Table of Contents

The raw material
Handling before freezing
Size grading
Freezing time
Unloading the freezer


This note gives advice on the handling and freezing of small pelagic fish, notably herring and mackerel, in quantity soon after capture, either on the fishing vessel or at the port processing plant. The information is also largely applicable to sprats and blue whiting. General advice on freezing and cold storage of fish is given in Advisory Notes 27 and 28, and advice on the freezing at sea of cod and allied species is given in Advisory Note 34, but small fish when being frozen at sea require somewhat different treatment; this note is therefore designed to supplement the earlier publications.

The raw material

Herring, mackerel, sprats and blue whiting are not normally gutted at sea; gutting is generally impracticable because the fish are small and they are often caught in great numbers within a short period. The gut is often full of food, and the action of digestive juices can quickly burst the belly wall and soften the flesh unless the catch is handled expeditiously and kept chilled prior to freezing. Burst bellies can occur witin 6 hours of capture if the fish are not chilled.

Small fish are more susceptible to physical damage than large ones, and bacteria can spread more rapidly through the flesh of damaged fish, thus accelerating spoilage; careful handling and adequate chilling prior to freezing are necessary to maintain quality.

Herring, mackerel and sprats are fatty fish which are often sought and captured when their fat content is highest; fish that are rich in fat requires a process that gives added protection in cold storage to retard the onset of rancidity.

Handling before freezing

The catching rate on a freezer trawler must be limited to suit the handling capacity of the ship; for example on a large freezer trawler single hauls of up to 50 tonnes are possible with a large midwater trawl, whereas the shipboard freezing capacity is unlikely to be more than 2 t/hour. A catch of this size would mean that either some of it going into the freezer would be of poor quality because it had been delayed too long or some of it would have to be discarded. Furthermore, small fish in the bottom of a full cod end are subject to tremendous pressure when the net is hauled up a stern ramp, and a high proportion of the catch can be damaged by crushing, particularly when the fish are inherently soft.

Wherever possible the gear should be hauled as soon as the ship's fish detection instruments indicate that a manageable quantity of fish has entered the net but, if too large a haul is inadvertently made, there are two possible ways of reducing damage and spoilage. Both depend on leaving the full net in the water, where the fish will keep better, and bringing on board small lots at a time. One way of doing this is to bring the cod end just far enough on board to enable a fish pump to be inserted, and then to lower the neck back into the water and pump out what is required for processing. The other way is to bring the full cod end around the stern to the side of the ship, still in the water, and to make several lifts of the cod end containing part of the catch each time, in a manner similar to that employed on side trawlers for splitting a large bag of fish.

Once the fish are aboard, they must be frozen as soon as possible, and kept cool while they are waiting. Ideally the fish should be held in chilled sea water prior to freezing but, where this facility is not fitted, ice can be used as an alternative. Herring with a high fat content stowed in ice in a ratio of 1 part of ice to 1 part of fish by weight will keep in first class condition for 30 hours or so, but the same fish held uniced are unlikely to remain in first class condition more than 6-8 hours; lean herring spoil less quickly. Strict control of quality is essential; the permissible delay depends on the intrinsic quality of the fish and the temperature of storage prior to freezing. Soft fish or fish with burst bellies are generally unsuitable for freezing whole.

Fish that are to be filleted on board can withstand a somewhat longer delay than fish that are to be frozen whole. Herring with burst bellies, for example, can still yield reasonable fillets until they reach the stage when they are so soft that they are difficult to cut by machine, and the resultant fillets are broken and ragged. Whole herring kept unchilled generally yield first class fillets after a delay of up to 8 hours, and second class fillets after a delay of 8-11 hours. Fish delayed more than 11 hours without chilling should be discarded. Once the fish have been on board for 6 hours, continual inspection of quality is essential to ensure a satisfactory frozen product.

Fillets cut from fish before they have gone into rigor will shrink; fillets cut from herring only 1 hour out of the water, for example, can shrink by as much as 20 per cent of their length within 1/2 hour in a surrounding temperature of 14°C. Chilling before and after filleting reduces shrinkage but does not prevent it altogether; whole herring need to be chilled for about 10 hours to eliminate shrinkage, a delay that is generally impracticable at sea. Fillets should be chilled by mixing them with ice; the use of refrigerated sea water is not recommended. The problems of handling fish before and during rigor are dealt with more fully in Advisory Note 36. Many kipperers accept that pre-rigor herring fillets shrink when thawed for smoking, and when the fillets are to be canned or marinated, shrinkage is relatively unimportant.

When fish from a new haul are brought on board, any fish remaining from the previous haul should be either discarded or put to one side for immediate processing; new fish should never be put on top of old ones.

Size grading

Buyers of blocks of frozen whole fish normally specify the size range offish required. Thus whenever fish of mixed sizes are caught together, some means of size grading is necessary before the catch is frozen. The size grades for herring and mackerel as laid down by the EEC are shown in Tables 1 and 2 respectively. Since UK processors buying herring for kippering require close control of size, the EEC grade 1 for herring can be subdivided as shown to give a suitable category of fewer than 5 fish to 1 kilogram.

