8. Chilling fish at sea

Contents - Previous - Next

Proper handling and storage of fish at sea ensures that the catch stays as fresh as possible until it is landed. The important requirements are to chill the fish rapidly as soon as they are caught, to keep them chilled, and to maintain a good standard of cleanliness on the fishing deck, in the handling area or shelter deck, and particularly in the fishroom or stowage area.

A well designed vessel can make handling of the catch easier, but few ships are ideal in this respect. Good stowage practice can help maintain freshness of the catch even in badly designed vessels, or in small boats where stowage facilities are primitive. Bad handling, even on a well designed ship, can only result in poor quality fish.

The importance of good practice at sea cannot be over-emphasised because, fish begin to spoil as soon as they die. Neglect on board, even on short fishing trips, can sometimes result in poor quality fish after only a few hours. Moreover, since the time the fish is on board ship is often longer than the time on shore between landing and consumption, the fisherman may bear much of the responsibility for the freshness of the fish reaching the consumer.

In many countries there are now schemes for inspecting and grading the catch on landing. Therefore, the care with which the catch is stowed as well as the length of the trip will affect the value put upon the catch. Under these conditions there is usually a financial incentive to the fisherman to bring back the catch in top condition, since the penalty for poor practice may well be downgrading or even withdrawal of the catch from sale.

The art of good stowage can vary to some extent with the species being handled, the type of fishery being pursued, the size of vessel and the length of voyage. There are, nevertheless, certain broad principles that apply to almost all fisheries and these are outlined here. Although the advice is based mainly on that given to the north Atlantic trawling industry, most applies equally to the smaller inshore vessel, whether fishing in temperate or tropical waters. However, where necessary the special problems of tropical stowage are discussed in more detail.

Methods of handling and stowage

Lean fish. Lean fish are those in which the fat content of the flesh is usually less than 1 %; the bulk of the fat is contained in the liver as opposed to fatty fish in which most of the fat is in the body tissues. Lean fish are also usually bottom-living species, caught principally by trawl, seine or long line. Fatty fish are typically surface living fish, caught for example by purse seine, midwater trawl, gill net or hand line.

Lean fish caught by large vessels in colder waters are usually gutted at sea, but in some inshore fisheries, the small number of crew may exclude the possibility of gutting large numbers of small fish; this is often true of many tropical fisheries. It is generally true that gutting is desirable in order to remove one of the main sources of bacteria and proteolytic enzymes, particularly if the stomach of the fish is full of food. Where stomachs are consistently found to be empty and the fish are not required to be bled, it is sometimes permissible to omit the gutting stage, particularly if this avoids any long delay before chilling.

However small the vessel and however short the trip, ice should be carried aboard and used to protect the catch. After gutting and washing, the fish should be stowed as soon as possible in crushed ice or in ice which has inherently small pieces. There are two main methods of stowage in general use, either bulk stowage in ice within the fishroom or stowage area of the ship, or with ice in containers, usually boxes of some kind, which can be removed from the ship on landing without disturbing the catch. Most methods of stowage in ice, sometimes with minor variations or refinements in practice, fall into one of these two categories.

On small boats in which the boxed iced fish has to be stacked on the open deck, an insulated cover will not only help to further protect the fish from adverse ambient temperatures, but also reduce the rate at which the ice melts and hence the demand for ice storage where space is at a premium.

Fatty fish. Most fatty fishes, for example herring, sardine, mackerel and so on (are usually caught in large quantities), are too small and too numerous to gut at sea. Because they are fatty and are left whole, they spoil much more quickly than lean fish and consequently there is even greater need to chill them as quickly as possible. Small fatty fish do not withstand bulk stowage in ice very well; they are fragile and easily squashed or damaged. Therefore iced stowage in containers, or in RSW, are the main methods of keeping them on board ship.

