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7.2 Fish handling in artisanal fisheries
Artisanal fisheries, existing both in developed and developing countries, encompass a very wide range of fishing boats from pirogues and canoes (large and small) to small outboard and onboard engine vessels, utilizing also a variety of fishing gears. It is difficult to find a common denominator; however, from a fish handling point of view, artisanal vessels handle relatively small amounts of fish (when compared with industrial vessels) and fishing journeys are usually short (usually less than one day and very often only a few hours).
In general, in tropical fisheries the artisanal fleet land a variety of species, although there are examples of the use of selective fishing gear. In temperate and cold climates artisanal fleets can focus more easily on specific species according to the period of the year; nevertheless, they may land a variety of species to respond to the market demand.
Although very often artisanal fisheries are seen as an unsophisticated practice, closer scrutiny will reveal that in many cases they are passing through a process change. There are many reasons for this process but very often the main driving forces are: urbanization, fish exports and competition with the industrial fleet.
This change in the scenario of artisanal fisheries is essential to understanding the fish handling problems faced by the artisanal and small sector of the fish industry, particularly in developing countries.
When the artisanal fleet was serving small villages, the amount of fish handled was very low; the customers usually bought the fish direct from the landing places, fishermen knew customers and their tastes, and fish was consumed within a few hours (e.g., fish caught at 06.00 h, landed and sold at 10.00 h, cooked and consumed by 13.00 h). In this situation, ice was not used, and gutting was unknown; very often fish arrived at landing places in rigor mortis (depending on fish species and fishing gear), and fish handling was at most reduced to covering the fish from the sun, keeping it moist and keeping off the flies. In Figure 7.7 two cases of landing un-iced fish by artisanal fishermen are shown.
With urbanization and the request for safer and more quality products (as a result of exports and competition with industrial fish) conditions changed drastically. Large cities also demanded increased fish supplies, and thus middlemen and fish processors had to go to more distant landing places for fish. The amount of fish handled increased, fishing journeys lasted longer and/or passive fishing gears like gillnets were set to fish for longer times, a chain of middlemen and/or official fish markets replaced the direct buyer at the beach and, as a result of growing business (fish for income), in some places the catch effort also increased with a consequent increase in the number of fishing boats and an increase in the efficiency of the fishing gears.
In one way or another, each of the new circumstances added hours to the time which passed between catching the fish and eating or processing it (e.g., freezing). This increase in exposure of un-iced fish to ambient temperature (or water temperature for a dead fish in a gillnet), even though brief (e.g., an additional 6-12 hours), dramatically changed the situation regarding fish spoilage and safety.
In the new situation, fish remained at ambient temperature some 13-19 or more hours. It could be already spoiled, at terminal quality and/or could present public health hazards (e.g., from the development of C. botulinum toxin to histamine formation). In addition to the safety and quality aspects, post-harvest losses, non-existent at subsistence level and very low at the village stage, become important. For instance, it is estimated that the post-harvest losses of Nile perch caught artisanally in Uganda amount to 25-30% of the total catch.
The situation described in previous paragraphs, and cases like those shown in Figure 7.7, moved extension services in developing countries and international technical assistance to focus on the problem of introducing improved fish handling methods at the artisanal level. The basic technical solution is the introduction of ice, proper fish handling methods and insulated containers, which is the approach utilized by most of the artisanal fleet in developed countries.
There are several examples where this approach was adopted by fishermen in developing countries and has become a self-sustained technology. Two very interesting cases to analyze are the introduction of insulated containers onboard of "navas", the traditional fishing vessels of Kakinada in Andhra Pradesh, India (Clucas, 1991) and the introduction of insulated fish containers in the pirogue fleet of Senegal (Coackley and Karnicki, 1984). The sketch of an insulated fish container for Senegalese pirogues is shown in Figure 7.8.
The insulated container of Figure 7.8 was designed to fit
existing pirogues, according to the type of catch and needs
expressed by fishermen. The materials and tools needed to
construct the insulated container are available to fishermen in
Senegal, even though some of them are imported
(e.g., foam sheets and resin).
The example of Senegalese fishermen is now spreading steadily to similar fisheries in Gambia, Guinea-Bissau and Guinea which are adopting the use of insulated containers similar to those of Senegal. However, the process of diffusion and adoption of a technology, even if relatively simple, is not as straightforward as could be supposed.
