Table of Contents

On the fishing deck
On the factory deck
Operating the freezers
Using the air blast freezer room
Stowage in the cold store
On fillet freezing trawlers
The main points once again


This note describes good practice in handling the catch on stem freezer trawlers, from the time the fish come aboard until the frozen product is stowed in the ship’s cold store. Most of the advice concerns the production of blocks of whole white fish, but some additional recommendations are made concerning practice on ships that freeze fillets.

Probably no two freezer trawlers are exactly alike; layouts differ considerably and some ships are much more mechanized than others. Nevertheless most of the advice given is believed to be applicable to all ships, supplemented where necessary by owners’ special instructions for individual ships.

The freezing of white fish at sea is now well established commercially in Britain, and the product can be equal to the best that is available from traditional trawlers stowing the catch in ice. Sometimes, however, the quality of sea-frozen fish can fall short of perfection due to incorrect handling and processing; this note attempts to give the best advice on how the catch should be handled. Quality is everyone’s responsibility; earnings and indeed the future of the industry depend on it.

On the fishing deck

As soon as the cod-end comes up the ramp, the fish should be dropped to the factory deck below; fish left lying in the net or in pounds on the fishing deck will quickly become crushed or bruised and will spoil rapidly.

On the factory deck

The catch has to be gutted, bled and washed, and then conveyed to the freezers as quickly as possible; the fish must be kept cool throughout.

Before gutting

Don’t let the fish slide about too much; they will become chafed and damaged, especially when there are rough-skinned fish among them. Pound boards or dividers should be used to stop excessive movement in rough weather.

Don’t put newly caught fish on top of older fish; keep the older fish separate and gut them first.

Keep ungutted fish cool; if cold sea water or ice is available, use it to chill the fish awaiting gutting. Just running the deck hose on the fish will help a little if the sea water is cold enough; check that the water temperature is not above 5°C.

Most important of all, don’t keep fish waiting about; gutting should begin as soon as the fish are put below. Delay before gutting in a warm factory space increases spoilage and makes it difficult to bleed the fish properly. Fish that have been kept more than six hours before gutting should be inspected by the factory manager and specially labelled when they are frozen.

... the fish must be kept cool...


Gut the fish quickly after capture so that they can bleed properly; fillets cut from fish that have not been thoroughly bled are discoloured and unattractive.

Gut the fish cleanly and neatly; a ragged cut into the flesh will reduce the value of the fish, and any bits of gut or liver left in the belly may cause the fish to spoil more quickly. Cut both napes to make washing and bleeding more efficient.

If the ship does not have a gutting bench, don’t trample on the fish or kick them about; every time a fish is bruised this can result in an unattractive blood mark on the fillet that may not show up until after the fish is thawed and filleted some months later.

Don’t throw guts down on top of other fish; juices from the guts will digest and soften other fish and spoil their appearance. Throw the guts into a basket or well clear of the fish.

Keep an eye on cod livers for signs of worms; if the liver is heavily infested there’s a fair chance the flesh may be affected. If there are more worms under the black skin of the belly flaps, a dozen or more fish should be cut open and inspected. The skipper should be informed when quantities of wormy fish are found in one haul; he may want to consider changing grounds. In any case the factory manager should ensure that frozen blocks of wormy cod are specially labelled.

If fish are unusually soft or in poor condition, this should be reported; it may be desirable to label the fish specially or to change fishing grounds. Throw away crushed or damaged fish.

Transfer the gutted fish to the washer as gently as possible; every time a fish bangs against the deck or the ironwork it can be badly bruised or damaged.

Sorting into species and sizes is often done during gutting; gut the more perishable species first; haddock for instance spoil more quickly than cod and should not be left lying until all the cod are gutted.

Be careful when sorting for size; the buyers don’t like to find small codling in a block that’s supposed to be cod, and this will affect the value of the catch.

Wash out the gutting pounds each time they are cleared of fish.

Washing and bleeding

Washing removes traces of gut and slime, and some of the bacteria that make the fish go bad. The fish also need time to bleed so that the final product will have white flesh, free from traces of blood discoloration. Ideally the fish should be kept cool and left to bleed for up to 30 minutes. Once the fish are put into the washer they are not normally handled again by the crew until they reach the freezers.

... don’t keep fish waiting about...

Awaiting loading into the freezer

The washed, bled fish are usually conveyed mechanically to loading bins beside the plate freezers. If the fish have already been properly bled, delay at this stage should be as short as possible. Alternatively the fish should be chilled in the holding bins until bleeding is complete.

Never leave fish lying in the holding bins when they can be loaded into the freezer. Even when there is not enough fish to fill a freezer completely, they should still be loaded and frozen in preference to leaving them lying in the warm factory space.

Make sure that new fish are not dropped on top of old fish in the bottom of a bin; the oldest fish should always be frozen first.

Wash out the holding bins each time they are emptied; don’t put fresh, washed fish into a dirty bin.

