A new study by the US National Institute of Standards and Technology (NIST) and the South Carolina Department of Natural Resources (SCDNR) has shown that raising farmed fish on a diet incorporating less than the usual amount of fishmeal may be cheaper, but the fish were less healthy.
Aquafarmers currently rely heavily on fishmeal as a protein source but it's expensive to produce and the resource from which it's derived - fish captured in the wild - is being rapidly depleted. One proposed remedy is to substitute cheaper and more environmentally friendly foods that replace some fishmeal content with other sources of protein.
The SCDNR designed a study to evaluate the efficacy of diets with reduced and full amounts of fishmeal fed to cobia during the period when juveniles mature to adults. One diet contained 50% and another 75% less fishmeal than that found in commercial food products. A third diet contained 100% of the conventional fishmeal content. A fourth group of cobia ate off-the-shelf fish food as a control.
Metabolically different
The results showed that cobia fed the reduced fishmeal diets were metabolically different from those fed either the full fishmeal diet or the control diet. Fish fed the reduced fishmeal diets had higher levels of two metabolites linked to physical stress, tyrosine and betaine, and lower levels of a primary energy source, glucose. This suggests that these cobia were not receiving the necessary nutrition to support healthy growth.
Overall, the researchers were surprised to find that cobia on the experimental 100% fishmeal diet showed the most growth by the end of the 100-day study period. Along with more normal tyrosine, betaine and glucose levels, tests revealed significantly higher levels of lactate in cobia fed 100% fishmeal compared to fish on the other diets.
This finding may be explained by the fact that the 100% fishmeal experimental diet has the highest percentage of the carbohydrate cornstarch, and lactate is produced by gut bacteria metabolising carbohydrates. In turn, since efficient breakdown of carbohydrates is essential to energy production, the researchers surmise that a diet enhancing gut microflora activity might be one of the conditions needed for optimal cobia health.
Although the reduced fishmeal diets in this study did not fare well, the NIST and SCDNR researchers say that the data from the NMR-based metabolomic analysis still provide insight into what might be needed for more successful formulations. They expect that future studies will eventually lead to alternative dietary products that are more cost effective, better for the environment and lead to high yields of healthy fish.
