During phase I of this project, in 2009, we identified an optimal diet for the replacement of fish meal (FM) with soy protein concentrate (SPC) and/or soybean meal (SBM) for summer flounder, based on a feeding trial in which six diets were tested. The diets included a FM control, one diet based on a mixture of FM and SBM, one diet based on a mixture of FM and SPC, and three diets based on a mixture of FM, SBM and SPC. Our work for phase 2 in 2010 was to compare the “best” diet (diet 6, all SPC replacement of FM, with no SBM) from that trial with a “standard” commercial diet in a six-month study using a quasi-commercial-scale rearing environment.

While that work was going on, we also had funding from the National Oceanic and Atmospheric Administration National Marine Aquaculture Initiative (NOAA-NMAI) to investigate different levels of FM replacement with SBM, and especially to examine if those levels affected the performance of the fish (summer flounder) in a bacterial challenge, which tests their resistance to disease. To our surprise, the fish survived best in the bacterial challenge after they had been fed the diet with the highest level of SBM (70% replacement of FM with SBM), even though their growth on that diet was significantly worse than that of fish grown on diets with lesser amounts of SBM. This unanticipated result suggested to us that something in SBM (but perhaps lacking in SPC) may serve as an immunostimulant to boost the immune system of fish.

Based on the results of the NOAA-NMAI work, we proposed to USB that we would examine the relationship between levels of FM, SBM and SPC during 2011. The graduate student involved in the project also had some separate funding for another experiment along these lines in early 2011. Our goal in these studies was to try to quantify the relationship between SBM and SPC levels in the diet and the survival of fish in a bacterial challenge. Fish were grown in our standard feeding trial prior to their use in the bacterial challenge, so we were also able to obtain data on survival, growth, and food conversion before they were challenged.