Round goby (Neogobius melanostomus) δ13C/δ15N discrimination values and comparisons of diets from gut content and stable isotopes in Oneida Lake

Gut content analyses have found that round gobies (Neogobius melanostomus) are highly dependent on dreissenid mussels but stable isotope analysis has often suggested that the dreissenid contribution is lower. However, estimation of dietary contributions with stable isotopes relies on accurate discrimination factors (fractionation factors). To test if discrimination values commonly used in aquatic food web studies are suitable for round gobies, we collected round gobies from Oneida Lake, raised them for 63 days under four different diets (Chironomus plumosus, Mytilus chilensis, Dreissenia spp., Euphausia superba) and measured the change in white muscle delta C-13 and delta N-15. Gobies were also collected throughout Oneida Lake for gut content and stable isotope analysis. Diets changed as round gobies grew, with small round gobies (17-42mm) feeding mostly on cladocera and chironomids, intermediate sized gobies (43-94mm) transitioning from chironomid to dreissenid consumption, and larger gobies (95-120mm) predominantly consuming dreissenids, similar to findings in other studies. Discrimination factors were obtained by fitting a commonly used asymptotic regression equation describing changes in fish delta C-13 and delta N-15 as a function of time and diet stable isotope ratios. The discrimination factor determined for delta C-13 (-0.4 parts per thousand +/- 0.32, SE) was lower than the standard value of 0.4 parts per thousand, while that of delta N-15 (4.0 parts per thousand +/- 0.32, SE) was higher than the standard value of 3.4 parts per thousand. Turnover rates for both delta C-13 and delta N-15 were estimated as 0.02 parts per thousand*day(-1). The use of experimentally determined discrimination factors rather than standard values resulted in model estimates that agree more closely with the observed increasing importance of dreissenids in gut content of larger gobies. Our results suggest that the importance of dreissenid mussels inferred from stable isotope studies may be underestimated when using standard isotopic discrimination values.

Automated summary

Gut content analyses show that round gobies are highly dependent on dreissenid mussels, but stable isotope analysis often suggests a lower contribution of dreissenids. To test this, round gobies were collected from Oneida Lake and raised under different diets. The results indicate that the importance of dreissenid mussels may be underestimated in stable isotope studies.