Don’t mix species or different sizes of fish in the same holding bin; divert the fish to another bin whenever there’s a change of type.

Keep separate any fish that are known to be of poor quality, so that they can be specially labelled as they are loaded into the freezer; these may include fish that were in poor condition when caught, wormy fish or fish that have suffered long delays or high temperatures somewhere along the line. Remember that a few bad fish among a lot of good ones may result in the whole catch being classed as poor quality fish; the reputation of the ship, and the price of future catches of frozen fish, may be badly affected.

Be careful when sorting for size...

Operating the freezers

British freezer trawlers are fitted with vertical plate freezers for making large blocks of whole frozen white fish; the following are some of the points to watch when using this type of freezer:

Make sure that the freezer plates are defrosted before loading; if fish are loaded between cold plates, some will stick before they are properly in position. A block that has gaps in it freezes more slowly, breaks easily and stows badly.

The fish should never be higher than the top of the freezer...

Pack the fish carefully between the plates, head to tail alternately, with the heads to the ends of the block, and with as few gaps as possible.

The fish in the block should never be higher than the top of the freezer plates; if you overfill the freezer the top fish won’t freeze properly and the block will be difficult to remove.

Don’t mix large and small fish together; buyers don’t like mixed sizes in the same block, and small fish tend to get crushed or distorted by the bigger ones when they are frozen together. If the fish are not sorted for size before they reach the freezer, have a simple measuring board handy for checking them.

Don’t try to straighten fish that have gone rigid in a bent position; the muscle structure will be damaged and this will result in a fillet that tends to fall apart.

Don’t try to force large fish between the plates; if heading is permitted this will make them easier to load, but if they are much too big for the plate freezer, put them in the air blast freezer room.

Don’t load too many freezers at once; this imposes a very large demand on the refrigeration plant to begin with, whereas later on, when freezing is nearly complete, the plant will have very little to do.

Load a few freezers at a time in strict rotation, so that by the time the last ones are loaded, the first ones are ready for discharging and loading again.

Never use unnecessary force on the blocks...

Don’t cut down your freezing time; make a note of the time or set the dummy clock on the freezer as soon as loading is complete, and don’t start to unload until the fish have been frozen for the given number of hours. If the blocks are removed too soon, even though they feel hard, they will not be properly frozen. The middle of the fish will still be soft, and this means that the cold store has extra work to do for which it’s not designed; the store temperature will go up, the fish will be slowly frozen, and produce ragged fillets when thawed.

Don’t leave frozen blocks in the freezer longer than necessary when other fish are waiting to be frozen; too long a freezing time means wasted freezer capacity and unnecessary delay to the other fish.

Defrost the plates when unloading just long enough to free the blocks from the plates, and then shut off the defrost valve; when the defrost is left on too long, the blocks will partially thaw before they are dropped and, on reloading, wet fish will be heated unnecessarily.

Don’t force freezer plates apart when unloading; this may damage the freezer. Wait until defrosting has freed the blocks before attempting to unload them.

Never use unnecessary force on the blocks or on any part of the freezer when unloading.

Don’t use hooks anywhere on frozen blocks except on the heads of the fish; wherever possible try to avoid using hooks at all.

Check the refrigerant temperature to the freezer each time the fish is loaded; on most ships the temperature should be - 40°C. If it’s above this, report it and get it put right. The fish won’t freeze properly in the time allowed if it’s not.

Keep an accurate log of freezer operations; note loading and unloading times, the number and type of block, and the refrigerant temperature.

Label every block as it is loaded into the freezer; ideally the label should tell the people on shore when and where the fish were caught, and whether they were in good condition or not before they were frozen.

Using the air blast freezer room

In addition to the plate freezers, most British freezer trawlers have a special room for freezing fish that are too big or too awkward in shape to be handled in the plate freezers. Cold air is blown over the fish, which are normally hung on hooks. The room is sometimes known as a halibut room or, less accurately, as a sharp freezer. Watch the following points when using this room:

Hang up the fish so that cold air can blow around them; they will freeze far too slowly if they are merely laid on the floor.

Don’t overcrowd the room; stow fish away in the cold store once they are frozen, to make room for more. When the room is too full the cold air cannot circulate properly, and fish left in too long will become dried out.

Don’t obstruct the fan openings; this will stop the air circulating properly.

Check the room temperature regularly; if it starts to rise this may be because there is too much fish in the room, or because the cooling system isn’t working correctly, probably due to a build-up of frost on the cooling pipes. If the room has a removable false wall, this should be taken down and the pipes brushed clear of frost to make them fully effective again.

Whenever permissible, large cod should be headed and frozen in the plate freezers; don’t use the air blast freezer for anything but extra large cod and large flatfish like halibut.

