Increasingly, plant protein sources including soybeans are being evaluated as replacements for fish meal in the diets of farmed carnivorous fish. Many plant protein sources contain factors that can exert nutritional and antinutritional effects in fish, and alter other developmental processes including reproduction.

Genistein is an isoflavone that is found in significant concentrations in soybeans. Genistein can have both estrogenic and antiestrogenic effects, either of which, in turn, can have stimulatory or suppressive actions on growth and reproduction in fish. The purpose of this study was to assess the effects of genistein on the growth, development and reproduction of rainbow trout and Atlantic salmon, to clearly elucidate the potential positive and negative effects of this isoflavone before large-scale attempts are made to incorporate increasing amounts of soybean products into commercial salmonid diets.

In rainbow trout, for a one year period fish were fed diets containing genistein at four concentrations: 0 (control), 500, 1,000 and 3,000 ppm. At all doses tested, genistein had no effect on growth or feed conversion. Serum levels of vitellogenin, measured at four times during the treatment period, were higher in genistein-treated fish (all doses) than controls. We found no effects of genistein on serum levels of reproductive hormones, morphology and histology of the gonads (both sexes), egg production, fertility rates, fry size nor viability. Accordingly, we suggest that the genistein content of soy products should not be a factor in determining their incorporation into the diets of rainbow trout.

In Atlantic salmon parr, for a six-month period fish were fed diets containing genistein at four concentrations: 0 (control), 500, 1,000 and 3,000 ppm, and a fifth group of fish were fed estradiol-17β (E2) at 20 ppm as a positive control. At all doses tested, genistein had no effect on growth or feed conversion. At the end of the treatment period, serum levels of vitellogenin were higher in the E2 and genistein treated fish than in controls. E2 and genistein at all doses tested inhibited the process of smoltification, as measured by 24-hr seawater challenge tests. Accordingly, the incorporation of genistein into the diets of Atlantic salmon parr may be contraindicated.

We suggest that further studies be conducted to evaluate the effect of genistein on Atlantic salmon smolts, to determine the appropriateness of high soybean meal content on Atlantic salmon grow-out diets.