Principles of good stowage

Immediately after capture, all fish should be chilled rapidly to keep spoilage to a minimum. Fish stowed in plenty of ice are typically at a temperature of -0.5C (see p. 53). The ice has to do more than just chill the fish; it usually has to remove heat from the surrounding structure of the box or the fishroom; it has to absorb the heat input through the structure during stowage from the warm air and sea outside, and it may also have to remove heat produced by the spoilage process in the fish themselves. It is therefore essential that plenty of ice is properly distributed throughout the catch to ensure efficient cooling.

Ideally, each fish should be in contact only with ice and not even with other fish. Fish touching one another do not cool as rapidly as those completely buried in ice. Apart from this, when a fish is stowed so that it is against a smooth surface, like the side of a box or a large area of another fish, air may be completely excluded. Some spoilage bacteria, in the absence of air, can rapidly produce foul smelling odours which spread throughout the flesh of the fish, resulting in what are known as stinkers or "bilgy fish" in north Atlantic fisheries. Fatty fish as well as lean fish are sometimes affected by this kind of spoilage. There are usually innumerable small pockets of air between small pieces of ice, so that fish properly surrounded by ice does not become spoiled in this way.

When the ice melts, some cooling of the fish is done by the melt-water running over the fish, probably because contact is much better between the fish and the ice cold water than between fish and ice; the meltwater acts as a carrier of heat as it flows from fish to ice and fish to ice again within the fish/ice mixture. Because of the shape and size of the pieces, flake ice usually melts more quickly in direct contact with the fish than crushed block ice, and therefore cools the fish more rapidly. A continuous flow of melt-water is desirable to preserve the fresh moist appearance of the fish.

Besides helping to cool the fish, the melt-water also washes away bacterial slime, spoilage products and traces of blood, and thus helps to preserve the fresh appearance and smell of the fish. It follows therefore that small fish should never be so tightly packed that the flow of melt-water is prevented. At the same time, it is important to provide adequate drainage so that the fish do not become immersed in dirty water.

The surrounding temperature should be kept just above ice temperature to permit the ice to melt, but not so high that ice is needlessly wasted. A fishroom temperature of 1-2C is usually suitable. If the air temperature around the iced fish is too low, say -1C to -2C, uncontrolled slow freezing of the outermost fish can occur, whilst those in the middle of the stowage may be inadequately cooled, since only fish in direct contact with the ice will be cooled quickly if insufficient melt-water flows over them.

Iced fish should not be in too thick a layer in a box or a fishroom without some intermediate support, because the bottom fish will be crushed and damaged, and also lose a significant amount of weight. Boxes should be shallow and bulk stowage and stowage in large containers should have supporting shelves at intervals of not more than 0.5 m.

Cleanliness is an important part of good stowage. Much of the care exercised in chilling can be wasted if the boxes or the stowage space are dirty, or if dirty ice from a previous trip is used on fresh fish. All unused ice should be discarded at the end of each trip. Although it may look clean, unused ice may be heavily contaminated with spoilage bacteria. Fish stowed in dirty ice spoil faster than fish stowed in clean ice. After discharging the fish at the port the fish room should be cleaned out with suitable detergent and disinfectant.

In fishing vessels which have a proper fishroom, some of the points to watch for in good stowage practice are as follows.

Bulk stowage. Always cover the bottom of the fishroom with a layer of ice 10 to 15 cm deep. The actual depth will depend on how well the fishroom is insulated, the length of the voyage and the temperature of the sea outside. If the fishroom is a metal one, or the floor is uninsulated, increase the thickness of the bottom layer of ice, especially if the floor is a bare steel tanktop. If no ice remains between fish and floorboards when the ship is discharged, then not enough ice has been used. The bottom layer of fish will have warmed and will probably be spoiled.