Once artisanal fishermen become aware of the rationale of insulated containers, they tend to favour large insulated fish containers rather than small ones. The reason is clear from Equations 7.e and 7.g, as for the same volume of fish and ice, large containers will present less external area than the area presented by several small containers. For example, a large cubic insulated fish container can be envisaged of a side measuring x m, and eight cubic insulated containers of sides equal to x/2 m presenting the same total volume as the large one. The eight containers will have an external area twice that of the big container, thus increasing the ice consumption by two, and decreasing the amount of fish that can be transported.
Other reasons are that small containers will cost more than a large one of the same total volume (simply because they need more material); small containers are not always easy to secure safely onboard small boats, and large containers allow for transport of large ice bars that can be crushed at sea (reducing stowage rate). However, large containers are difficult to handle and sometimes canoes and pirogues are very small or narrow and they cannot accommodate large insulated fish containers. This is the case for relatively small insulated fish containers.
A serious constraint in many artisanal fisheries is the relatively high cost of industrial containers and the difficulty in finding appropriate industrial materials to construct them. For this reason, efforts have been made to develop artisanal containers made from locally available materials (Villadsen et al, 1979; Govindan, 1985; Clucas and Whitehead, 1987; Makene, Mgawe and Mlay, 1989; Wood and Cole, 1989; Johnson and Clucas, 1990; Lupin, 1994).
In some cases, the correct approach could be to add insulation to local fish containers; in other cases it could be necessary to develop a new container. In general, artisanal fish could be cheaper than industrial fish containers, but they will not last as long. An artisanal insulated container developed at Mbegani (Tanzania), based on the local basket container ("tenga") is shown in Figure 7.11.
A key factor in the construction of artisanal insulated containers is the selection of insulation material. There are a number of materials available: inter alia, sawdust, coconut fibre, straw, rice husks, dried grass, old tires and rejected cotton.
However, the use of such materials presents problems: the materials become wet very quickly (with the exception of old tires), losing their insulating capacity and increasing the weight of the container. When wet, most of them tend to rot very quickly. The solution is to put them inside a plastic bag (waterproof); however, in this case they tend to settle, leaving part of the walls without insulation.
With a view to overcoming these problems, the concept of "insulated pillows" was developed in various FAO/DANIDA fish technology workshops. This concept is very simple: the insulating material (e.g., coconut fibres) is placed inside one plastic tube of the type usually found to produce ordinary small polyethylene bags (10 cm in diameter); the insulating material is pressed before sealing the tube; the tube is sealed by heat at both ends (e.g., every 20 cm), and with some practice it is possible to produce a strip of "pillows". It is advisable to utilize a second tube to reduce the incidence of punctures due to fish spines and bones.
The strip of "insulated pillows" can then be placed between the internal and the external walls of the container. Once the container is finished with an insulated lid and handles, fish and ice can be put in a large resistant plastic bag, as shown in Figure 7.11. The use of the plastic bag extends the lifespan of the container and improves fish quality.
This example indicates the type of practical problems found when developing an artisanal insulated fish container, and the possible solutions.
Why is ice not always used to chill fish when necessary?
Despite the knowledge on the advantages of fish chilling, ice it is not as widely used as it should be, particularly at artisanal level in developing countries. Which are the main problems found in practice? Some of the problems that can be found are as follows:
(i) Ice should be produced mechanically
This obvious statement implies, inter alia, that it is not possible to produce ice artisanally for practical purposes (machines and energy are required). To produce ice under tropical conditions, from 55 to 85 kWh/1 ton of ice (depending on the type of ice) are necessary whereas, in cold and temperate countries from 40 to 60 kWh are required for the same purpose. This may be a large power requirement for many locations in developing countries, particularly in islands and places relatively far from large cities or electricity networks. Ice plants require maintenance and hence trained people and spare parts (in many cases this requires access to hard currency).
A cold chain will also require chill rooms (onboard and on
land), insulated containers, insulated trucks and other auxiliary
equipment (e.g., water treatment units, electric generators).
Besides increasing the cost, all this equipment will increase the
technological difficulty associated with the fish cold chain.
(ii) Ice is produced and used within an economic context
In developed countries ice is very cheap and costs only a fraction of the price of fresh fish. In developing countries ice is very often expensive when compared with fresh fish prices.
