system should be so designed that it can support the combined weight of the frame, the people working on the raft and the wet net with a full load of fish at harvest. The design of anchors, including weight and number, should be related to the weight and size of cages, nature of bottom, tides, currents, winds and waves of the location.

Apart from all these factors, the design of a floating cage involves consideration of other requirements associated with cost, availability of materials, ease of construction and maintenance, and its effectiveness and efficiency as an enclosure system for that particular locality. In arriving at suitable sizes for rafts and cages, various factors such as cost, labour and handling, have to be taken into consideration. It is true that with larger cages the cost per unit volume of water enclosed is considerably less, and similarly the cost for perimeter floating support is less for larger cages than for a series of smaller ones with equal total surface area, but one has to bear in mind the problems of currents, winds, waves, maintenance, stock-taking and harvesting. It is virtually impossible to derive a perfect system that can fulfill all these requirements. There are thus no definite standards of raft and cage construction. The basic approach to cage design is to adjust structures and materials to local supply and conditions, and there exists a great need for engineering study and cost analysis in order to arrive at a satisfactory system.

6. REFERENCES

Hunter, C.J. and W.E. Farr. 1970 Large floating structure for holding adult Pacific salmon (Oncorhynchus spp.) J.Fish.Res.Board Canada. 27: 947–50.

International Workshop on Pen and Cage Culture of Fish, 1979 Iloilo, Philippines. IDRC. 164p.

Joint SCSP/SEAFDEC 1977 Workshop on Aquaculture Engineering, Iloilo, Philippines. SCS/GEN/77/15, Vol. 2. 453p.

Landless, P.J. 1974 An economical floating cage for marine culture. Aquaculture 4(4): 323–8.

Milne, P.H. 1972 Fish and shellfish farming in coastal waters. Fishing News (Books), Ltd., London. 208p.

Milne, P.H. 1976 Selection of sites and design of cages, fishpens, net enclosures for aquaculture. FAO Technical Conference on Aquaculture, Kyoto, Japan. FIR: AQ/Conf/76/R. 26. 15p.

FIG. 1 LAYOUT OF A 4-CAGE RAFT MADE OF WOOD AND IRON DRUMS

FIG. 2 LAYOUT OF A 4-CAGE RAFT MADE OF WOOD AND PLASTIC CONTAINERS. (18 - LITRE CAPACITY)

SCS/PCC/WP-12
SPECIES OF FISH CULTURED IN HONG KONG
by
C. Wong¹

1. SOME CONSIDERATIONS IN THE SELECTION OF FISH SPECIES

Profitable fish culture aims at the production of the maximum quantity of edible fish flesh from a given quantity of organic matter. The species selected should have a rapid growth rate, and it should be able to use the natural food resources efficiently. Another consideration is the need to utilize all the available food resources for the production of fish flesh. The usefulness of shortening the food chain to achieve this purpose will be obvious. A fish which can convert decaying organic matter or the next link in the chain, namely, algae, directly into edible fish flesh is from the food utilization viewpoint, superior to others. Pond fishes have distinctive feeding zones and food preferences, so it is not entirely profitable to base the cultural operations on a single species. Chinese fish culturists have evolved the polyculture system rearing a group of different species of fish which, when present in a pond together, will use up all the food resources.

Even though the culture of non-predaceous fishes is more economical, it may not be advisable to restrict cultivation to those species in all areas. Human preferences are important in this respect. Carnivorous fishes are said to have a better flavour than herbivorous ones.

The efficiency of fish production varies with species and environmental factors. Each species has its own preferences as to the organic matter it uses as food, besides the oxygen content, the temperature and the acid-alkaline range of the water it can inhabit. A particular species, though fast-growing, may be wasteful of food. On the contrary, a slow-growing species may be able to convert a given quantity of food into a greater weight of fish flesh than other species. Further, a fish food may be such that it cannot be provided economically. One fish may not withstand adverse conditions or its flesh may deteriorate too rapidly to permit easy marketing, even though it may be highly relished. Similarly, a fish of indifferent flavour may have maximum sales appeal on account of its ability to remain alive out of water for considerable periods. Indigenous fishes, which are naturally suited to the environmental conditions, may be uneconomical or unfit for cultivation in their home countries. So when suitable species are not available, the introduction of exotic species may be considered.

¹ Fisheries Development Division, Agriculture and Fisheries Department, Hong Kong.

2. WHY FISH FARMING IS BETTER THAN OTHER ANIMAL HUSBANDRY

1. Fish live in the medium of about the same density as their own, therefore, less energy is required for its osmoregulation.

2. Fish have less skeletal structure for support than birds and other animals.

3. Fish are poikilothermic and expend less energy for body temperature than warm-blooded animals.

4. In fish production, e.g. carp production, the increase in weight per unit weight of feed intake is 2–2.5 times more than cattle and sheep.

5. One man-year for: pig production is 25 tons
   oyster production is 40–60 tons
   Danish trout production is 40 tons
   carp production is 30 tons
   trout production is 40–50 tons

6. There is no or little food competition between man and fish.

7. Fish can grow in varying environmental conditions.

8. Fish produce great numbers of offspring (as much as 100 000 eggs/kg fish).

9. Fishes can spawn naturally and/or artificially, making possible genetic improvements.

10. Fish eggs and larvae can be hatched and reared under controlled environments.

11. Fishes are suitable for different methods of culture and can be mixed with other species (polyculture, monoculture, running water, cage culture and tank culture).

3. TYPES OF FISH CULTURED IN HONG KONG

1. Low value fishes mainly for low cost protein production, e.g. tilapia, carps, and grey mullet for pond fish culture.

2. High priced fish, e.g. grouper, snapper, seabream for marine fish culture.

3. Restocking fish for fish farming in reservoirs.

4. Petfish for tropical fish and goldfish culture.

4. CRITERIA FOR THE SELECTION OF MARINE FISHES FOR CAGE FARMING IN HONG KONG

1. Spawn naturally and/or artificially.

2. Large production of offspring.

3. Eggs and larvae can be hatched and reared under controlled environment.

4. Grow in different conditions, disease resistant and hardy.

5. Little or no food competition with man.

6. Fast growing and production per unit area is high, but culture period is short.

7. Stable market demand.

8. Ability to remain live out of water for considerable periods.

5. SPECIES OF MARINE FISH CULTURED IN HONG KONG

Marine fish are cultured in floating cages and impoundments mainly in the sheltered bays of the eastern waters of the New Territories and outlying islands. The cultured species are tabulated below:

5.1 Major cultured species of marine fish in Hong Kong

5.3 A total of about 17 million number of fry and fingerlings were estimated to have been stocked in cages by Hong Kong mariculturists between January and December, 1980. (Table 1) Seabreams (Sparidae) and groupers (Serranidae) account for 80 percent and 13 percent, respectively. Sea perch (Lates spp.) represents 3 percent which is more than all the snappers (Lutjanidae) together at 2 percent. Most of the fry are collected locally, but also imported from China, Taiwan, Philippines and Thailand. Fish fry for commercial stocking are not produced artificially in Hong Kong.

