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Fish fillet

Article index
 Process overview
 Filleting of white fish
 Filleting of oily fish


The fish processing industry is very widespread and quite varied in terms of types of operation, scales of production and outputs. The species of fish processed include cod, tuna, herring, mackerel, pollock, hake, haddock, salmon, anchovy and pilchards. Marine fish accounts for more than 90% of fish production, with the remainder being fresh water fish and fish produced by aquaculture.
In general, fish processing operations are located close to commercial fishing areas. However in some cases catches may be transported long distances or exported for processing. The Northwest Pacific region is by far the most important fishing area in terms volumes caught and processed. China, Peru, Chile, Japan, the United States, the Russian Federation and Indonesia (in that order) are the top producing countries, together accounting for more than half of world fish production. Aproximately 75% of world fish production is used for human consumption and the remaining 25% is used to produce fish meal and oil. Fish meal is a commodity used as feed for livestock such as poultry, pigs and farmed fish. Fish oil is used as an ingredient in paints and margarine.
Currently, only about 30% of fish produced for human consumption are marketed fresh. The supply of frozen fish fillets and fish, in the form of ready-to-eat meals and other convenience food products is growing in both developed and developing countries. The end products from fish processing may be fresh, frozen or marinated fillets, canned fish, fish meal, fish oil or fish protein products, such as surimi. Surimi is an important fish product, with the majority of catches for some species used solely for its production.
Fish processing most commonly takes place at on-shore processing facilities. However some processing can take place at sea, on board fishing vessels—for example the gutting of fish. In some regions of the world, where large sea fleets operate, processing can also take place on board of the fishing vessels. For some sea fleets, 100% utilisation of the catch may be required by legislation. This means that the entire processing operation, including fish meal and oil production of offal and fish waste, takes place on board the fishing vessels.
Some sectors of the industry are very seasonal. Salmon processing, for example, may operate fewer than 100 days per year during the salmon harvesting season. During this time, plants operate at full capacity with little opportunity for down time and little incentive for waste reduction.

Process overview

Filleting of white fish

Filleting involves a number of unit operations: pretreatment, fish filleting, trimming of fillets, packing and storage. These processes generally take place within separate departments of the fish processing plant. White fish species have a low oil content and, unlike their oily fish counterparts, are generally gutted, cleaned and sometimes de-headed on board the fishing vessel. The fish are kept on ice in boxes before being delivered to the fish processing plant. On arrival at the plant, fish may be re-iced and placed in chilled storage until required for further processing. Pretreatment of the fish involves the removal of ice, washing, grading according to size and de-heading, if this has not been done previously. Large fish may also be scaled before further processing. The next step in the process is filleting, which is generally done by mechanical filleting machines. The filleting department is generally separated from the pretreatment area by a wall, to prevent workers and goods passing from the non-sterile pretreatment area to the sterile filleting high care area. The filleting machines comprise pairs of mechanically operated knives which cut the fillets from the backbone and remove the collarbone. Some fish fillets may also be skinned at this stage. In the trimming department, pin bones are removed and operators inspect the fillets, removing defects and any parts that are of inferior quality. Offcuts are collected and minced. Depending on the final product, the fillets may be cut into portions according to weight or divided into parts such as loin, tail and belly flap. As a final step before packaging, the fillets are inspected to ensure they meet product standard.
Fresh products are packaged in boxes with ice, the ice being separated from the products by a layer of plastic. Frozen products can be packed in a number of ways. Fillets or pieces can be individually frozen and wrapped in plastic, but the most common method is for them to be packed as 6–11 kg blocks in waxed cartons. The blocks are typically frozen and then kept in cold storage.

Filleting of oily fish

Oily fish species are characterised as those having oils distributed throughout the fillet and in the belly cavity around the gut. Fillets from these species may contain up to 30% oil. Oil content varies not only between species but also within species. Oily fish species are very rarely gutted or cleaned on board the fishing vessels, due to the high oil content and the consequent risks associated with oily surfaces. Keeping the skin of the fish intact also reduces oxidation of the oil and thus maintains the quality of the flesh. Oily species can be filleted like white fish species, but they are also used for canning.


The fish are off-loaded at the plant, weighed and loaded into water flumes. The flumes transport the fish to holding vessels, where they remain until required for processing. Fish are flumed, as required, onto cutting tables, where the heads, tails and other inedible parts are removed.
Some fish species, such as mackerel, need to have the skin removed by immersion in a warm caustic bath. The effluent generated from this process has a high organic load and has to be neutralised before being discharged.
The canning process depends on the size of the fish. Small fish species such as sardines and pilchards are generally canned whole, with only the heads and tails removed. These whole-fish products are cooked in the can after it has been filled with brine or oil. Medium-sized fish species are cut into pieces and pre-cooked in the can before the can is filled with brine or oil. For large fish species such as mackerel and tuna, the fish are filleted, cut into pieces of suitable size and also precooked in the can. Bones and inedible parts are removed when large fish species such as tuna are canned. After precooking, the liquid is drained from the cans and oil, brine or sauces are added. The cans are then sealed, sterilised and stored.
Most large canneries also operate a fish meal plant, in which fish not suitable for canning is combined with offal and processed into fish meal.
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process description


Introduction The fish processing industry is very widespread and quite varied in terms of types of operation, scales of production and outputs. The species of fish processed include cod, tuna, herring, mackerel, pollock, hake, haddock,... read full description