A survey conducted in 1986 by the FAO/DANIDA Project on Training on Fish Technology and Quality Control on current fish and ice prices in fourteen African countries demonstrated that in all cases and for all the fish species, 1 kg of ice increased the fish price at least twice the rate recorded in developed countries. The cheaper the fish the worse the situation. For instance, in the case of small pelagics, the percentage of increase in the fish cost per kilogramme of ice added, was 40% for the "yaboy" of Senegal, 16-25% for the sardinella of Congo, and 66% for the sardinella of Mauritania and the anchovy of Togo. The market price for fish, in this case, acts as a deterrent for the use of ice.
According to the relative cost of ice to fish, ice may or may not be used. For instance, in Accra, Ghana in 1992, it was found that using ice to chill small pelagics (Ghanian herring) in a proportion of 2 kg ice: 1 kg fish would increase the cost of fish by 32-40%. However, in the case of snapper, for the same ratio of ice to fish the cost increase would be in the range of 4.5-5.7%. The result is that ice chilling of snapper is relatively common in Accra, whereas ice is not utilized to chill small pelagics.
Very often fish compete with other sources of demand (soft drinks, beer), even if the ice machine was initially installed to supply ice for chilling fish. This and energy losses at the ice plants contribute to increase the market price of ice.
In addition to producing and utilizing ice on a sustainable basis, economic aspects must be considered (e.g., depreciation, reserves, investment). Moreover, in the case of ice manufacture there is a strong influence of the scale of production. Low ice prices in developed countries are also the result of large ice plants located at the fishing harbours that supply a large number of companies and fishing boats.
(iii) Practical constraints
Introduction of ice into fish handling systems that are not accustomed to using it can create practical problems. For instance, from Table 7.1 it is clear that the introduction of ice will increase the volume required for stooge and distribution, and will reduce the effective fish hold in vessels. The use of ice will also increase the weight to be handled. This will have a number of implications such as an increased workload for the fishermen, fish processors and fishmongers, and an increase in costs and investment.
From Figures 7.3 and 7.4 it is clear that the total amount of ice needed per 1 kg of fish, in the complete cycle from the sea to the consumer will be much higher in tropical countries than in cold and temperate regions. As an indication, the average consumption of ice in the Cuban fishery industry was estimated at around 5 kg of ice per 1 kg of fish handled (including ice losses), although higher values (up to 8-10 kg of ice per 1 kg of fish) have been recorded in single industries in tropical countries; this necessitates large storage and transport capacities.
Freshwater or seawater utilized for producing ice should comply with standards (microbiological and chemical) for potable water and should be readily available in the volumes required. This is not always possible particularly in countries with energy problems (blackouts) and without (or with erratic) public tap-water distribution. If water has to be treated, this implies additional costs and additional equipment to operate and maintain.
Properly trained personnel are required to operate the ice plant and auxiliary equipment efficiently, and to handle ice and fish properly. Although many developing countries have made efforts to train people, in many cases there is a lack of technical personnel ranging from well trained fish technologists to refrigeration mechanics or electricians, or simply plant foremen.
Moreover, in many developing countries it is increasingly difficult to keep technical and professional schools operating in this field, thus jeopardizing the possibility of self-sustained training, and hence fishery industry developments.
(iv) Ice is not an additive
Knowledgeable people (e.g., fishmongers) are quickly aware of the fact that ice is not an additive. Therefore, when there is a delay in icing, ice is not usually utilized (even if available) because it will not improve fish quality. Consumers could also be intuitively aware of this fact, and they prefer to be presented with the fish as it is (e.g., at the terminal state of its quality) rather than in ice, because in this case ice will increase the price of fish but not enhance its quality. Due to the above and to the problems associated with the transition between artisanal and industrial or semi-industrial fisheries, already discussed, consumers in some countries (e.g., in Saint Lucia and Libya) tend to believe that iced fish is not fresh fish.
A need for chilled fish can develop if a market for iced fish (not just a market for "fresh fish") is developed, and to develop a market for iced fish where it does not already exist may be a very difficult and expensive endeavour as is the introduction of any other food product.
(v) Need for appropriate fish handling technologies
To chill and keep fish with ice is a very simple technique. A more complicated picture emerges when actual fish handling systems are analysed, including the economic aspect.
