| Trophic interactions between jellyfish and fish are complex and not clearly defined. Most studies focus on trophic interactions using gut content analysis alone, an approach time-limited and prey-type-dependent. The application of stable isotopes to jellyfish-fish interactions is rare. Therefore, I developed a standard protocol to measure carbon and nitrogen stable isotopes in jellyfish. I determined that: 1) the bell could replace the whole medusa in case of large organisms, 2) a specific correction for scyphozoan tissues was required to normalize their delta13C for lipid content, and 3) fractionation ranges of the scyphomedusa Aurelia sp. and the ctenophore Mnemiopsis leidyi were species-specific and different from those currently used in stable isotope analysis. To assess the effect of jellyfish predation, I focused on the unclear trophic ecology of Aurelia sp.5 in the enclosed saltwater lake Veliko Jezero (Mljet, Croatia). I suggested that the role of carbon-rich prey and diel differences in feeding were underestimated in Veliko Jezero and, potentially, in the ecosystems where medusae appeared food limited. To find a potential explanation for the decrease in landings of the forage fish, Brevoortia patronus, when the biomass of the scyphomedusa Aurelia sp. was high in the northern Gulf of Mexico, I determined that the diets of the two species overlapped, using a combination of stable isotope and gut content analyses. Both approaches indicated that mesozooplankton was the dominant prey in their diet. I highlighted an almost unknown aspect of the early-life stages of another forage fish, Chloroscombrus chrysurus, in the northern Gulf of Mexico. Measuring carbon and nitrogen stable isotope ratios of juvenile C. chrysurus associated with scyphomedusae indicated that the diet of juvenile fish was based on their hosts' tissue, potentially increasing food availability and decreasing mortality rates of fish. Overall, I suggest that stable isotopes are a necessary complement of gut content analysis to elucidate the complex trophic interactions between jellyfish and fish. My findings provide evidence that jellyfish can impact food web dynamics at several levels, with broader effects on the ecology and economy of the ecosystem. |
