F. Müller
L. Váradi
Fish Culture Research Institute
Szarvas, Hungary
1. INTRODUCTION
2. SITE SELECTION FOR CAGE CULTURE
3. THE MAIN PARTS OF A CAGE FARM
4. RESULTS OF THE CAGE FISH CULTURE EXPERIMENTS IN HUNGARY
Different terms have been used for fish culture in an artificially enclosed area of a natural water body. The term 'Enclosure' is the general term applied to a culturing unit in such types of culture.
'Cage' refers to the type of culturing units consisting of a framed net open at the top and floating on the surface, or when completely enclosed, the cage is kept below the water surface by adjustable buoyancy or suspending from the surface.
'Pen' refers to that type of culturing unit where one part of the natural water body is enclosed by a fence-like wall resting on the bottom.
The increasing popularity of cage culture world-wide can be attributed to the following advantages:
- The applicability of cage culture is extremely varied. Rivers, backwaters, reservoirs, lakes, strip-mine lakes, irrigation canals etc., are all suitable for culture without any alteration in their state or function.- The production in small units (cages) as well as their quick and simple harvest render the method capable both for flexible adaptation to the actual market demands and for continuous supply.
- Cages are a convenient means of wintering, thus they save the vast investments for separate wintering ponds.
- Due to the high stocking density of enclosures, the direct observation of fish and immediate intervention, if necessary, are possible.
- Harvest is simple and quick, and the technological steps can be mechanized.
- Investment necessary to produce a unit of fish meat is 30-40 percent of that invested in a conventional fish pond system.
- Production of carnivorous fish, first of all sheatfish (Silurus glanis) in monoculture is also feasible.
- By producing seed for stocking natural waters in cages, building nursery ponds can be avoided.
- An indirect advantage is that by utilizing existing natural waters, land areas for fish ponds can be used for other rural activities.
- Cage culture can be well associated with sport fishing.
- The in situ construction of cages is quick and simple, so both the location and dimension of cages are easy to change.
Naturally, as with every technology, it has certain disadvantages as well:
- Since in cage culture natural feed is partly or completely ruled out of the fish diet, complete feed of higher protein content is required which significantly increases the feeding expenses in the net cost of the fish meat produced.- Due to high stocking density fish are more susceptible to bacterial and parasitic infection and are more sensitive to the decrease of DO (dissolved oxygen) content of water. This latter may result in lack of appetite and mortality in serious cases.
- From the viewpoint of environmental protection, the rules of applied technology should be kept in mind.
- When choosing the site, the foreseeable environmental stresses (pollution, oxygen depletion) should be considered.
2.1 Characteristics of the Site
2.2 Environmental Considerations
Prior to the introduction of cage culture to a natural water body three major factors have to be taken into account as follows:
- water quality
- water depth
- water current
First of all temperature, dissolved oxygen, pH and ammonia content have to meet the requirement of that species of fish desired to be cultured.
In addition to these parameters the total mineral content, different forms of N and P, free CO2, fenol, oil and tar content should be measured as well.
The water should not contain poisonous materials for fish. The expected change in the water quality parameters during the growing season has to be taken into account.
There should be a minimum of 1 m of clear water below the bottom of the cage in order to keep it away from the mud and sediment. Generally deeper water areas have to be used for siting of the cages.
When the water depth is considered, the expected minimum water level should be taken as a decisive value.
Although cage culture can be carried out successfully even in still water, some water current (10-20 cm/sec) has a good effect on the oxygen supply of fish, ensuring permanent water exchange between the water body inside and outside of the cage.
The water current helps to remove the solid wastes from the cage quickly. However a high water velocity is disadvantageous for cage fish culture for the following reasons:
- a large part of the food can be washed away
- the fish is forced to swim causing energy waste
- the regular shape of the cage is deformed (the useful water volume is decreased by the current)
According to some authors, water velocity in cage fish culture should not exceed 40 cm/sec.
The prevailing wind has to be taken into account during site selection as well. It may have a good effect on the water exchange by generating surface water current, however if it is too strong the cages should be placed in a sheltered water area or have to be protected by breakwaters.
2.2.1 Effects of cage fish farming on the environment
2.2.2 Effects of the environment on cage fish farming
2.2.3 Service and operational considerations
When a cage fish farm is established, one has to consider the effect of the farm on the environment and on the other hand the effect of the environment on the farm activity.