From a comparative study on the same fish handling operation,
utilizing ice and insulated containers, carried out in both a
developed and a developing country, it was seen that in developed
countries, the more "appropriate" technology would aim
at reducing wage costs (e.g., chutes to handle ice and fish,
special tables to handle containers and boxes and conveyors to
move them, machines that mix ice and fish automatically); in
developing countries the main concern would be to reduce ice
consumption, and to increase the fish:ice ratio in the containers
(Lupin, 1986 b).
The same study found that a twentyfold difference in wage costs between developing countries and developed countries cannot offset a tenfold difference in the cost of ice. There is no "comparative advantage" in low wages in developing countries with regard to fresh fish handling. Advanced technology on fish handling from developed countries could make work easier for people in developing countries, but might not improve the economics of the operation as a whole.
There is obviously no single solution to the problems discussed above. However, it is clear that it is the problem to be solved in the coming decade in the field of fresh fish handling. With total catches having reached a plateau, losses due to the lack of ice utilization could be ill- afforded, and developing countries and artisanal fishermen in particular should not be deprived of potential market opportunities.
7.3 Improved catch handling in industrial fisheries
The aims of modern catch handling are the following:
- to maximize the quality of the landed fish raw material. It is of particular importance to provide a continuous flow in handling and to avoid any accumulation of unchilled fish, thereby bringing the important time-temperature phase under complete control.
- to improve working conditions onboard fishing vessels by eliminating those catch handling procedures which cause physical strain and fatigue to such a degree that no fishermen need to leave their occupation prematurely for health reasons.
- to give the fisherman the opportunity to concentrate almost exclusively on the quality aspects of fish handling.
To meet these aims, equipment and handling procedures that will eliminate heavy lifting, unsuitable working positions and rough handling of fish must be introduced. By doing so, the catch handling time is accelerated and the chilling process initiated much earlier than was previously the case (Olsen, 1992). The typical unit operations in catch handling are shown in Figure 7.12. earlier than was previously the case (Olsen, 1992). The typical unit operations in catch handling are shown in Figure 7.12.
Figure 7.12 Typical unit operations in catch handling of pelagic and demersal fish
Important general aspects in modern catch handling are:
- phase one, which covers the time used for the necessary handling onboard, i.e., the time until the fish is placed in chilling medium, must be as short as possible. The fish temperature at time of capture can be high with consequent high spoilage rate.
- phase two - the chilling process - must be arranged so that a fast chilling rate is obtained for the whole catch. Maximum chilling rate will be obtained by a homogeneous mixing of fish and ice, where the individual fish is completely surrounded by ice and the heat transfer therefore is maximum, controlled by the conduction of heat through the meat to the surface. This ideal situation can be obtained during chilling of small pelagics in a chilled seawater (CSW) system; but by chilling demersal food fish in boxes with ice it is not always possible to obtain homogeneous fish/ice mixing. However, the appearance of fish completely surrounded by ice is often deteriorated due to discolorations and impression- marks. In practical life, icing is therefore often done by placing a single layer of fish on top of a layer of ice in the box even if it is bad practice from a temperature control and therefore shelf life point of view. Cooling is primarily achieved by melt-water dripping from the box stacked on top. This type of chilling will only function satisfactorily if fish boxes are shallow and have a perforated bottom.
- in phase three, which covers the chilled storage period, it is important that a homogeneous temperature at -1.5°-0°C is maintained in the fish until first hand sale. As this period may be extended for several days, this aspect has top priority.
Catch handling can be done in several ways ranging from manual methods to fully automated operations. How many operations will be used in practice and the order in which they are done depends on the fish species, the fishing gear used, vessel size, duration of the voyage and the market which has to be supplied.
Transferring catch from gear to vessel
Midwater trawlers and purse seiners fishing pelagic fish use tackling in lifts of up to 4 t, pumping or brailing for bringing the catch onboard. When lifting huge hauls (100 t or more) onboard by these methods, the danger of losing fish and gear always exists if the fish start to sink after having been brought to the surface. The speed of which the fish may sink depends on the species, catching depth and weather conditions during hauling.
Pumping the catch onboard using submersible pumps without bruising the fish can be difficult, as it is not easy to control the fish-to-water ratio during pumping.