5.4 Groupers and red pargo are marketable in 18 months while seabreams and other snappers are marketable in two years.

Total: about 17 million

SCS/PCC/WP-13
FEEDS AND FEEDING FISHES IN CAGES
by
T. K. Mok¹

1. INTRODUCTION

In cage culture, fish are raised in a relatively small enclosure at high density. The fish must receive their nutrients and energy from food. In general, the food supplied to the fish can be classified into two types: (1) natural feeds and (2) prepared feeds. Natural feeds consist of various meat, organs and by-products from animals, poultry and fish, e.g. meat scraps and trash fish, while prepared feeds are comprised of animal meals, various grains and possibly vitamins and minerals.

In countries where trash fish can be cheaply supplied by the marine fishing industry, the commercially farmed fish are often raised on a diet of trash fish. Since trash fish are high in protein and in most vitamins there is often little need for supplementation of other diets. But handling and transport of trash fish can be troublesome and laborious, and the greatest disadvantage of the trash fish diet is that it is extremely variable in nutrient composition and often high in indigestible substances. On the other hand, prepared feeds offer a number of advantages over natural feed. Processing and handling of prepared feeds are much easier and storage is simple. The most important advantage is that they are relatively uniform in composition, and the ingredient content of the diet can be altered to adapt the feed to the specific needs of the species of fish.

2. COMPOSITION AND FORMULATION OF FEED

Prepared feeds, also referred to as artificial feeds or diets, can be divided into three groups, namely practical diet, semi-purified diet and purified diet. The practical diet is the one commonly used by fish farmers for feeding fish. The diet is formulated from natural ingredients such as meat by-products, fish meals and cereal grains, usually supplemented with vitamins and minerals. The semi-purified diet contains some natural ingredients in relatively pure form, while the purified diet contains only ingredients of accurately known composition. Both the semi-purified and purified diets are used in nutritional research to test dietary components in relation to growth and health.

¹ Fisheries Research Division, Agriculture and Fisheries Department, Hong Kong.

In the practical diet, proteins, lipids and carbohydrates are energy-containing components while the vitamins and minerals are essential for normal metabolic functions, but the requirements for many of them are only little known for most species of fish. Very little work has been done on the quality of protein, carbohydrates and fats in fish feeding. There is a great difference in the utilization of the proteins, fats and carbohydrates from different sources. It is not yet possible to recommend a perfect diet for fish. Most of the work in the field of feed technology has been limited to feeding on salmon, trout and channel catfish.

For most species, a great deal of research is required in order to determine their nutritional requirements. To establish the nutritional requirements of any fish species, a series of experiments must be run in which the known composition of the diet is varied. The first step is to study the quality and quantity of protein that can give the most rapid growth rate. This is done by preparing a series of isocaloric diets containing different commercial protein sources at various percentages.

When the desired nutrient content of a diet is set, a selection is made of practical feed ingredients that will give those nutrients at the required levels when mixed together. As proteins, fats and carbohydrates can be supplied by a variety of grains and animal products, a least-cost formulation may be designed according to the price of each product.

3. PHYSICAL PROPERTIES OF FEED

The physical properties of a pellet feed must be considered in the development and formulation of a diet for a fish, and they should be adapted to the physiological and anatomical development at different growth stages of the fish. These properties include texture, water stability, size, flavour, odour and colour.

Texture is an important aspect of a diet which often affects the acceptance of a pellet directly. Most species of fish appear to avoid pellets that are too tightly compressed. Hard pellets are often not immediately swallowed but are first crushed in the mouth, then spat out and swallowed again. On the other hand, poorly-bound pellets cause excessive dissolution of nutrients into the water.

Water stability of a feed is related to the intactness of the pellet when it is submerged in water. Pellets with good stability should have minimum disintegration and loss of fine particles in water. Both texture and stability affect the leaching of nutrients and thus the availability of the food to the fish.

The pellet size is important in the feeding of fish and must be adapted to the different stages of growth in order to achieve optimum acceptance. If the pellet is too large the fish will be unable to ingest it immediately and will have to wait until it partially dissolves in the water to an acceptable size.

Very little is known about flavour, odour and colour in relation to the acceptability of feeds.

4. MANUFACTURE OF FEED

Prepared feeds can be classified as dry, moist or wet depending on the water content. Dry feed has a water content of less than 20 percent, moist feed between 20 and 50 percent, and wet feed more than 50 percent.

Commercially prepared feeds are mostly dry diets in pelleted form. The pellets can be manufactured so that they either sink or float. Sinking pellets are produced in a pellet mill by forcing the ground and mixed dry feed ingredients through a die of selected diameter under pressure. High temperature and pressure are not required. As the material leaves the die, a knife blade cuts the pellets into the desired length. The pellets produced generally have a water stability of a few minutes to an hour.

Binders are generally used when the diets are difficult to make into pellet or are unstable in water. In pelleted practical diets, starch serves as the binding agent. Other examples of binding agents in experimental diets are algin, gelatin and carboxymethyl cellulose. In most cases, 1 or 2 percent of the diet is composed of a binder. An increase in the proportion of the binder can increase water stability.

Floating pellets are manufactured by the extrusion process. The ground and mixed feed ingredients are also forced through a die, but they are subjected to much higher temperature and pressure than the pellet mill. As the material leaves the die, the drop in pressure causes the starch in the pellets to expand. The resulting feed pellet will often float in the water for up to 24 hours.

5. FEEDING PRACTICES

Feeding can be done either by hand, or by automatic or self-feeding devices. The choice of the method should be assessed on the basis of the cost involved. The main disadvantage associated with automatic or selffeeding devices is the inability of the farmer to observe the fish whilst feeding. When the fish are fed by hand, the farmer can see that they are consuming the feed actively. The feed should be spread out evenly to ensure that all fish have access to feed without much competition.

The fish can get accustomed to a particular feeding schedule; they are fed at about the same time of the day, usually in the morning.

The feed is physically adjusted to the requirement of the fish by varying the particle size of the feed to give improved food acceptance at different stages of growth with minimum wastage.