The first layer of fish should be placed on the bed of ice, more ice sprinkled over the fish, and additional ice placed against the fishroom lining, particularly when the ship's sides are uninsulated. As each further layer of fish is added, a sprinkling of ice should be put over it until the space is almost full. A top layer of ice about 5 cm thick should be added. For long trips in temperate climates, about one tonne of ice should be used for every 2 tonnes of fish; in tropical waters about one tonne of ice to one tonne of fish is usually necessary for safe keeping. There should always be some ice remaining throughout the fish on landing (Fig. 22).

Where the shelves for bulk stowage are made of portable boards, they should be filled so that the weight of the shelf of fish above is resting on the shelf supports rather than on the fish below. The advantage of keeping bulk-stowed fish in shallow layers not more than 0.5 m deep will be lost if fish and boards rest on top of other fish, and the catch will be crushed, damaged, provide lower yield and spoil more rapidly.

Successive shelves of fish and ice should be added in the correct manner until the stowage is full; the topmost fish should be covered with 10 to 15 cm of ice to protect them against heat coming through the deckhead.

Corrugated shelf boards have the advantage that dirty melt-water at the bottom of the stowage is carried away to the sides and does not run through onto the fish stowed below.

Bulk stowage can be improved by making the layers as shallow as possible, the ideal being a single layer of fish on each shelf, with ice above and below the fish so that crushing is virtually eliminated, and every fish is adequately protected by surrounding ice. This ideal method of storage can only be achieved at the expense of a poorer stowage rate of about 4.5 m/tonne of fish.

Bulked fish often suffer a certain amount of rough handling during discharge from the fishroom into some kind of container at the port market.

Fig. 22 Bulk stowage

Boxed stowage. Boxing at sea can produce better quality fish on landing than bulking, with less weight loss, and can also help to ensure that the fish continues to be well protected after landing, by remaining in the same box with ice.

Design of the box is important; first and foremost it must be big enough to hold the required weight of fish and sufficient ice to chill the contents and keep them chilled at least until landing. It should not be so deep that the bottom fish are crushed and should be long enough to accommodate, without bending, most of the larger fish which are caught. At the same time, it should not be so unwieldy that it cannot be handled by one or two men as required, both at sea and in port. The boxes should also nest/stack when empty, so that there is plenty of working room at the start of stowage.

Drain holes in the box should be so arranged that melt-water drains down the sides or the ends of the box below, rather than down through the fish in the box below. Although the melt-water helps to cool the fish quickly it eventually becomes dirty so it is undesirable to have it flowing over too many layers of fish. The box should be capable of being cleaned easily, and should not taint or contaminate the fish. It should be robust enough to withstand rough handling on board ship and should where necessary be suitable for onward transit on shore, leaving the contents undisturbed. In tropical fisheries, where it may not be possible to keep the boxes in an insulated fishroom, it is an advantage to insulate the box itself and fit it with a lid, so that it can be used for onward carriage on land without the need for an insulated road vehicle.

Stowage in a box should consist of a bottom layer of ice, about 5 cm deep, layers of fish sprinkled with ice, and a final top layer of ice again about 5 cm deep. As with bulk stowage, the test of whether enough ice has been used is how much is left when the box is discharged; in tropical fisheries, particularly, the thickness of the top and bottom layers may have to be increased if little or no ice remains on and among the fish on landing.

Some fishermen tend to overfill fish boxes with a resultant loss in quality, yield and shelf life. Overfilling boxes to the extent that fish are protruding above the edges of the box results in the fish being crushed when the boxes are stacked. Overfilling also means that the space available in the box for ice is reduced and insufficient ice may then be added to last the entire storage period (Fig. 23).

Fig. 23. Box stowage

The species of fish, seasonal variability and intrinsic quality are factors which will greatly affect the extent of losses incurred by overfilling so that the following results from boxing trials are only indicative of likely losses. The results are from one trial with haddock, but in another with similar fish of a poorer intrinsic quality the results were significantly different.

Standard nest/stack type plastic boxes designed to hold 30 kg of fish and 15 kg of ice were Overfilled with 50 kg of fish and stacked 7 to 8 boxes high for a period of 6 to 7 days and a comparison made with good boxing practice.