In flowing water (especially when large structures are placed in shallow water) the cages tend to act as a floating barrier causing differences in speed, direction and eddies as well.
However the most discussed question is the effect of cage fish farming on water quality. It is a fact that in those waters where cage fish farms are operating, deposition of bottom sediment can occur, including the undesirable build-up of waste materials (uneaten food, metabolic wastes).
Cage fish farming activity however, can be done in conformity with the requirements of the environment, taking into account the following considerations:
- The size of the cage culture plant and thus the volume of the production has to be adjusted to the conditions provided by the given water body.- Proper feeding technology should be applied with special respect to the following:
feed quantity
feed quality
feeding method
The quantity of feed should be adjusted properly according to the water temperature and the condition of the fish. Overfeeding should be avoided.
The feed should be water stable and easily acceptable by the fish. First of all dry, pelleted food is recommended. In cage fish culture automatic feeders are recommended.
- Aeration can also be applied in order to improve the environmental conditions.- The water body outside of the cages can be stocked with certain species of fish in order to control the growth of plankton and benthic organisms.
- The sediment accumulated under the cages can be removed by a submersible pump.
Among the different effects of the environment on cage fish farming, two major factors have to be noted, namely the damage caused by predators and poaching.
The cage can be protected against birds with a top cover. A double net can be installed in order to protect the fish stock from predators like rats, otters, turtles, etc.
These animals can be kept away by a dog as well if there is a walkway for the dog along the cages.
The cage fish farm has to be protected against poachers by ensuring permanent guarding. Some electrical anti-poaching devices can also be used.
The shore facilities of a cage fish culture plant should be accessible by a paved road. Electric energy supply and the market as well have to be taken into account. The required labour has to be available at the site.
3.1 Netting
3.2 Floats
3.3 Frame
3.4 Mooring
3.5 Shore Facilities
Several types and sizes of cages have been designed and used widely, as shown in Figures 1 to 5. However there are general considerations that have to be taken into account during the design.
Figure 1. Cages with flexible nylon netting
Figure 2. Cages with rigid netting a)
Figure 2. Cages with rigid netting b)
Figure 3. Octagonal cage designed in Norway
Figure 4. Submersible cages
Figure 5. Rotating cages
One consideration is the fish species because of its general behaviour, feeding response, tolerance to high stocking density etc. Its age and size influence the design of the cage as well. On the other hand the cage must allow for easy feeding, grading treatment and harvesting of stocks.
The netting has three major functions as follows: keeping the fish stock together; protecting the stocks against harmful external influences; allowing free water exchange between the inside and outside water.
The most commonly used netting material is flexible nylon since it is relatively inexpensive and it can be treated with chemicals against anti-fouling (Figure 1). Rigid netting material (e.g., rigid plastic, galvanized or plastic coated steel) are also used in some cases (Figure 2).
The mesh size should be as large as possible, taking into account the fish size. The larger the mesh size the better the oxygen supply of the stocks and the fouling problems are less as well. The fouling of the net should be avoided by regular cleaning or by replacing it. Heavy fouling reduces the water exchange through the net wall and thus causes oxygen depletion inside the cage; increases the net drag requiring large and more expensive mooring; increases labour requirement in cleaning and replacing the net.
The floats should keep all the floating devices (netting with frame, feeder, walkway, etc.) safely on the water surface. Empty barrels, plastic containers or styrofoam bodies can be used for that purpose, but specially designed air or foam filled plastic floats are also readily available. In Figure 6 an inflatable plastic float can be seen. There are special floats with variable buoyancy so that the position of the cage can be changed in the water body.
The frame can be made of galvanized steel, aluminium, timber and different plastic materials. The frame should be mechanically strong, resistant against corrosion, and easily repairable or replaceable. For fixing the different frame elements together special joints have to be used. A special joint can be seen in Figure 7. The applied rubber and hemp hose ensure flexible connection between the adjoining cages. Although the feeder is not a real part of the frame structure, one has to provide for the mounting and easy operation of a feeder during the design.
Although the criteria of the mooring system of fresh water cages are not as high as in the case of heavy sea conditions, the mooring system should be strong enough to hold the cages and the connected floating facilities in position in all weather conditions.
In shallow water the cages can be tied to a pile driven into the bottom. In deep waters the cages can be moored with various configurations of anchors, cables and floats as illustrated in Figure 8.
