Diet effects on fatty acids and mercury in carp

The objective of this translational research proposal is to study effects of essential (polyunsaturated fatty acids, PUFA) and potentially toxic (methyl mercury, MeHg) fish diet composition on PUFA and MeHg concentrations, and somatic growth condition of farm-raised common carp (Cyprinus carpio), a frequently consumed diet fish in Austria. This proposal bridges scientific ecological, ecotoxicological, and nutrition questions. Based on dietary supply of various biochemical fish feed compositions, the following hypotheses test that, A) increased biochemical quality of fish diet, as measured by omega-3 and -6 PUFA, results in increased PUFA concentrations of C. carpio (effect of selective essential nutrient retention); and, B) increased concentrations of the contaminant MeHg cause increased MeHg concentrations in C. carpio (effect of bioaccumulation). Moreover, it will be examined how higher dietary PUFA concentrations enhance somatic growth of C. carpio. For this effect it is expected that high RNA:DNA ratios relate to higher somatic growth condition, as RNA content is connected with protein synthesis rate and DNA content with cellular multiplication, which provide a general measure of the condition of C. carpio. Results of this project will thus further our understanding on how diet composition improves fish quality. These findings are directly applicable for fish feed producers and fish farmers to further their understanding on how to increase highly desirable PUFA concentrations and to decrease MeHg bioaccumulation in C. carpio. Dietary benefit-risk assessments for PUFA-MeHg profiles will finally lay the groundwork for diet recommendations (desired for preventive medicine). The basic and applicable scientific outcome of this proposal will be greatly enhanced through close collaboration between a fish feed producer (GARANT Austria), the long-standing knowledge of fish farming (Teichwirtschaft T. Kainz; not related to the proposer!), and the scientific expertise of aquatic lipid ecology and ecotoxicology of the principal investigator.

The objective of this study was to assess the role of diets on the bioaccumulation of the potentially toxic methyl mercury (MeHg) and essential polyunsaturated fatty acids (PUFA) in carp. Results of our 3-year feeding experiments clearly demonstrate that all carp, retained very low MeHg concentrations, but increased their PUFA contents when feeding on PUFA-rich diets. Carp feeding on high amounts of dietary PUFA resulted in even lower MeHg concentrations, suggesting that diet-induced increasing somatic growth caused MeHg to biodilute in carp. It was evident that zooplankton, natural diet in fishponds, contained higher MeHg concentrations than any of the carp feeds. As a fundamentally new finding we could show that up to 90% of the variance of MeHg in zooplankton was predicted by aqueous MeHg and not by particulate MeHg supply via bacteria or algae. These results dramatically shift our long-standing perception that MeHg in aquatic consumers is predicted by food consumption. This project clearly demonstrates that MeHg is more efficiently transferred by water than by particulate diet to aquatic metazoans. Carp feeding on omega-3 PUFA-rich diets resulted in high retention of omega-3 PUFA. This demonstrates that precious omega-3 PUFA at the base of aquatic food sources are efficiently preserved in consumers at higher trophic levels and that the non-carnivorous carp have increasing potential to supply growing amounts of omega-3 fatty acids to humans, provided that ecological conditions of fish ponds support algae that synthesize such long-chain omega-3 PUFA. Finally, this project enabled us to determine that short-term feeding (30 days), rather than continuous feeding during the entire growth periods, on omega-3 PUFA-rich diets caused carp to double their long-chain PUFA content. Results of this FWF-project show that common carp retain little MeHg, but bioaccumulate biochemically high diet quality in a very short period of time. These findings have direct application on future and more sustainable carp farming.