In recent years, the so-called P/V pump (P/V - pressure/vacuum) has found increasing use. The P/V-pump principle is that an accumulation tank of 500-1 500 l size is alternately put under vacuum and pressure by a water-ring vacuum-pump (Figure 7.13). The fish, together with some water, are sucked through a hose and a valve into the tank of the system. When the tank is full, it is pressurized by changing the vacuum and pressure side connections from the tank to the pump and the fish/water mix flows through a valve and a hose into a strainer. The P/V-pump is claimed to handle the fish more gently than other fish pump types, but the capacity is generally lower, mostly because of the alternating way of operations. This problem can be solved by having two P/V-tanks running in phase opposition using only one vacuum- pump.
Figure 7.13 Working principle of a P/V pump
Small gillnetters (10-15 m) haul the nets with the net hauler, and very often store their catch in the net until landing. Here the net is drawn through a net shaker by two men in order to free the fish from the gear. It has been shown that the violent way in which the shaker works can be harmful to the men's hands, arms and shoulders. Ergonomic precautions have therefore been suggested to overcome this problem.
Trawlers and seiners (Danish and Scottish) tackle the catch into pounds. Commonly used pounds are those with a raised bottom which can be hoisted hydraulically. The purpose of these designs is to provide good working conditions for the crew (Figure 7.14). Also gillnetters may use a work-saving pound system, which is often connected with a conveyor to bring fish to the gutting-table.
Figure 7.14 Deck lay-out for trawler using machine gutting of demersal fish
1. Tackle pound, 2. Hoisting pound, 3. Gutting table, 4. Bleeding/washing machine, 5. Gutting machine, 6. Chair.
Holding of catch before handling
When large catches are to be handled, or if for other reasons catch handling cannot start immediately, it is convenient and necessary to prechill the catch during holding in deck-pounds using ice or in tanks using Refrigerated Sea Water (RSW) or a mixture of ice and sea water (Chilled SeaWater, CSW).
Prechilling holding systems are mostly used on pelagic trawlers which grade their catches in size before storing in boxes or in portable CSW-containers. It is also essential to prechill when the pelagic fish are soft and feeding and therefore very prone to belly-burst. Prechilling tanks are unloaded by elevator or P/V-pumps. If no sorting is done onboard, the fish is conveyed directly for chilled storage in the hold.
A system for holding demersal fish in tanks is shown in Figure 7.15.
Sorting/grading
Pelagic fish are sometimes sorted or graded onboard according to size. The equipment used operates on the basis of thickness of fish using principles such as:
- vibrating, inclined diverging bars
- contrarotating, inclined, diverging rollers
- diverging conveyors where fish are being transported along a power driven V-belt.
Grading by thickness can meet the demand for the high capacity needed in pelagic fish handling, but it is generally accepted that the correlations between thickness and length or weight are not too good (Hewitt, 1980). The most important point, often forgotten, for making a grader function at its optimum is even feeding. This could be done with an elevator delivering to a (vibrating) water sprayed chute leading to the inlet guide chute of the grading machine.
Sometimes it is necessary to install a manual sorting conveyor before the grading machine for removal of larger fish and debris, e.g., in the fishery for argentine with by- catch of grenadier.
Sorting and grading of demersal fish by species and by size is normally done by hand. However, some automatic systems of sorting according to width are in use. Static or dynamic weighing by marine weighing systems are also in use with good results. Research is under way using a computerized vision system for species and size grading.
Bleeding/gutting/washing
In order to obtain optimal quality in a white fillet, many white-fleshed demersal fish (but not all) need to be bled and gutted immediately after capture. The best procedures from an economic, biological and practical point of view are still under discussion (see section 3.2 on bleeding and section 6.4 on gutting).
The vast majority of fishermen are handling the fish in the easiest and also the fastest way, which means the fish are bled and gutted in one single operation. This may be done manually, but gutting machines have been introduced to obtain even more speed. The fish are transported to and from the fisherman by suitable conveyor systems. Using machines, round fish can be gutted with a speed of approximately 55 fish/minute for fish length up to 52 cm and 35 fish/minute for fish length up to 75 cm. Gutting by machine is 6-7 times faster than hand-gutting.
Existing gutting machines for round fish of the type using a circular saw blade for cutting and removing the guts destroy the valuable roe and liver. A new type of gutting machine which copies the manual gutting procedure is now available on the market. Gutting speed of this machine is 35-40 fish/minute and the roe and liver can be saved (Olsen, 1991). Flatfish can also be gutted by a recently developed machine. The speed of this machine is about 30 fish/minute.