The shore facilities usually involve:
building or shed for feed storage and feed preparation
store for equipment and materials with a separate compartment for fuel storage
building for the staff with small size workshop
landing stage for boats
harvesting place
Figure 6. Inflatable plastic float
Figure 7
Figure 8. Mooring system
4.1 Culture of Common Carp (Cyprinus Carpio) in Net Cages
4.2 Net Cage Culture of Sheatfish (Silurus Glanis)
4.3 Net Cage Culture of Bester (a hybrid of Huso huso × Acipenser ruthenus)
4.4 Biculture of Sheatfish (Silurus glanis) and Silver Carp (Hypophthalmichthys molitrix) in a Net Cage
4.5 Net Cage Polyculture of Carp and Herbivorous Fish
4.6 Fish Polyculture in an Enclosure
Experiments with cage culture of common carp showed that different Hungarian carp land races can adapt well to these new, crowded conditions.
In experiments with common carp fry performed in 1975 two cages were used. The production during the 172 day-long culturing period was 24.2 kg/m3 and 42.5 kg/m3 in the two cages placed in faster (50-60 cm/sec) and slower (3-4 cm/sec) water current respectively (Table 1).
Pelleted feed with identical composition (raw protein, 28.85 percent; starch value, 743 g/kg, 12.525 kJ) was applied in both cages.
Feed loss, however, was higher in the case of the first cage, due to the fast current which, at the same time unfavourably changed the shape of the net cage in the water, reducing its useful capacity. The fast current forced the fish to swim constantly which required increased energy, resulting in an energy consumption 20 percent higher for 1 g/kg weight increase. (20 cm/sec water current was found to be optimal by German researchers.)
During experiments to culture market-size common carp a stocking density of 1 200-1 500/cage was found optimal in the case of healthy, sturdy carp fry with 250-500 g body weight.
The fish adapt to the new conditions within 5-10 days. Feeding must be done regularly in the same hour of the day, avoiding any excess movement or activity which may irritate the fish (stepping on the plank, drawing of the feeding tray or net, bumping of the boat to the buoys etc.).
Feed can be given on feeding trays twice or three times a day, or with demand feeders. Application of demand feeders in our experiment performed in 1979, resulted in a 0.8 kg increase in feed conversion rate. In another experiment in 1982, a production of 576 kg could be achieved with automated feeder.
The initial value of daily food ration is expressed in percent of fish body weight. The daily food ration is 2-5 percent of the body weight of fish, which naturally depends on environmental factors too, first of all on the temperature and dissolved oxygen (DO) content of water.
Sheatfish fry can be adapted to the highly stocked enclosure in 10-12 days. Feeding was made twice a day with pelleted feed of 3 mm particle size, manufactured at the Feed Mill of the Institute (Szarvas). The pellet consisted of 10 percent trash fish, 40-50 percent slaughterhouse waste material (liver, spleen, lung, etc.), 40-50 percent concentrate (vitamins, mineral premix, cereals - wheat, rye, rice extracted soy and alfalfa meal - according to our own prescription). The slaughterhouse waste material not only ensured cheap animal protein, but increased the water stability of the pellet by 2-4 hours.
The daily food ration was 2-4 percent of the actual body weight of the fish. The polyethylene feeding trays (800 × 500 × 100 mm) were bordered with a 300 mm net, so that the fish could not sweep the pellet off the tray. The tray was fixed in the middle of the cage, 1-1.2 m in the water, so that the fish could swim freely under it.
The optimal feed consumption and weight gain was experienced at 20-24°C and 6-10 mg/l oxygen content.
In our experiment of 1974, sheatfish fry of 23 g reached 275 g body weight in a 165 day-long culturing period, while in 1975 sheatfish fry with 49 g initial body weight were 250 g at the end of a 175 day-long culturing period.
In another experiment in 1976 (Table 2) fish were cultured in two phases. Ninety sheatfish fry stocked in 1 m3 on 10 May were transferred to another bigger cage on 31 June, where the density was 30 individuals/m2 only.
The best weight gain, i.e., 20.1 kg/m2 was experienced at a stocking density of 94 individuals/m2.
The netting of the cages was cleaned of algae and bio-fouling with a strong brush once a fortnight. To prevent Ichthyophtirius multifillius infestation regular (monthly) flush was applied in a solution of malachite-green (1 mg/l) and formalin (0.2 ml/l). In our experiments performed in 1976 and 1977, Flexibacter columnaris caused disease, which, it was discovered, could be overcome with feeding medicated feed. The feed contained 1% Neo-Te-Sol (a Hungarian-made antibiotic complex of 18% oxytetracycline, hydrochlorocid, 12% neomycinum sulfuricum and 70% saccharose).