Stowage in the cold store

The freezers may be top-unloading, end-unloading, or bottom-unloading. Blocks discharged through the bottom of the freezer usually fall direct onto racks inside the store, but end-unloading or top-unloading freezers discharge the blocks into the freezer loading area; these should be transported immediately to the cold store; any delay will warm the blocks and thus may cause the cold store temperature to rise.

Always handle the blocks as gently as possible; broken blocks are a nuisance to stow and to discharge, and damaged fish are useless fish. Stow broken blocks and single fish separately.

Make sure the blocks are all labelled when they are put away, and wherever possible put fish of the same kind and size together.

Keep a stowage plan and a log of the number and kind of block so that the catch can be discharged as efficiently as possible.

Don’t overcrowd the air blast freezer room...

Don’t jam blocks in too tightly; this may damage the fish, and further harm may be done to them when forcing them out again during landing.

Don’t leave cold store doors open any longer than is absolutely necessary; and never have more than one door open at a time.

Check the cold store temperature at regular intervals; it is essential that it remains steady at - 30°C at all times. Every time the store temperature rises, the quality of the frozen catch may be affected, and long spells at too high a temperature will make the fish useless for distribution later as wet or smoked fillets.

Never put unfrozen or partially frozen fish into the cold store; the store is not designed to freeze fish; any extra work imposed on the store in this way will result in a rise in temperature and thus some loss of quality.

Wherever mechanical aids such as chutes or roller conveyors are provided, use them; the less handling the blocks receive, the less they will be broken or damaged.

Don’t work in the cold store alone; slight accidents at these low temperatures can prove dangerous if help is not quickly available.

Never switch off the cold store lights without first making sure that there’s no one inside.

Keep the cold store at - 30°C

People working in the store should leave immediately if they feel drowsy, dizzy or light headed; there may have been a leak of refrigerant. Everyone should know how to work any alarm that is fitted in case they are unable to get out quickly enough.

On fillet freezing trawlers

Most of what has been said about handling and freezing of whole fish is equally relevant to fillet freezing, but there are some additional points to watch.

Keep an eye on the initial quality of the fish as it comes aboard; as when freezing whole fish, raw material in poor condition will probably yield an inferior product. Fillets taken from soft or spent whole fish should be marked as such when packed and frozen.

Gut the fish as soon as possible after capture; this is very important when taking fillets, because once the blood has started to clot the flesh becomes discoloured and it is almost impossible to obtain an attractive white fillet. If possible keep the fish chilled while awaiting gutting; this helps to stop the blood clotting too quickly.

Chill the fish after gutting and give them plenty of time to bleed properly, 20 minutes at least.

Filleting will probably be mostly by machine; inspect the fillets for signs of worms, blood discoloration and bruises; discard damaged fillets and make sure that second-grade fillets are labelled accordingly.

When fillets are cut from newly caught fish that have not become stiff after death, the fillets begin to shrink after they are removed from the bone. When the factory deck is warm, the shrinkage and distortion may be considerable, making the fillet corrugated and unattractive to look at and tough to eat; washing the fillets in fresh water can make their condition worse. It is very important, therefore, to load the fillets into the freezer as soon as possible after they are cut. If there has to be a delay, keep the fillets chilled. Ideally the filleting machine should then be stopped until the hold-up at the freezer has been cleared; it is far better to keep back the fish after they have been gutted and bled, but before they go through the filleting machine.

... the fillets begin to shrink after they are removed from the bone...

Fillets are usually frozen at sea in horizontal plate freezers; use the freezers in strict rotation in the same way as for whole fish, so that there is an even demand on the refrigeration plant, and to keep delays to a minimum.

When a freezer is ready for loading, put in any fillets that are waiting, even though they do not make a full load. It is better to stick to the time schedule and freeze a part load than to risk leaving fillets lying for too long. If only part of a shelf is loaded, space the fish out to prevent the plate becoming bent.

Trays used for making blocks of fillets should be kept clean and free from lumps of ice and frozen blood; keep the freezer plates clean and free of ice also, so that they make good contact with the fish. Good contact means faster freezing and this in turn will give a better product and a greater output.

Fillets are much more susceptible to damage and contamination than whole fish; they should be handled carefully and hygienically at all times, and packed properly before putting them in the cold store. Fillets with faulty wrappings will dry out rapidly in the store; they won’t taste as good and they will lose weight before they are landed.

Keep all machines, conveyor lines, bins and benches scrupulously clean at all times; remember that highly perishable food is being handled in a modern floating factory. Freezing and cold storage can’t improve the quality of fish that is already dirty, contaminated or spoiled when it goes into the freezer; it will be just as bad when it’s thawed out, and will rapidly get worse.

The main points once again

When handling the catch on a freezer trawler, the most important things to remember are:

1. Keep the catch cool from the time it comes on board until it reaches the freezer.
2. Gut and wash each haul before starting on the next one.
3. Leave the fish to bleed for at least 20 minutes.
4. Never cut short the freezing time.
5. Store the fish at - 30°C as soon as they are frozen, and keep them at this temperature until they are landed.

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