Trophic Ecology of Sargassum-Associated Juvenile Fishes

Holopelagic Sargassum (S. natans and S. fluitans) provides refuge and foraging opportunities for a diverse assemblage of species, including a suite of recreationally and commercially targeted fishes, such as Gray Triggerfish, Tripletail, and Greater Amberjack. To better manage and conserve these fisheries resources, it is important to understand the role of Sargassum in open ocean environments, and the spatial and temporal variability in Sargassum nursery habitat function within its native range. As part of a NOAA RESTORE project, the goal of this study is to quantify the variability in the trophic ecology of Sargassum-associated juvenile fishes, and the environmental drivers that contribute to this variability. Juvenile fishes were collected from Sargassum habitats during four research cruises in the Gulf of Mexico (2017-2019). A combination of gut content and bulk stable isotope analyses was used to characterize diets and trophic linkages for juvenile fishes, with an emphasis on managed species. Results from these analyses were then used to characterize variability in juvenile fish trophic ecology with respect to a range of environmental variables (e.g., proximity to Loop Current, Sargassum biomass, water depth). Preliminary results suggest Amberjacks (Seriola spp.) frequently consumed caridean shrimp and other fishes. Tripletail diets were more diverse and included caridean shrimp, portunid crabs, copepods, bivalves, and gastropods. Gray Triggerfish fed primarily on epiphytic organisms, copepods, and amphipods. Stable isotope results support those of gut content analysis with Gray Triggerfish and Tripletail exhibiting a wide range of isotope values, implying a more varied diet than other fish species which clustered more tightly in isotope space, suggesting more specialized diets. Results from this study will be paired with remote sensing observations to develop a Sargassum "habitat index" for inclusion in population assessments of managed species.