A 5-day-long feeding period with the medicated pellet was followed by a 5-day-break and feeding again proved to be a successful strategy, the mortality rate being 33% in the untreated cages, and 1-1.5% in the treated ones.
For the undisturbed growth and prevention of stress effects, reference may be made to the section on carp culture.
One-year-old bester - having been reared in a net cage for 1 year - were stocked with an average individual weight of 80 g (58-113 g) into a floating net cage locally constructed from galvanized metal bars and synthetic netting of 5 mm mesh, with a frame of 2 × 2 and placed into the water 1.6 m deep.
Feed pellet with 3 mm particles was put on synthetic feeding trays, as described with sheatfish, twice a day. Feed ration was 2-5% of the actual body weight of the fish as a function of temperature and oxygen content of the water. The feed consumption was satisfactory and the growth can be fully attributed to artificial feed. The pellet contained trash fish paste, boiled slaughterhouse wastes (liver, spleen, lung, etc.) mineral and vitamin premix, extracted soy-meal, fish and meat-meal, with the following values: raw protein: 53,4%, fat: 8.22%, ash: 9.44%, moisture content: 10.7%.
The best weight gain of bester was at 18-23°C temperature and an oxygen content of water higher than 6 mg/l. There was a dramatic decrease in the appetite of fish, however, if DO content decreased below this value.
DO content of 3 mg/l or less resulted in considerable mortality. The best production in a culturing period was achieved with a stocking density of 44 individuals/m2, where the weight was the highest (40 kg/m2) as well. An extremely favourable feed conversion rate was obtained (2.78), together with good weight growth rate (9.42).
Bester fry adapted well to this intensive culturing system. No bacterial, parasitic or other type of diseases led to any losses. The average loss during the culturing period, i.e., 13%, was caused by a temporary oxygen depletion in two cages.
The bester with an average stocking weight of 86 g reached 583 g with an individual weight gain of 3.2 g/day/4% during a 5-month-long culturing period in net cages.
Favourable experience was obtained in an experiment performed in 1978 with sheatfish fingerling and 2-summer-old silver carp in biculture. During the 160 days of the culturing period, the sheatfish and silver carp fingerlings grew from 62.5 to 297 g, and from 300 to 600 g, respectively. Stocking rate was 80% sheatfish and 20% silver carp, altogether 37 individuals.
Table 1 Results of the Cage Culture Experiments with Two-Summer-Old Carp in 1975
|
|
Cage in fast current 50-60 cm/sec |
Cage in slow current 3-4 cm/sec |
|
Date of stocking |
7 May |
7 May |
|
Stocking rate (fish/m2) |
125 |
167 |
|
Stocking rate (fish/m3) |
83 |
111 |
|
Initial average weight (g) |
107.8 |
107.8 |
|
Stocking weight (kg/m2) |
14 |
18 |
|
Stocking weight (kg/m3) |
9 |
12 |
|
Date of harvesting |
22 October |
22 October |
|
Harvested number (fish/m2) |
124 |
161 |
|
Harvested number (fish/m3) |
83 |
107 |
|
Harvested weight (kg/m2) |
50 |
81 |
|
Harvested weight (kg/m3) |
33 |
54 |
|
Average weight (g) |
407 |
505 |
|
Weight gain (kg/m2) |
36 |
63 |
|
Weight gain (kg/m3) |
24 |
42 |
|
Individual daily increment (g/day) |
1.79 |
2.38 |
|
% |
1.66 |
2.2 |
|
Number of feeding days |
167 |
167 |
|
Feed conversion rate |
6.0 |
4.7 |
|
Weight gain coefficient |
3.8 |
4.7 |
|
Survival rate (%) |
99.4 |
96.3 |
Number of cages: 2 floating cages, 3 × 4 × 1.6 m impregnated perlon netting
Table 2 Results of Cage Culture Experiments with Two-Suimner-Old Sheatfish Fingerlings in 1976
|
|
Phase I |
Phase II |
Total | |
|
|
of growing season |
| ||
|
Date of stocking |
10 May |
1 August |
| |
|
Stocking rate (fish/m2) |
90 (56-108) |
32 |
| |
|
Stocking rate (fish/m3) |
58 (37-72) |
18 |
| |
|
Initial average weight (g) |
127 (90-180) |
282 |
| |
|