Feeding rations are normally expressed as a percentage of the fish weight. The daily percentage of feed to be offered to the fish must be determined independently for each species. The optimum rate of feeding can be determined by examination of the food conversion ratio, which is the quantity of food required to produce a unit weight of the fish. Food conversion ratio becomes lower as the efficiency of feed utilization increases. The lowest food conversion ratio value occurs at the feeding level when food conversion efficiency is highest. For economic feeding, it is vital to subsample the stock, preferably at intervals of a month, in order to determine the conversion ratio of the feed and to adjust the feeding requirement accordingly. Too frequent sampling should be avoided as this will cause injury to fish and increase susceptibility to diseases.

6. REFERENCES

Halver, J.E. and K. Tiews. 1979 Finfish Nutrition and Fishfeed Technology, Vol. I and II. Proceedings of the EIFAC Symposium, 1978. 593 p and 622p.

Symposium on New Developments in Carp and Trout Nutrition. 1968 Rome, 1968. EIFAC Technical Paper No. 9. 213p.

SCS/PCC/WP-14
FISH DISEASES: DIAGNOSIS AND CONTROL
by
C. Wong¹

Fish are prone to hundreds of parasitic and non-parasitic diseases, especially when grown under artificial conditions. Adverse hydrological conditions often precede parasitic attacks, as the resistance of the fish is lowered. Mechanical injuries sustained by a fish when handled carelessly during farming or transport may also facilitate parasitic infection.

1. CLASSIFICATION OF MAJOR PATHOGENIC BACTERIA OF FISH

1.1 Order Eubacteriales - straight or curved rods and cocci

1.1.1 Family - Pseudomonadaceae

(a) Aeromonas spp. - straight rods with one polar flagellum; has many different bio-types; a most important fish pathogen. The best known aeromonad pathogen is the nonmotile Aeromonas salmonicida, the cause of furunculosis of salmon, trout and other fish in most parts of the world. Furunculosis can be treated with sulfonamides. antibiotics, or nitrofurans. Other aeromonad pathogens include three closely related motile species, A. liquefaciens, A. punctata and A. hydrophila, which many investigators, consider as one species. They cause hemorrhagic septicemia of trout, pondfish and European carp; fin rot and tail rot in aquarium fish; red leg of frogs; and ulcerative stomatitis of snakes.

(b) Pseudomonas spp. - more than one flagellum. Pseudomonas fluorescens or P. putida produce disease in trout, aquarium fish, catfish and other pondfish, such as hemorrhagic septicemia (“Red Pest”) in eels and carps and are treated with antibiotics.

¹ Fisheries Development Division, Agriculture and Fisheries Department, Hong Kong.

1.1.2 Family - Brucellaceae

(a) Vibrio spp. - curved and spiral-shaped rods. Diseases caused by Vibrio anguillarum, and to a lesser extent V. parahaemolyticus, affect principally marine and estuarine fishes. Freshwater species may become diseased, especially if fed on a diet containing marine fish or fish products containing vibrios. For prevention of vibriosis in fishes, oral vaccines appear to be promising. Oxytetracycline, sulfamethazine, or a combination of a sulfamethazine and furazolidone are useful in the prevention or treatment of outbreaks of vibriosis.

(b) Hemophilus piscium - gram negative, non-motile, cause ulcer disease of trout and is treated with oxytetracycline.

1.1.3 Family - Corynebacteriaceae

(a) Corynebacterium - small, gram positive bacilli, occurring in pairs as diplo-bacilli; asporogenous; non-motile; cause bacterial kidney disease. Kidney disease of salmonids is a chronic, and at times acute, disease caused by a Corynebacterium. The disease is characterized externally by exophthalmos and blebs or blisters on flanks. Internally the kidney may be swollen and may contain greyish-white areas. Hemorrhages may occur in reproductive organs. Microscopic examination of gram-stained material from external lesions or kidney lesions show small, gram-positive diplobacilli, the causative agent. This disease can be treated with erythromycin.

1.2 Order Myxobacteriales

1.2.1 Family - Cytophagaceae

(a) Cytophaga spp. - long, thin flexible; gram negative; no microcysts. Peduncle disease, also known as cold water disease, caused by Cytophaga psychrophila affects juvenile salmonids, particularly fry and fingerling coho salmon and fingerling brook trout, in water temperatures below 10°C (50°F). The disease begins as an external infection in the caudal area and then becomes systemic.

Benzalkonium chlorides are useful in controlling external infection, and sulfisoxazole and oxytetracycline are used for systemic infections; best results may be obtained by using both. Raising water temperature to above 12°C (53°F) is also effective if the disease has not progressed too far.

1.2.2 Family - Myxococcaceae

(a) Chondrococcus spp. - long, thin, flexible; gram negative; form microcysts; arranged in “columns”. Columnaris disease caused by Chondrococcus columnaris was the first described disease of any animal caused by a myxobacterium. The disease affects warm- and cold-water, freshwater fishes when temperatures are above 15°C (60°F). It begins as an external infection producing greyish-white areas on the head, lips or fins. These areas superficially resemble a fungus infection. The lesions gradually spread and become necrotic. A localized necrotic lesion often develops on gill tissue. If untreated, the disease usually becomes sytemic. In early stages, external treatment with diquat is beneficial. For systemic infection, oxytetracycline and nifurpirinol, a new nitrofuran, are effective in prevention.

2. CLASSIFICATION OF MAJOR PATHOGENIC VIRUSES OF FISH

(a) Infectious pancreatic necrosis (IPN) is a severe disease of very young salmonids caused by a rhabdovirus and is characterized by fish frantically whirling on their long axis then lying quietly on the bottom. Affected fish become dark, exhibit swollen abdomens, exophthalmos and hemorrhagic areas on the ventral surface. Histologically, necrosis occurs in acinar and islet cells of the pancreas and in cells of adjacent adipose tissue.

(b) Infectious hemopoietic necrosis (IHN) was formerly known as Sacramento River Chinook disease (SRCD) and Oregon Sockeye disease (OSD). IHN is an acute disease principally affecting the fry and fingerlings of rainbow trout and salmon. The disease is caused by a rhabdovirus. Affected fish may have long fecal casts, ascites, exophthalmos, and hemorrhages at the base of fins, in the peritoneum, air bladder, lateral body wall and membranes covering brain and heart. Histologically, extensive degeneration and necrosis occurs in the hemopoietic tissues of kidney and spleen.

(c) Viral hemorrhagic septicemia (VHS) is probably the most important disease of rainbow trout, affecting both fingerling and larger trout. VHS is caused by a rhabdovirus, and affected trout show alternate periods of frantic swimming and quiescence. There are several forms of the disease - the acute form, characterized by reduced appetite, erratic swimming, multiple hemorrhages in skeletal muscle and viscera, and hyperemic and swollen kidney and liver; the chronic, in which trout seek quiet areas, show exophthalmos, swollen abdomen and anemia; and the nervous form, which may be the last stages of an epizootic and in which trout usually show frenzied swimming and leaping.