After gutting, the fish are conveyed to the washing or bleeding operation. This may be done in pounds, often with raised bottom or in special bleeding tanks, frequently with a hydraulically- operated tilting system and rotating washing drums are also used (Figure 7.15); and special equipment such as the Norwegian and British fish washer may be used.
After catch handling (sorting, grading, gutting, etc.), the fish may be passed to an intermediate storage silo or batch holding system for the different sizes or grades before being dropped by chute to the hold, or the chutes may lead directly from the grading machines to the hold (Figure 7.16).
Figure 7.16 "Polar"-system. Mechanized sorting and boxing of herring
1. Herring sorting machine, 2,3,4. Conveyors, 5. Flexible dosing tube.
Chilling/chilled storage
Demersal fish have traditionally been stored on shelves or in boxes. Boxing has a big advantage over shelf storage as it reduces the static pressure on the fish and also facilitates unloading.
Shelf storage is done by alternating layers of ice and fish from one layer of ice and fish (single shelving 25 cm between shelves) up to ice/fish layers 100 cm deep. In practice, shelving often allows better temperature control than boxing and therefore also a longer storage life. Because excessive handling during unloading and excessive pressure on the fish have a negative effect on quality, e.g., appearance, boxing is preferable to shelving, given proper icing.
In pelagic fisheries, boxed fish will be untouched until processed, but in demersal fisheries the catch is often only sorted by species onboard and not graded by size and weighed. These operations are carried out after landing before auction whereby some of the handling and quality advantages of boxing are lost.
In the near future when integrated quality assurance systems have been introduced, these unit operations will be carried out onboard and an informative label on each box will give details of factors of importance for first-hand sale (including freshness).
In general, two types of plastic fish boxes are used: stack-only and nest/stack boxes.
To overcome some of the space problems in using stack-only boxes, the nest/stack type has been developed. These occupy only approximately a third of the space needed when stored empty compared to when the boxes are loaded with fish and ice.
This type of box is widely used in France, the Netherlands and Germany and also in some Danish ports.
When a system tailor-made for a certain type of plastic box is designed, the quality advantages of using boxes can be fully utilized onboard. The key points to consider are:
1) The handling rate necessary to prevent quality loss because of delayed icing. Prechilling can be of advantage to compensate lack in handling rate.
2) Handling methods which make it possible to guarantee that the icing procedure is sufficient to chill the fish to 0°C and maintain this temperature until landing.
3) The hold construction must be constructed such that safe and easy stacking of the boxes can take place.
4) Hold insulation of a relatively high quality should be considered. A small mechanical refrigeration plant can be of advantage. Air temperature in the hold should be + 1°-3°C.
RSW-storage (Refrigerated SeaWater) is a well established practice which has been refined both theoretically and practically since its introduction in the 1960s in Canada where it was developed for salmon and herring storage (Roach et al., 1967). At the beginning, most RSW vessels were salmon-packers and because of some failures in design which were attributed either to insufficient refrigeration or circulation systems, a standard for control of RSW-systems was established. Since vessels are different, the RSW-installation has to be studied carefully in every fishery to determine its real capability. Therefore, methods for rating each individual system and vessel and providing general specifications and guidelines for the proper installation have been suggested by the Canadian technicians (Gibbard and Roach, 1976).
In order to obtain maximum shelf life from RSW-systems,
temperature homogeneity in the region of -1°C is very important.
The factors affecting temperature homogeneity were recently
studied in Denmark (Kraus, 1992). The most important conclusions
were that the inflow of the chilled seawater in the bottom of the
tank must take place over the whole tank bottom area, and that
filling capacity for securing water circulation and temperature
homogeneity is dependent on fish species.
The necessary chilling rate was suggested to be: fish temperature
must be below 3°C within four hours and below 0°C after 16
hours, and the temperature should be kept between -1.5°C and
0°C until unloading.
The CSW system has also been developed in Canada as a much cheaper means from an investment point of view - to obtain rapid uniform chilling of fish. The most popular method used is the so-called "Champagne" method where rapid heat transfer between fish and ice is obtained by agitation with compressed air introduced at the bottom of the tanks, instead of using circulation pumps as in RSW and some earlier CSW designs (Figure 7.18) (Kelmann, 1977; Lee, 1985). An indication of the chilling rate for herring could be: reduction of fish temperature from 15°C to 0°C within two hours. The concept of a CSW system is to load well insulated tanks at the harbour with the amount of ice necessary to chill the catch to between 0° and - 1°C and maintain this temperature until unloading.
