Physiology of Finfish and Shellfish (2+1)

4.1.13. Effects of Carnivory

In terms of both biomass and diversity, the terrestrial environment is dominated by herbivorous animals. Compared to herbivores, carnivores are in the minority. In the aquatic environment, fish are the dominant vertebrate and most are carnivorous. Herbivorous fish exist, but are quite rare compared to carnivorous and omnivorous fish. Moreover, there are no ruminant fish, while many terrestrial mammals are ruminants.

In the U.S., most fish culture is directed toward intensely carnivorous trout and salmon, or somewhat omnivorous channel catfish. Fish that are more fully omnivorous, e.g. tilapia, are less commonly produced in the U.S., but commonly imported form tropical countries. In salmonid rations, you have to feed very high percent protein (30 to 40%). Not only is the percentage of protein high but the majority of this protein must come from animal sources. This is because the amino acid balance is different between animal and plant protein, i.e. all the essential amino acids are present in plant protein and they are all present in animal protein, but the percentage of individual amino acids tends to differ between animal and plant sources and less total protein needs to be fed if the amino acid profile of the feed is similar to the body of the animal. Moreover, carnivorous fish like trout do not tolerate much carbohydrate in their diet (it cannot exceed 20% starch and sugar), and it is difficult to feed enough plant protein to meet their amino acid requirements without exceeding this limit, since plants are so rich in carbohydrates. As a consequence, the best protein source for carnivorous fish is fish meal, which means feeding fish flesh to grow fish flesh. This is obviously expensive and not a very ecologically sound way of growing food. An irony of carnivorous fish aquaculture is that it increases the use of wild ocean fish. Growing trout and salmon is like raising lions and tigers for food. In fact, in the years before scientific ration formulation allowed a complete trout ration to be made from dry feedstuffs, trout were successfully raised on ground beef liver. Channel catfish are more tolerant of plant material in their diet, but most catfish rations include a sizable percentage of fish meal.