(d) Channel catfish virus disease (CCVD), caused by one of the herpesvirus group, and affects catfish. Affected fish whirl on their long axis, or swim convulsively and lie quietly on the pond bottom. Characteristically, just before death fish hang vertically in water with their heads at the surface. Diseased specimens show hemmorrhages at the base of fins and abdomen, distended abdomen, pale or hemorrhagic gills, ascitic fluid and pale and swollen kidneys. Histologically, necrosis and oedema occur in the kidneys and gastrointestinal tract.

(e) Spring viremia of carp (SVC) is a recently described viral disease of fish. It is caused by a rhabdovirus in cultured carp. SVC is probably a form of infectious dropsy of carp (IDC), a disease with complex syndromes. Affected carp may show distended abdomen, exophthalmos and bleeding in the anterior eye chamber, protruding and inflamed anus, petechiae in kidneys, liver, pericardium, heart, intestine, gas bladder, skeletal muscles, and oedema of all internal organs.

(f) Lymphocystis is a disease of worldwide distribution which affects freshwater and marine teleosts. Nodular masses resembling warts occur on the fins and body. The causative agent is a polyhedral virus.

Diagnosis of all these virus diseases is based on isolation of the virus and identification by means of a serum neutralization test. Fish which have recovered from these disease become carriers and shed the virus in feces, eggs and sperm. There is no specific chemotherapy useful in the treatment of viral fish diseases. Temperature manipulation as a means of control is apparently effective in contagious stomatitis and IHN. However, strict hygiene, avoidance of known diseased carriers or eggs from carrier fish are the only means of general control of viral fish diseases.

3. MYCOTIC DISEASES OF CULTURED FISH

(a) Saprolegniasis - The fungus Saprolegnia attacks fish that have sustained mechanical or other injuries. It often attacks eggs and hatchlings and this may cause serious mortality in hatching ponds. Over-abundance of algal growth, mechanical injuries, overcrowding and fouling of water are considered to be the factors that encourage the attacks of this fungus. Malachite green, potassium permanganate, common salt and copper sulphate are used for the treatment.

(b) Ichthyosporidium (Ichthyophonus) - This disease is a systemic granulomatosis and is found in both freshwater and marine fishes. The infection is evident as a roughened skin texture described as the “sandpaper effect” occurring principally in the lateroventral tail region. Measures aimed at preventing the disease, including steam sterilization of trash fish as quoted by others as an essential treatment is not practicable.

(c) Branchiomycosis - This disease, known as gill rot, is characterized by areas of infarctive necrosis in the gill due to intravascular growth of the fungus Branchiomyces spp. Two species are involved, B. sanguinis and B. demigrans. B. sanguinis is usually localized in gill blood vessels and has non-septate branched hyphae. B. demigrans differs in having thicker hyphal walls and larger spores. This disease is encouraged by waters rich in organic matter, algal blooms and high water temperature.

4. PROTOZOAN DISEASES OF CULTURED FISH

The protozoan subphyla and the genera most responsible for diseases of fishes are: Sarcomastigophora (Amyloodinium, Costia, Cryptobia, Hexamita and Oodinium); Ciliophora (Brooklynella, Chilodonella, Cryptocaryon, Ichthyophthirius, Trichodina and Trichodinella); Sporozoa (Eimeria and Hemogregarina); Cnidospora (Ceratomyxa, Henneguya and Myxosoma).

4.1 Sacromastigophora - Amyloodinium and Oodinium are dinoflagellates responsible for velvet disease of aquarium fish. Amyloodinium invades primarily the gills and in heavy infections also the skin. Oodinium normally invades both gills and skin. Transmission of both is by the motile infective-dinospore stage. Low concentrations of copper sulphate or acetate eliminates these protozoa from the water.

Costia, Cryptobia and Hexamila are parasitic flagellates. Costia, an external parasite, is the most ubiquitous and troublesome, but is controlled easily with dilute formalin. It invades the skin and gills of the cultured fish and has direct transmission. Cryptobia is a hemoflagellate, and Hexamita lives within the intestinal tract of the cultured fish.

4.2 Ciliophora - Brooklynella and Chilodonella, marine and freshwater forms, respectively, are ectoparasitic ciliates. Chilodonella can be a serious pathogen in freshwater fishes, and Brooklynella can produce severe lesions on gills of marine fish kept in aquaria. Dilute formalin or acetic acid will usually remove these protozoa from the fish.

Two of the most ubiquitous pathogenic ciliates are Ichthyophthirius of freshwater fishes and Cryptocaryon of marine fishes. Ichthyophthirius has caused epizootics in nature, while Cryptocaryon has not been recorded as causing epizootics in nature, but does so in aquarium conditions. Both of these ciliates invade and burrow under the epidermis, they feed on host cells, cause irritation as evidenced by the fish flashing and rubbing against the aquarium and cause hypersecretion of mucus and loss of osmoregularity.

Trichodina and Trichodinella are also external ciliates which occasionally cause epizootics in cultured and aquarium fishes. They are easily controlled by formalin.

4.3 Sporozoa - Eimeria and Haemogregarina are coccidian parasites. Eimeria is found in both freshwater and marine fish. In freshwater fish it is found within the intestine and visceral mass, and believed to be pathogenic. Eimeria of marine fish are known to be pathogenic, producing liver lesions, parasitic castration, and partial or total occlusion of kidney and air bladder.

4.4 Cnidospora - Ceratomyxa, Henneguya and Myxosoma are histozoic myxosporidians which are serious pathogens of fish. Ceratomyxa affects anadromous and freshwater salmonids and has been responsible for total losses of rainbow trout. Myxosoma species parasitize many species of fishes. The most important species is Myxosoma cerebralis, the causative agent of whirling disease of salmonids.

Henneguya, a myxosporidium of warm-water fishes, has its greatest effect on catfish culture, producing cysts within the skin and gills. It has caused mortalities of small fingerling catfish.

5. TREATMENT OF FISH DISEASES

In recent years, there has been a rapid development of marine fish culture in Hong Kong. Densely cultured fish can die as a result of disease, pollution or wrong management of the culture practice. The fish can be subjected to many disease, especially when farming is by intensive methods. The outbreak of diseases in fish farms can be the result of high density of stocking, poor conditions of farming and the small number of species farmed which favours the development of specific disease. The methods of treatment and chemicals and drugs recommended for monoculture farms in Hong Kong are:

5.1 Immersion method

External chemical treatments to deal with external parasites and/or infections.

5.1.1 Bath

In this method, fish are bathed in situ in a chemical solution of low concentration for one hour.

5.1.2 Dip

Fish are dipped into a chemical solution of high concentration for a few minutes.

5.1.3 Flush

A slug of chemical is added to the inflow and diluted through the holding unit by the water flow.