The following losses were incurred:

3.3% extra drip loss
8.8% less fillet yield, mainly due to the need to trim damaged fillets
2 days loss in potential shelf life based on a taste panel assessment

The above are overall losses but there were significant differences depending on the location of a box within the stack. The best results were for fish in boxes on top of the stack and the greater losses were evident in boxes at the bottom.

Boxes at the bottom of a fishroom should be on battens to keep them off the floor, and the intervening air space should be filled with ice. Alternatively, the bottom layer of boxes can be charged only with ice to provide the necessary thermal barrier. Similarly, boxes against ship's sides or bulkheads should be supported by battens with ice between the boxes and the lining, particularly when the fishroom is uninsulated. Robust boxes can be stowed full height in a fishroom without any supporting structure in the form of stanchions and shelves, so that full use can be made of the stowage space.

If full advantage is to be taken of boxed, iced stowage, the buyer at the landing place has to be prepared to buy by sample. If every box has to be emptied and the contents checked for quality and weight, the catch may be handled and disturbed as much as when stowed in bulk, However, if a sample box (or boxes) is truly representative, the balance of the catch can be taken on trust and the whole operation of discharge and sale can be simplified.

How much ice should be used at sea?

A simple rule of thumb has already been given, one tonne of ice to 2 tonnes of fish in temperate waters, one tonne of ice to one tonne of fish in tropical waters. Many factors can affect the amount of ice required. Sea and air temperatures, the insulating efficiency of the fishroom or the container, the size and temperature of the fish as they come out of the sea, delays in handling the fish, the efficiency of icing, the mean temperature of the fishroom and the length of the voyage all have an effect. The only safe way of judging the ice required in a particular fishery is to examine the catch on discharge. The core temperature of the fish should be close to 0C and there should be a reasonable amount of ice remaining amongst the fish. Otherwise insufficient ice has been used. Particular attention should be paid to vulnerable parts of the stowage such as, fish close to ship's side or tanktop, boxes against a bulkhead and so on.

Further points on good stowage practice

After completing the stowage of a haul of fish, the top fish should be covered with a protective sprinkling of ice, even though the shelf or box is not full.

Draughts of warm air, for example from open hatches, should be excluded from the fishroom.

Extra ice should be used wherever the insulation is known to be poor or for example, against an engine room bulkhead, to absorb the incoming heat.

Additional ice should be used on the topmost layer of fish to protect it against heat from lights, warm air and a hot deck. Even where cooling grids are fitted, extra ice on the top of the fish will reduce the risk of unwanted drying and partial freezing. Lights should be switched off, and hatches closed, when not in use.

Remote reading thermometers should be fitted at appropriate points in the fishroom so that air temperature can be monitored throughout the trip. Deck awnings should be rigged when the deckhead temperature is high, and if necessary, the deck should be hosed to reduce the amount of heat entering the fishroom.

Ice which becomes contaminated during the trip, by fish or dirty boots for example, should be discarded and not used for stowage.

Fishroom design and equipment

Provided sufficient ice is available, fish can be stowed satisfactorily in almost any kind of container. It is obviously desirable to have a structure that is durable, hygienic, convenient for stowage and discharge, and resistant to entry of heat.

Insulation. The ice in an unrefrigerated fishroom has to absorb the heat coming into the fishroom as well as cool the fish. Insulation can help to keep the ice consumption at a reasonable level, especially in warm seas. Insulation should be all round the fishroom or stowage space, not just on the deckhead. About two thirds of the fishroom is surrounded, more often than not, by warm water so that insulation on ship's sides and tanktop is just as important as on the deckhead and bulkheads. The insulating material should not absorb water, and should be capable of being inserted in all the awkwardly shaped spaces between beams or frames, around drains and stanchions and so on. Cellular expanded plastics are often most suitable for fishroom insulation. Where wood is used for, or behind, the lining of an insulated fishroom, it must be protected by a suitable preservative against rot, since damp air will inevitably penetrate behind the lining. In temperate waters, about 5 cm of an expanded plastic insulant is usually sufficient for a chilled fishroom, but in tropical waters a layer of insulation 10 cm thick may be necessary to give adequate protection.

