Aquaculture for all

Difficulties In Digesting Pea/ Soybean Mix


NORWAY - Researchers from the Norwegian University of Science and Technology (NTNU) have discovered that fish feed, with combined pea/ soybean proteins in it, causes digestive complaints in fish.

Marie Hillestad, Senior Scientist at BioMar in Trondheim, has headed the project “Sources of plant protein for fish feed: Understanding and controlling anti-nutritional factors”, which received funding under the HAVBRUK programme and is presented in the latest programme newsletter (“Nytt fra HAVBRUK” no. 3-4/2010, available in Norwegian only).

The project mapped various protein-rich plants as potential feed ingredients for salmon and rainbow trout. Part of the project involved researchers at the Norwegian School of Veterinary Science in Oslo, who studied the fish intestinal tract and fish health. The project featured collaboration with researchers in the US and Canada as well, two countries which are major suppliers of soybeans and peas, respectively, for fish feed. BioMar’s North American research partners included the University of Saskatchewan (Canada), the Agricultural Research Service (the research arm of the US Department of Agriculture), and the Hagerman Fish Culture Experiment Station (Idaho/Montana, USA). The Catholic University of Temuco (Chile) also collaborated.

Anti-nutrients are a key strategy in the plant world for protection from being eaten by animals or destroyed by fungus or other microorganisms. These anti-nutrients give a bitter taste to the plants and can even cause illness in animals that eat them.

“In our project we studied saponins in particular, which are well-known anti-nutrients in soybeans,” explains Dr Hillestad. “Fish feed with relatively moderate proportions of soybeans with saponins intact can trigger serious intestinal reactions in salmon. Saponins cause irritation of the mucosa and leakage in the intestinal membranes.”

“We observed marked impacts on the fish in our trials, depending on the type of protein source the saponins were combined with. Saponins and pea protein, for example, are a very bad combination for both salmon and rainbow trout. But if we feed the fish pea protein without combining it with saponins, the peas are an excellent protein substitute for fishmeal.”

“It’s no problem for the fish to eat feed in which pea protein comprises a large proportion of the protein content. But feed with pea protein must not contain soybeans or other vegetable ingredients that are high in saponins,” she cautions. “Alternatively, the saponins can be removed from the soybeans.”

The researchers do not yet know why this pea protein/saponin combination spells trouble for the fish. There are a number of reasons why peas are an ingredient that feed producers seek to use in greater quantity. One is that pea plants are capable of nitrogen fixation, which reduces the need for artificial fertiliser, making peas a particularly sustainable crop for agriculture.

“We need different, better methods to understand why this combination of soybean saponins and peas has such ill effects. We must be able to quantify the saponins in the vegetable ingredients, which we have not yet succeeded in doing,” confides Dr Hillestad, adding that more than once the team thought it was on the verge of a breakthrough, only to be disappointed.

The researchers did discover that saponins in themselves do not necessarily cause appreciable problems in the fish. It is the combination of saponins with certain other potential vegetable feed ingredients that create the major difficulties.

Dr Hillestad has also received funding under the HAVBRUK programme to examine the effects of supplementing fish feed with the naturally-occurring amino acid tryptophan, which can both increase appetite and reduce aggression in fish. Her research team has studied the optimal amounts of tryptophan for salmon and cod feeds.

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