5.1.4 Flowing treatment

A constant volume of chemical is added to the inflowing water over a fixed period to give the required concentration using constant-head siphons and constant volume delivery pumps.

5.2 Systemic treatment

This method refers to the incorporation of drug into the food to treat systemic bacterial diseases or gut parasites.

5.3 A combination of immersion and systemic treatment

This is suitable for fish infected by bacterial diseases with ulcers and wounds on the skin. The drug is used as a skin treatment. It can also be absorbed by the gills.

5.4 Swabbing

The infected fish is taken out of water, then the wounds are swabbed; but this method is labour intensive.

5.5 Injection

In this method, the drug is injected into the fish; it is, however, limited by its high labour cost.

5.6 An example using the immersion treatment

5.6.1 Maintain quality of the environment in terms of:

1. oxygen

2. ammonia

3. pH

4. temperature

5. salinity and hygiene

5.6.2 Balance with toxicity and efficacy of the treatment, i.e.

5.6.3 Procedures for immersion treatment:

1. Starve the fish;

2. Check gills, suspended solids, tank hygiene, stocking density and water temperature

3. Treatment trial;

4. Check mathematical calculations;

5. Treatment with record;

6. Monitor oxygen and observe;

7. Check results; and

8. Retreat if necessary.

5.7 The range of chemicals and drugs recommended for :

5.8 Choice of method/chemical or drug

Procedure

5.8.1 Diagnosis

Investigate temperature, water quality, cause of disease, husbandry method.

5.8.2 Action

Choose the range of chemicals/drugs and method of treatment
Numbers and age of fish
Layout and type of holding unit employed
Cost

Q. Is it worth treating?
A.    (i) Leave disease to run course and accept mortality of x%.
       (ii) Harvest rapidly and count losses.
       (iii) Treat.

Q. What is the cheapest and most effective way?

External Chemical Treatment, i.e.

Dip Bath Flush Flowing

5.8.3 Choice of method/chemical or drug

Systemic Treatment,i.e.
	Sulphonamides Nitrofurans Antibiotics Injection

5.8.4 Treatment trial

5.8.5 Treatment

SCS/PCC/WP-15
THE ENVIRONMENTAL IMPACT OF CAGE CULTURE OPERATIONS
by
T. K. Mok¹

1. INTRODUCTION

The effects of pollutants on fish have been widely studied but the effects of fish culture on the environment have received little attention. The setting up of floating cages and the rearing of fish will undoubtedly modify the environment both physically and biologically.

2. POTENTIAL ENVIRONMENTAL IMPACT

2.1 Reduction in water currents

With the setting up of floating cages, water currents between the cages inside the culture area are reduced. A further reduction in water current occurs inside the net cages even in the absence of marine fouling. This reduction in water currents will result in decreased dissolved oxygen available to the fish, particularly during slack water.

2.2 Production of organic wastes

Cage culture produces a relatively large amount of organic wastes from uneaten feed and metabolic by-products. The wastes from the cultured fish occur in two forms: a soluble form and a flocculate that settles readily in still water. Also, algae and other fouling organisms detached during the cleaning of the cages throughout the year may re-enter the water.

2.3 Transmission of parasites and pathogens

Parasites and pathogens pass from the fish into the water. The release of these parasites and pathogens into the water affects other fishes, aquatic fauna and the whole ecosystem, but the effect is not known. Also, drugs and chemicals used to control diseases and parasites are also introduced into the water mostly through the fish food. Little is known of their effects, but in general their applications are sporadic.

¹ Fisheries Research Division, Agriculture and Fisheries Department, Hong Kong

2.4 Change of water quality

Decomposition of organic wastes from fish excreta, unconsumed feed and fouling organisms from cage cleaning, contributes to the degradation of the water quality. Dissolved oxygen is reduced, and biochemical oxygen demand (BOD), the levels of carbon dioxide, ammonia, nitrite and nitrate are increased. Particles of wastes increase turbidity and level of suspended solids which eventually deposit on the bottom. This is a possible significant source of pollution.

2.5 Increase of pollution-associated diseases

The degradation in water quality may cause environmental stress to the fish resulting in increased occurrence of stress-associated fish diseases, e.g. fin-erosion and ulcers.

2.6 Influence on bottom fauna

Organic wastes may cause anaerobic conditions and the formation of hydrogen sulphide in the bottom. The composition of benthic communities will change with increased growth of those organisms which can tolerate the conditions, e.g. polychaetes and gastropods.

2.7 Enrichment of water

Suspended solids, turbidity, nitrite, nitrate and ammonia levels increase with the degradation of water quality. This increase in nutrients will enrich the water which in turn favours the growth and propagation of many fish food organisms. This is beneficial from a fishery point of view. However, this enrichment, when evaluated from a public health and water quality standpoint, may not be desirable.

2.8 Eutrophication

The enrichment of water by organic wastes may upset the balance of nutrients and under certain conditions may stimulate the excessive growth of phytoplankton to an extent which interferes with the physiology of the fish. Under eutrophic conditions, oxygen is initially produced in excess by photosynthesis, but when the phytoplankton die, their decomposition will rapidly consume oxygen from the water. This usually leads to a state of oxygen depletion, which is aggravated during slack water, and may consequently cause mortality of fish by suffocation.

3. DISCUSSION

Of the various potential changes to the environment associated with cage culture operations, the most significant source is the continuous production of organic wastes from fish excreta and unconsumed feed. The extent to which this may upset the balance of the ecosystem of the site is dependent on the intensity of fish culture. In addition, the setting up of floating cages causes a reduction in the volume of waterflow, resulting in a decrease of dissolved oxygen supply to the fish. This can, however, be minimized through adequate spacing of cages and frequent cleaning of nets to remove fouling organims.

Organic wastes from cage culture are biodegradable through microbial activities which recover the nutrients from the wastes to the ecosystem. Bottom deposits beneath cages may be employed as an index to determine whether a cage culture operation has any noticeable impact on the environment. Thompson and Phillips (1980) analyzed core samples of bottom sediments beneath cages of one of the small-scale culture sites in Hong Kong, and found that the values of organic carbon were very low with negligible difference between the top surface layer and older sediments at 10–15 cm depth, indicating no adverse effect of the culture operation on the marine environment.

Since cage culture is practised in open areas with tidal range, there is significant water exchange through tidal currents which will normally assist in maintaining a certain level of oxygen and avoiding a build-up of wastes and parasites. The detrimental effect on the quality of water will occur only as a result of a long term release of wastes through high density of cage set-up and poor feeding practices. If the cage density is optimal and feeding practice is well-managed, the effect of tidal currents may nullify the detrimental effects from culture, and degradation of the water quality may not occur at all. Studies on environmental impact of cage culture operations will provide information required for the control and treatment of pollution sources to meet the environmental quality standards for fish culture.