For boxed stowage, no internal fishroom structure is normally necessary; for bulk stowage, the shelves and partitions should be portable and of simple design. Boards should as far as possible be of the same size, easily cleaned, and sufficiently strong. As much as possible of the internal structure should be removable, so that ice can be easily placed wherever it is needed without having to manoeuvre round intervening partitions.

Refrigeration. The simplest form of fishroom refrigeration, other than the use of ice, is the fitting of cooling grids on the deckhead and sometimes also on bulkheads and ship's sides. The ability to precool the fishroom on the way to the fishing ground without using too much ice is particularly useful in tropical waters. But the cooling coils cannot be expected to chill the catch effectively, only ice can do that job. Once fishing has begun, the main function of the cooling coils is to cope with heat leaking into the fishroom, and to cool warm air coming in through open hatches, leaving the ice to do the main job of chilling the fish. The refrigeration system should be controlled by a thermostat, with the bulb carefully placed so that it is representative of the average fishroom air temperature. It should be set to cut out at about 0.5C and to cut in at about 2C. Fans should not be used to blow air about the fishroom, since the fish will dry out rapidly wherever they are exposed to moving air.

Reliable and sufficient insulation, together with ample ice, augmented where necessary by a simple deckhead cooling coil, usually suffice to protect the catch in any climate.

Dos and Don'ts of good stowage practice: A summary

If the fishing vessel is decked and has a proper fishroom, stow the fish below in ice as quickly as possible. If not, do not delay chilling.

Adequate icing is essential even on the shortest voyages; fish begin to spoil as soon as they are dead, and they go off four times more quickly at 1 0C, the temperature on a cool day, than at the temperature of melting ice.

Always use clean fresh ice; discard dirty ice and ice left over from a previous trip.

Use small pieces of ice; large pieces mark the fish and may not chill as quickly.

Use plenty of ice; a layer below the fish, more ice among the fish and another layer on top. This applies whether stowage is in boxes or in bulk on supporting shelves.

Even when fish have not been gutted, do not delay, ice them quickly.

Don't overfill a box or shelf; the next box or shelf on top will squash the fish below.

Don't leave out shelves even when fishing is heavy; fish will be crushed and lose weight.

Use too much ice rather than too little; there should always be plenty of ice left among the fish when you land.

Put extra ice against linings and bulkheads where most of the heat leaks in.

Put a thick layer of ice on top of fish close to the deckhead; this protects them from warm air and stops then drying out.

Don't pack fish so tightly that melt-water cannot flow; the fish are chilled more rapidly when ice cold water runs over them, but fish in a stagnant puddle of water and blood can spoil quickly.

Lay gutted fish belly downwards, so that puddles of dirty water cannot lie in them.

Lay battens under the bottom-most boxes to keep the fish off the warm floor, and to prevent bilge water contaminating them; put some ice between the battens.

Keep boxes off the engine room bulkhead by fitting vertical battens; put ice between these as well.

Put off fishroom lights whenever they are not required. Open only one hatch at a time, and close it whenever the work is done.

Do not operate the fishroom below 0C or above 2C.

If the vessel is undecked, box the fish with ice and protect the boxes with a cover of some kind, preferably with good insulating properties; ice can be carried out to the fishing grounds in the empty boxes.

Use insulated boxes where practicable in the tropics, particularly if the fish can remain in them for onward inland transport.

If ice is not available at your port, find out if there is sufficient support among other catchers and merchants for a small ice plant, perhaps on a co-operative basis; the improvement in quality is always very much worthwhile.

Contents - Previous - Next