Figure 7.18 Chilled seawater system: piping layout
The Canadian west-coast fishermen are achieving this in practice by using a minimum of seawater when they start loading the tank and by forcing air through the ice-seawater-fish-mixture only during loading, and stop forcing air immediately when the tank is full. Thereafter they will force the air only for 5-10 minutes with 3-4 hours' interval. The air agitation therefore only serves as a method to overcome local temperature differences in the tank. The objective is to obtain a uniform mixture of fish and ice in order to secure temperature homogeneity.
A proven rule-of-thumb for estimating the amount of ice necessary is simply to observe the amount of ice left in the tank at unloading, and compare it with temperature readings, which should be in the -1°C range measured in the landed fish. The starting situation should be conservative, which at sea-temperature around 12-14°C, for a trip lasting 7 days and with 10 cm polyurethane insulation, is 25% ice by weight of the tank capacity. The amount of ice is adjusted according to the observations on the following trips.
An analytical approach to estimate necessary ice quantities in a CSW tank system has been developed. The quantity of ice required takes into consideration tank size, catch volume, time at sea, water temperature, hold insulation and hold flooding strategy (Kolbe et al., 1985).
CSW "Champagne" systems can also be used in small
coastal vessels, e.g., in a fishery for small pelagic fish with
vessels of 10-14 m length with a fish carrying capacity from 3 to
10 t fish
(Roach, 1980).
Another way of loading a CSW tank, which is in practical use in Denmark, is to add the necessary amount of ice to the fish during loading by mixing a controlled stream of fish with a controlled stream of ice. The greatest amount of ice is added to the fish during loading. When the tank is full the voids are filled with seawater from a hose and the tank is left undisturbed, except for watercirculation by pumping or compressed air blowing for 5-10 minutes of 4-hour intervals. The ice is bulk-stored in the forward hold and the ice is shovelled into a conveyor flush with the floor. The conveyor then leads the ice to the mixing point at the deck.
The use of portable CSW containers for pelagic fish handling was tested in the early 1970s (Eddie and Hopper, 1974). The approximately 2 m³ heat insulated containers were loaded with the necessary amount of ice from the harbour and agitated with compressed air in a similar way as for CSW-tanks. The main advantages with this method are that the fish will be undisturbed until processed and easily unloaded. The disadvantages are: marketing problems and reduced pay-load on existing vessels (Eddie, 1980). Portable 1.1m³ CSW containers are used to a limited extent in combination with the earlier mentioned conveyor system originally laid-out for boxing without the above-mentioned reduced pay-load compared to boxing (Anon., 1986). Also, small coastal vessels can use insulated portable CSW containers (Figure 7.19).
Unloading
Shelfed fish are unloaded, using baskets or boxes which are filled as the shelves are removed. The fish are tackled from the hold and emptied on a conveyor leading to the manual grading and weighing process.
Boxed fish iced in 20 or 40 kg boxes at sea will normally be unloaded in pallet loads of, for instance, twelve 40 kg boxes per pallet. Swedish boats use hydraulic deck-mounted cranes and a special pallet fork during unloading. An unloading rate of approximately 30 t/h is possible by this method.
Danish coastal vessels, landing their pelagic catches daily, use quay mounted P/V- pumps for unloading their catches, which often are iced in pens in layers up to approximatly 1 m height. It is necessary only to add small quantities of water to make the pump function properly. The fish is delivered to a strainer from where a conveyer leads the fish to a size grader. The strained water is recirculated to the hold. Grading machines with up to 30 t/h are often installed.
In Scandinavia the 30-50 m RSW/CSW vessels still use brailing to a limited extent when unloading their catches at a rate of 30 to 50 t/h. The main disadvantage of this method is that very big hatches are needed to obtain reasonable unloading rates.
P/V-pumps have recently been introduced for unloading herring and mackerel. Thus vessels with small tanks, e.g., 30 m³, and small hatches can also be unloaded at a rate similar to or higher than the above-mentioned brailing rate. P/V-pumping rates will typically be around 40-50 t/h. The fish can be transported directly in a tube system into the factory where representative samples are taken for quality assessment.