4. REFERENCES

Assessment of the effects of pollution of fisheries and aquaculture in Japan. FAO Fisheries Technical Paper No. 163. 105p.

Coastal Aquaculture and Environment. 1972 Indo-Pacific Fisheries Council Proceedings, 15th Session, Wellington, New Zealand. 99p.

Milne, P.H. 1972 Fish and shellfish farming in coastal waters. Fishing News (Books), Ltd., London. 208p.

Report of the Seminar on Methods of Detection, Measurement and Monitoring of Pollutants in the Marine Environment. 1970 Rome. FAO Fisheries Reports No. 99. Suppl. 1. 123p.

Thompson, G.B. and D.J.H. Phillips. 1980 Report on a visit to the fish rafts at Tap Mun on 31 August 1979. Agriculture and Fisheries Department, unpublished report. 1p.

SCS/PCC/WP-16
CONTROL OF FOULING ORGANISMS IN MARINE CAGES
by
C. Wong¹

1. TYPES OF FOULING ORGANISMS

Fouling organisms include:

1. Bivalves - e.g. mussels, Mytilus spp, Teredo spp, Anomia spp

2. Algae - mainly Ectocarpus spp, Scytosiphon spp, Enteromorpha spp (green) and other mixed diatoms, Alaria spp, Dictyopteris spp

3. Hydrozoans - e.g. tufts, Tubularia spp

4. Nudibranchia - Facelina spp

5. Lichens - Verrucaria spp, Arthopyrenia spp, Roccella spp

6. Crustacea (barnacles) - Chthamalus spp, Semibalanus spp, Balanus spp

7. Gastropods- e.g. snails and slugs, Hydrobia spp, Littorina spp

2. MATERIALS USED FOR FISH NETTING

1. Natural fibers (cotton and hemp)

2. Synthetic fibers (nylon, terylene, ulstron, polythene and courlene)

3. Rigid polymeric fabrics (netlon and plastabond)

4. Metallic fabrics (galvanized steel, aluminium, stainless steel, brass, copper, cupro-nickel and nickel)

¹ Fisheries Development Division, Agriculture and Fisheries Department, Hong Kong

3. CHOICE OF NETTING MATERIALS TO MINIMIZE THE ATTACHMENT OF FOULING ORGANISMS

The natural and synthetic fibers have been used for many years for fish netting, mainly of the knotted diamond mesh type, but which can also be produced in square mesh. Nets made from these fibers are easy to handle and store. The rigid polymeric fabrics have been developed mainly for fencing - netlon in a plastic extruded square mesh and plastabond in a chain link diamond mesh with a galvanized steel wire core, both of which can be rolled up for easy handling and storing. The metallic fabrics are usually manufactured as a cold drawn wire which can either be woven into chain link diamond mesh or welded into square mesh. Chain link mesh and the finer square meshes can be rolled for handling, but the larger gauges of square mesh can only be handled in sheet form.

Hemp and cotton are known to foul very quickly in the sea, and the modern synthetic fiber is more resistant to biological attack. But the synthetic fiber nets are more difficult to clean as the rougher surface texture of these materials offers a better means of attachment for byssus threads. Some of the metallic fabrics prevent the settlement of fouling organisms, e.g. 90/10 cupro-nickel (Cu/Ni) chain link fencing and manganese/ aluminium (Mn/Al) square mesh. Recently a new polymeric material (Parafil) has been developed for mooring cables with an anti-fouling coating. These materials are quite resistant to attachment of fouling organisms.

4. CONTROL OF FOULING ORGANISMS

(a) Chemical method - Use chemicals to destroy and/or minimize the growth of fouling organisms, e.g. CuSO₄, bleaching powder and formaldehyde. Fish nets are taken out of water for a long bath of about 2–3 hours in a chemical solution to destroy all fouling materials. This method is not commonly used in Hong Kong as the chemicals used may have ill effects on the cultured fish.

(b) Mechanical method -

(i) With a high pressure water pump - nets are taken out of water and high pressure water is sprayed on the nets for clearing away the fouling organisms; and

(ii) With a wooden stick - nets are taken out of water and sundried for 2–3 days. Then, the fish culturists beat the nets with wooden sticks and knock off the fouling materials after which they are soaked in the sea water for a day. This method is widely practised in Hong Kong.

The nets are changed once a week for small mesh size of 0.5 cm (1/4 inch), once a fortnight for mesh size between 1.3–2.5 cm (1/2–1 inch) and once a month for mesh size above 3.8 cm (1–1/2 inches). Cages located in the south coast are not fouled as rapidly as those located in the coastal waters of the east coast. The nets in the south coast are usually cleaned once a month, as the salinity of the coastal water is much lower and fouling organisms are not as abundant or demonstrate restricted growth. Under our subtropical condition, nets can be fouled very easily and rapidly. If the fouling organisms are not removed regularly, they may block the passage of water through the net thereby reducing the dissolved oxygen content inside the cage and may impose respiratory stress on the cultured fish. The cage could sink from immense weight of the foulers or tear due to the sharp edges of bivalves or barnacles.

Regular checking of the conditions of the nets for wear and tear is important. Very often the netting might be torn by predator fishes or by the sharp edges of barnacles or bivalves, and this will cause escapement of the cultured fish.

SCS/PCC/WP-17
HARVESTING AND MARKETING OF CULTURED MARINE FISH IN HONG KONG
by
C. Wong¹

Cultured live marine fish are produced in Hong Kong by cage culture and impoundment methods. The largest culture areas are in Sok Kwu Wan, Ma Nam Wat, Loo Foo Fat, O Pui Tong, Po Toi O and Tap Mun. In 1980, there was a total of 1 553 large and small mariculture units with 2 339 fish rafts covering a total area of 17 hectares, and 11 impoundments occupying a total area of about 16 hectares. The culture units are situated in about 50 locations and provide employment for some 4 000 persons. The annual yield from impoundments is low at 0.3 kg/m² and compares unfavourably with the cage culture yield of 7.6 kg/m² of cage area. Basically, fish are cultured in cages made of galvanized steel wire or synthetic fiber netting and suspended from wooden rafts. Fish of the same species and size are generally kept in the same cage. Aerators are not required as water circulation resulting from tidal movements is usually adequate.

The most popular species cultured are:

The local market has a strong preference for these species which are suitable for culture under local conditions.

The demand for live marine fish is high in Hong Kong. The price for live marine fish is about four times that for fresh fish. The pricing of cultured marine fish is generally dependent on fish size, species and consumer demand.

