Fishmeal is a commercial product mostly made from fish that are not generally used for human consumption; a small portion is made from the bones and offal left over from processing fish used for human consumption, while the larger percentage is manufactured from wild-caught, small marine fish; either unmanaged by-catch or sometimes sustainable fish stocks. It is powder or cake obtained by drying the fish or fish trimmings, often after cooking, and then grinding it. If the fish used is a fatty fish it is first pressed to extract most of the fish oil.
Fish meal is primarily used as a protein supplement in compound feed. As of 2010, about 56% of fish meal was used to feed farmed fish, about 20% was used in pig feed, about 12% in poultry feed, and about 12% in other uses, which included fertilizer.
Fishmeal is made by cooking, pressing, drying, and grinding of fish or fish waste into a solid. Most of the water and some or all of the oil is removed. Four or five tonnes of fish are needed to manufacture one tonne of dry fishmeal. Of the several ways of making fishmeal from raw fish, the simplest is to let the fish dry out in the sun before grinding and pressing. This method is still used in some parts of the world where processing plants are not available, but the end product is poor quality in comparison with ones made by modern methods.
Today, all industrial fish meal is made by the following processes:
Cooking: The fish are moved through a commercial cooker — a long, steam-jacketed cylinder — by a screw conveyor. This is a critical stage in preparing the fishmeal, as incomplete cooking means the liquid from the fish cannot be pressed out satisfactorily and overcooking makes the material too soft for pressing. No drying occurs in the cooking stage.
Pressing: The cooked fish is compressed inside a perforated tube, expelling some of its liquids, leaving “press cake”. Water content is reduced from 70% to about 50% and oil down to 4%.
Drying: The press cake is dried by tumbling inside a heated drum. Under-drying may result in the growth of molds or bacteria; over-drying can cause scorching and reduction in the meal’s nutritional value.
Two alternative methods of drying are used:
Direct: Very hot air at a temperature of 500 °C (932 °F) is passed over the material as it is tumbled rapidly in a cylindrical drum. While quicker, heat damage is much more likely if the process is not carefully controlled.
Indirect: The meal is tumbled inside a cylinder containing steam-heated discs.