Stocking weight (kg/m2) |
11.4 (9.7-13.5) |
9 |
| |
|
Stocking weight (kg/m3) |
7.4 (6.2-8.7) |
5 |
| |
|
Date of harvesting |
31 July |
4 September |
| |
|
Harvested number (fish/m2) |
88 (56-106) |
29.9 |
| |
|
Harvested number (fish/m3) |
57 (37-71) |
16.6 |
| |
|
Harvested weight (kg/m2) |
26 (18-30) |
11.9 |
37.9 | |
|
Harvested weight (kg/m3) |
17 (11.6-19.3) |
6.6 |
23.6 | |
|
Average weight (g) |
298 (270-322) |
397.5 |
| |
|
Weight gain (kg/m2) |
14.8 (8.4-16.8) |
3.3 |
18.1 | |
|
Weight gain (kg/m3) |
9.6 (5.4-10.8) |
1.8 |
11.4 | |
|
Individual daily increment (g/day) |
2.08 |
1.71 |
1.95 | |
|
% |
1.6 |
0.6 |
1.5 | |
|
Number of feeding days |
82 |
65 |
147 | |
|
Feed conversion rate: |
|
|
| |
|
|
average |
3.3 |
6.5 |
4.5 |
|
|
extremes |
2.5-4.8 |
5.5-9.6 |
4.5 |
|
Weight gain coefficient |
|
|
3.13 | |
|
Survival rate (%) |
88.5 |
98.0 |
88.5 | |
Number and type of cages:First phase - 5 floating cages, 2 × 2 × 1.6 m impregnated perlon netting
Second phase - 5 floating cages, 4 × 4 × 1.8 m impregnated perlon netting
Sheatfish fingerling in monoculture with the same density grew from 58 to 245 g. In biculture, i.e., sheatfish together with silver carp, there was a better survival rate (+ 5-10%). Moreover the biofouling was eliminated by silver carp.
In this type of polyculture mostly young fish were cultured. Of the stocked fish, 33% was common carp (Cyprinus carpio), 17% bighead carp (Aristichthys nobilis), and 50% silver carp (Hypophthalmichthys molitrix), altogether 188 fish/m2. Feeding was done twice daily as previously described.
During the 169 day-long culturing period the carp fry grew from 123 to 500 g, the bighead carp from 18 to 115 g and the silver carp from 17.3 to 126 g.
Growth of common carp was the same as experienced and described with monoculture.
The weight gain of herbivorous fish, however, was 50% behind the value obtained with fish cultured in a fish pond owing to the lack of natural food. The survival of carp was 98-100%, that of the silver carp and bighead carp, 83-97% (average 90%) and 88-99 (average 94%) respectively.
The production calculated for 1 m2 ranged between 22.3.35.3 kg, 35-50% of which was herbivorous fish. Lower value than expected was obtained with silver carp because some of them escaped by jumping over the 40 cm high netting of the cage above the water surface. When culturing silver carp in an enclosure, the cage should either be made with a higher netting wall or it should be covered.
A 0.2 ha-sized part of the experimental oxbow lake of the Institute was surrounded by a solid wall on the long side and wire-mesh fences on the shorter sides. The water depth during the growing season was 1.2 to 1.3 m and about 1 m of water flowed through this enclosure per second.
As the flow rate depended on the needs of irrigated farming in the region, artificial aeration was also ensured in order to prevent damage caused by oxygen deficiencies. Two demand feeders with a volume of 100 1 each were provided for the distribution of the pellet feed. The feed contained 23.8% of crude protein, 5.6% of crude fibre, 4.9% of crude fat, its starch value was 668 and its calculated energy content was 2 354 kcal.
Polyculture was used for raising market fish in this enclosure. The stocking density was 46 870 fish/ha, 75% of which was common carp, 12.8% silver carp, 8.5% bighead, 3.2% grass carp and 0.5% tench, wels, and pike perch.
After a 152-day growing season the gross fish yield reached 40.45 tons/ha and the net yield reached 29.62 tons/ha. The feed conversion rate was 3.22 and the survival rate, 98.9 percent.
In previous years, hand feeding, and later, automatic feeders, were used in the same enclosure. The fish became accustomed to using the demand feeders in 10 days. Compared to the results achieved in previous years, it can be demonstrated that the introduction of demand feeders improved the feed conversion rate by 0.5 to 0.8.