¹ Fisheries Development Division, Agriculture and Fisheries Department, Hong Kong

At harvest, fish are transferred by means of a scoop net from the culture cage to a carrier boat equipped with holding tanks. In the case of impoundment culture, the fish are herded by means of a seine net and selected for sale. The carrier boat transports the fish to a central landing point where they are weighed and sold to wholesalers at mutually agreed prices. Sometimes, the fish are collected by the floating restaurants directly from the fish culturists. If land transportation is needed, the fish culturist will hire a 1.5-ton lorry equipped with fish tanks, aerator and water pumps with additional extra costs.

The fish sold to wholesalers are kept in cages and wooden barges, the latter having water circulating and aerating systems. The fish are normally fed with minced trash fish. When purchase orders for live fish from the retailers and restaurants are received, the required species are loaded and delivered on lorries which are fitted with aerated tanks. During the whole of the above process the handling of the fish is reduced to the minimum so as to avoid excessive injury. At the retail destinations, these fish are transferred to and kept in aquaria until sold. Fish weighing over 300 g are generally considered marketable. A list of the common species cultured in Hong Kong, with their average market sizes and wholesale prices is tabulated below.

The 1980 annual production of the industry was 760 metric tons with a value of some HK$37 million, 0.5% and 5% by weight and value, respectively, of the total landings of the capture fishery. Some statistical data on the marine fish culture (cage culture) industry in Hong Kong and its production during 1974, 1976, 1978 and 1980 are tabulated in the following page.

¹ Figures refer to the total annual yield.
² Figures refer to the value at average annual wholesale price.

A map showing the locations of live marine fish landing sites and culture sites in Hong Kong

SCS/PCC/WP-18
ECONOMICS OF MARINE FISH FARMING IN HONG KONG
by
John Cheng¹

1. INTRODUCTION

Commercial marine fish farming in Hong Kong started over a decade ago. Two methods are practised, viz. cage culture and impoundment culture. The former is by far the most common and the latter is of limited development. The notes below refer to cage culture farms only.

The majority of the fish farms are owner-operated and about 60 percent of them have a size of less than 100 m² of total raft area each. About 80 percent of the operators are also active small-scale fishermen engaged in trapping, purse seining, gill netting, hand lining and shrimp trawling. Fish farming offers them a better value for the live juvenile fish they catch by rearing them to marketable size, and provides an additional source of income.

2. CAPITAL INVESTMENT

The initial capital investment on an average size mariculture farm with a total raft area of 115 m² and nine cages each measuring 7 m² × 2.7 m deep costs about US$3,000. In addition, a mechanized sampan used as a tender boat costs another US$600.

3. WORKING CAPITAL

As most farmers collect fish fry for self use and work on their own farms, working capital covers mainly the purchase of fish feed which costs about US$1,000 to US$2 000 per annum for an average size farm, depending on the size of the stock. However, in recent years, many fish farmers have been importing fry of red grouper, green grouper and sea perch from China, Taiwan, Thailand and the Philippines.

¹ Economics and Marketing Division, Agriculture and Fisheries Department, Hong Kong

4. FEEDING

Fish in stock are fed with trash fish daily, although some farmers feed the fish less frequently, particularly in winter. Fish farmers normally feed their fish in the morning. They work about 2–4 hours a day for feeding and maintenance, depending on the size of the farm.

5. HARVEST

It takes about 12–24 months for the juvenile fish to reach marketable size. Although harvesting may occur at any time of the year as fish farmers usually stock fish of varying sizes at different times, the peak harvesting season is between December and February when demand and prices are relatively high.

The average survival rate of stocked fish less than 150 g in weight could be as low as 20 percent, disregarding fishkills caused by adverse hydrographic parameters and diseases. However, for fish of over 150 g, the average survival rate is about 70–75 percent.

6. COSTS AND EARNINGS

The costs and earnings position of an average size farm is assessed on the basis of a survey carried out during December 1980. First, the average yield and fish prices are used to establish the revenue of the farm. Second, the average physical inputs and current costs are used to establish the operating expenses. The details are shown as follows:

7. PROFIT

The net income of a farm of 63 m² (cage area) is US$1 480, or a profit of about 20 percent (i.e. net income over revenue). However, most farmers who collect fish for stocking and utilize unpaid family labour may cut expenses by US$3 190 and thus increasing profit to US$4 670 per year. Given a net cash flow of US$1 480 per year, it will take less than four years to recover the initial investment of US$3 600 on the farm, at current interest rate (i.e. 20 percent per annum).

In Hong Kong, there is a strong preference for live, fresh fishery produce. The mariculture industry which caters for live, prime species has been developed along this trend for profit reasons. With limited supply and a strong market demand, the price of live marine fish is about four to seven times greater than that of fresh marine fish. Given such a price incentive, marine fish farming will continue to be a profitable proposition.

8. ADVANTAGES

In general, marine fish farming is a viable undertaking for most of the small-scale fishermen. Apart from creating employment opportunities, it requires relatively inexpensive capital input. The almost self-sufficient nature of the farm operation keeps the cash outlay at a low level. The profit margin compares favourably with the other fisheries sub-sectors.

9. DISADVANTAGES

Marine fish farming is a high risk venture, particularly for the small-scale operators. The uncertain supply of fish for stocking from their own catch and the sudden fishkills and outbreaks of diseases can often upset their expected earnings from fish farmings. Although in any cases better management helps to reduce the occurrences of fishkills and diseases, there are red tides, typhoons, oil spills and other natural disasters which pose a threat to fish farming and are often beyond the control of the farmers. At present, no insurance scheme is available to cover all these accidental losses which, therefore, make many of the small operations very vulnerable.

10. LARGE SCALE FARMS

There are some ten large commercial farms in Hong Kong each with a total raft area of over 200 m². One of the largest of these farms has an annual turnover of some US$2.0 million. With a strong financial backup, these farms can stock higher-priced species of fish. The stocking, feeding and harvesting of fish are properly controlled, but the basic culture techniques show little difference compared with the small farms.

In general, the large commercial farms do not exhibit any significant advantage on the economy of scale in yield rate and earnings relative to their farm size and capital investment. However, most of these farm enterprises have an established distribution network, e.g., the owner of a farm is also a fish retailer or a restaurant operator. Farming then becomes an integral part of the business.

11. DEMAND ANALYSIS OF LIVE MARINE FISH

In economics, it is understood that the aggregate consumer's demand makes the market demand. However, consumption demand particularly for food may be divided into domestic and non-domestic.

The domestic consumption of live marine fish, apart from income and price, may be very much a matter of taste and preference or as an occasional treat in the family meals. It is difficult to isolate the underlying patterns of regularity, if any, as the commodity in question is relatively insignificant in the family budget compared with staple food, clothing, housing and services.