Table 1 Size grades of herring

EEC grade

UK subdivision


Weight of single fish kg

Fish/10 kg



under 5

over 0·2

under 50










over 12

under 0·085

over 117

Table 2 Size grades of mackerel

EEC grade

Weight of single fish kg

Length cm


over 0·5

over 37·5








under 0·1

under 24·0

TABLE 3 Makers of size grading machines



Baader Holms

West Germany Denmark





Makers of size grading machines are shown in Table 3. Some machines are designed for grading a particular species, and the manufacturer should be consulted about any proposed application to other species. Furthermore, a machine that works satisfactorily on shore may not work as well on board ship.

Loading the freezer

Existing conveyors used to move small whole fish from the holding area or the grader to the freezers on a trawler should be made suitable for the job; conveyors designed to handle larger fish sometimes have gaps around diverters that are large enough to allow small fish to spill out. Where fillets are cut by machine, a mesh conveyor should be provided on which they can be inspected between filleting machine and loading point; inspection allows damaged fish and unwanted species to be removed before they reach the loading point. At some point on the conveyor system there should be a seawater spray to wash the whole fish or fillets prior to freezing, but fillets must not be immersed in water for any length of time. Washing removes surface slime and blood, and helps to give the frozen block a clean appearance.

Herring frozen in a vertical plate freezer make a strong homogeneous block without the inclusion of water, but it is recommended that water be added to encase the fish in ice as protection against rancidity and to prevent physical damage to the outside fish. Mackerel pack together less well, and the addition of water is necessary to strengthen the block. Since vertical plate freezers are not watertight, a bag has to be inserted between freezer plates to hold the fish and the added water. Polyethylene bags are suitable, but the wrapped frozen blocks are slippery to handle, and a supporting pound structure is required for safe stacking in the cold store. Paper bags with a polyethylene lining are available as an alternative in a range of sizes; the outer surface is less slippery, thus making open stowage safer and the paper can be readily marked to identify the contents of the bag.

The method of filling bags of herring in a vertical plate freezer is as follows. A bag is inserted between a pair of plates, the mouth of the bag opened, and the top edges of the bag folded over the tops of the freezer plates and held in place with metal clips. Fish are poured into the bag until they are about 3 cm from the top; an adjustable chute and stopper boards can readily be made to guide the fish into several adjacent bags at once, in order to reduce loading time. Each filled bag is topped up with water, using a trigger operated nozzle on a sea-water hose, and a label is laid on top of the fish. The clips are removed, the mouth of each bag folded shut, and freezing begun.

The filling procedure is the same for mackerel, except that water should be put in the bottom of the empty bag to a depth of about 20 cm in order to cushion the fall of the fish and prevent them from puncturing the bag.

When freezing blocks of fillets at sea, packing and presentation are important. The most acceptable sizes of block are 3 -5 and 7 kg, frozen in cartons in a horizontal plate freezer with polyethylene interleaving between layers of fillets. Herring fillets can also be frozen in 25 or 50 kg blocks in a vertical plate freezer, using lined paper bags with fresh water added to protect the fillets against oxidation, in the same manner as for whole fish. Large blocks of fillets wrapped in this way, however, generally suffer some damage during handling and storage, and thus spoil more readily than small cartons of fillets from a horizontal plate freezer.

Freezing time

The freezing time of a block of small fish in a thin bag with added water is about the same as an unwrapped block of larger fish; any increase in freezing time due to the presence of the bag, usually 0·10-0·15 mm thick, is offset by better contact with the freezer plates due to the added water. Where an existing vertical plate freezer has an established freezing cycle for large white fish in unwrapped blocks 5 cm or 10 cm thick, this cycle should be maintained for small fish with water in wrapped blocks.

Other kinds of wrapping can alter the freezing time; for example, blocks of fillets 10 cm thick packed in waxed cartons 1 mm thick take longer to freeze than the same blocks in lined paper bags, as is shown in Figure 1. The thickness of the block is usually fixed in an existing freezer, but when any change is made in block thickness or the kind of wrapper being used, the freezing time should be checked and the cycle altered if necessary. Methods of measuring freezing time are described in Advisory Note 94.


Unloading the freezer

Too long a defrosting time or too high a temperature can be detrimental during the unloading of blocks of fatty fish from a vertical plate freezer. The fatty layer immediately beneath the skin of the fish can spoil if the surface of the block thaws, and in addition the thawed surfaces of the blocks stick together when they refreeze in cold storage, thus causing damage when the blocks are prised apart during discharge. Defrosting must be done quickly and at as low a temperature as possible.

Blocks in bags should be marked before they are stowed in the cold store. A felt pen can be used to record species, size, date of capture and haul number on paper bags, provided they have not been wetted by too long a defrost.

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