The three major sources of fish meal are
- Fish stocks harvested specifically for this purpose: small, bony and oily fish such as anchovy, horse mackerel, menhaden, capelin, sandeel, blue whiting, herring, pollack
- by-catches from other fisheries;
- Trimmings and offal left over from fish processed for human consumption.
- Fish meal is an excellent source of highly digestible protein, long chain omega-3 fatty acids and essential vitamins and minerals. Fish meal quality depends on the raw material used and on the processing method involved.
The best quality fish meal is obtained from raw fish. However, in order to prevent protein and oil breakdown, raw fish is often processed by draining, chilling (chilled water systems, mixing of ice with fish) or chemical preservation (with sodium nitrite or formaldehyde).
Raw material (raw fish or preserved fish) is composed of 3 major fractions: solids (fat-free dry matter), oil and water. Fish meal is manufactured by a series of actions involving cooking, pressing, drying and milling. After cooking, generally at around 85-90°C, the cooked fish is pressed through a screw press where liquids are removed and a “press-cake” is obtained. The liquids are decanted, the supernatants centrifuged to obtain “stick-water”, which is concentrated through mild evaporation. The press cake and stick water are mixed together before entering a dryer to obtain fish meals with a final moisture of about 10%. At each of these processing steps, there can be variations, leading to fish meals of variable qualities.
Good quality fish meals contain crude protein levels above 66%, fat content around 8 to 11%, and ash generally below 12%. In some of the tropical developing countries, “fish meal” is sometimes produced after sun-drying and grinding, and can have very high levels of ash and relatively low protein levels. Other fishery by-products include fish protein concentrates with high protein levels (more than 70%).
In the 1980s, acid-preserved “fish silage” was very much promoted as one of the means of conserving trash or raw fish and for making farm-made feeds for aquaculture by mixing such silages with other feedstuffs (Disney et al., 1980), although this practice is not widely applied.
Due to the ever increasing demand for fish meal and fish oil to be used in feeds for farmed fish and crustaceans, there has been concern that the over-reliance on capture fishery-derived fish products for aquaculture would contribute to the over-exploitation of certain types of fisheries, with concomitant effects on the stocks of other wild fish (Naylor et al., 2000). However, time-series data show that there has been no upward trend in the catch of fish for feed since the 1980s. Based on current developments in fish feed formulations, it is now recognized that aquaculture contributes to global fisheries supply and does not deplete the marine fishery resources. Besides, the fish meal industry has committed itself and set forth several stringent measures to ensure that the feed-grade fisheries respect sustainability criteria. Another issue of concern is the poor management of rejects.