In Hong Kong, about 70 percent of the live marine fish is consumed in the restaurants, hotels and other food establishments, a proportion of which caters for the tourist. Most people who dine out may wish to try different foods from those that can be prepared at home. Indeed, much of the non-domestic consumption of live marine fish can be said to be “created”. That is, institutional factors influence demand and the eating of marine fish which can be selected live from an aquarium or tank may become a social fad, a status symbol, or a speciality cuisine.

12. ASSESSMENT OF MARKET POTENTIAL

The demand for live marine fish as a relatively high-priced commodity may have very different consumption patterns for individuals dependent on cross-sectional characteristics. The assessment of prospective market demand by using the “per capita” concept is, therefore, unrealistic because under this method the cross individuals' variations are effectively held constant. Elaborate cross-classifications exhibit much more variability than do the averages in income and prices of a time-series analysis. Income, age, taste, education, city type, market location, ethnical or other characteristics of family type, even weather and seasonal variations, such as festivals are examples of plausible variables that may have independent effects on consumption.

In dealing with the cross-sectional approach, very often a large scale sampling on the consumption behaviour of the individuals is required in order to identify the pattern of regularity related to the demand for the commodity in question, i.e. live marine fish. In simple terms, one is trying to study the factors that influence the consumers' purchase decisions on the commodity during a single period. The most common type of survey for collecting cross-sectional data is the household expenditure survey.

13. EXPORT POTENTIAL

Assessments of demand from overseas markets are often based on the statistical analysis of past trends. Trade missions or commissioned studies are also useful approaches to evaluate the export potential of a specialized commodity, such as live marine fish.

SCS/PCC/WP-19
MARICULTURE LEGISLATION IN HONG KONG
by
T. Y. Yim¹

1. THE PRESENT SITUATION OF THE MARICULTURE INDUSTRY

Mariculture started in Hong Kong over 10 years ago and arose from the maintenance of fish stock in cages suspended from floating restaurants to meet the demand for live marine fish from restaurant patrons. The industry is dynamic and has expanded considerably over these years. There are now 50 areas with mariculture farms.

The rapid and unregulated expansion of the industry has given rise to a number of undesirable situations. These include:

1. Overcrowding of fish rafts which adversely affects productivity;

2. Fish rafts encroaching upon approaches to piers;

3. Fish rafts being established in areas planned for reclamation for land development use or close to population or industries where pollution risk is high;

4. Fish rafts competing with pleasure crafts for sheltered waters;

5. Fish rafts not moored in an orderly manner resulting in an impairment of the visual amenity of the place.

Therefore, it is necessary that the industry be regulated through legislation to enable it to develop in an orderly manner. It should also be protected from pollution and interference from other activities.

2. MARICULTURE LEGISLATION

The legislation on mariculture, called the Marine Fish Culture Ordinance, was passed in 1980. It provides for the regulation and protection of the industry. The Director of Agriculture and Fisheries is entrusted with the responsibility of administering this Ordinance.

¹ Fisheries Development Division, Agriculture and Fisheries Department, Hong Kong

On the regulatory side, the Marine Fish Culture Ordinance stipulates that mariculture may only be practised in areas of water specifically designated for that purpose, known as fish culture zones. Persons who wish to practise mariculture in these zones are only allowed to do so under licence and in specified sites in the zones. The Director of Agriculture and Fisheries is empowered by the Ordinance to designate suitable areas of water to be fish culture zones, to issue mariculture licences and at the same time to specify sites for the licences to practise mariculture.

Mariculture outside fish culture zones is prohibited. But, the holding of marine fish and the culture of marine fish for scientific research purposes may be done outside fish culture zones with permit from the Director.

To ensure that licensed mariculturists will carry out their operations properly and that space in fish culture zones will be efficiently utilized, the Director has the power to cancel or refuse to renew a mariculture licence if a licensed mariculturist is found to be incompetent or fails to manage his operation adequately.

On the protection side, the Ordinance makes it an offence to deposit on land or in the sea substances that may injure the fish in a fish culture zone, or may pollute the waters in the zone. Out of the same concern, the Director is given the power to instruct a licensee to dispose of or destroy his fish in a site that are infected by disease, and to dispose of any noxious or waste materials resulting from the harvesting of fish.

The Ordinance also makes it an offence to interfere with rafts in a fish culture zone.

Vessels are not allowed in a fish culture zone unless they are authorized by the Director. This provision aims at keeping vessels not related to mariculture out of a fish culture zone.

In order that the provisions of the Ordinance can be carried out effectively, the Ordinance also provides for the making of regulations for the inspection of sites, rafts and fish in fish culture zones, for the licensees to submit returns and reports, to keep accounts, registers and records, and for other matters as necessary.

3. THE IMPLEMENTATION OF THE ORDINANCE AND PROBLEMS ANTICIPATED

As a first step in the implementation of the Ordinance, the designation of the fish culture zones will be published in the Hong Kong Government Gazette, the official government publication. Plans for these fish culture zones will be available for inspection by the general public in appropriate government offices. The boundaries of the zones will be marked by spar buoys which will remain in position regardless of the state of the tide. This will enable people at sea to recognize the zone boundaries easily. Signboards will also be planted on land close to zones to inform people of the location of the zones.

Mariculturists will have to apply for sites in a fish culture zone. Because of the distribution of the future zones which will differ considerably from the distribution of existing mariculture farms, it is expected that many mariculture farms will have to move from their existing areas into fish culture zones in other areas. This will result in the loss of some fixtures and fittings on the rafts, such as sinkers and mooring ropes which may not be recoverable or may not be suitable for use in the new sites. It will possibly result in the damage of fish stock during the physical removal. Government has given consideration to give cash allowances to these mariculturists to make good the loss.

It is expected that the demand for sites in some conveniently located zones will exceed what are available. It will be necessary to allocate these sites to mariculturists by ballot. Mariculturists already operating in the same area where the zone is will be given priority over those operating in other areas. This will help to minimize the disruption caused to mariculture in an area where a zone is designated.

4. REGULATORY MEASURES AND RESOURCES REQUIRED

To enforce the Ordinance effectively after the initial implementation stage, staff members of the Department will inspect each mariculture operation at least once a month. They will check the validity of the licences, whether the rafts are securedly moored in the site, whether the operation is properly managed, and for any activities or other matters that contravene the provisions of the Ordinance and the regulations made under it.

A team of 10 technical grade staff members will carry out these duties on some 1 700 mariculture operations which are expected to be established in some 23 fish culture zones. Except a few fish culture zones which are accessible by public transport, two fast launches will provide transport of personnel to fish culture zones. These launches will be equipped with radio telephone to facilitate communication in the event of an emergency, such as an oil spill, that may threaten a fish culture zone. It is envisaged that, with these resources, the Ordinance will be effectively enforced.