A spatial impact gradient of open-sea-cage fish farm wastes: diagnosed by stable isotope and fatty acid analyses

Abstract

Environmental impact of wastes from an open-sea-cage fish farm on marine abiotic and biotic environments was assessed using dietary tracers, carbon (__13C) and nitrogen (__15N) stable isotopes and fatty acid profiles. Multiple organic sources and consumers representing different trophic guilds in the food web were examined. Results showed that feed wastage and fish feces form the base of the pelagic food chains, supplying organic matter to plankton, filter and suspension feeders. A transect-based study (0 - 2000 m) was conducted to determine an impact gradient away from the fish farm. Multivariate analyses demonstrated a clear spatial gradient of stable isotope ratios and fatty acid profiles from the fish farm. Canonical analysis of principal coordinates (CAP) showed that __13C and __15N in the gastropod grazer (Tectus pyramis) and deposit-feeder (Holothuria leucospilota) were the most important variables associated with the observed gradient, suggesting that these taxa can serve as potential bioindicators for the fish farm impacts. A spatial gradient of monounsaturated fatty acids (MUFA) in sediments, plankton and Holothuria, was also demonstrated, which may be used to indicate the dispersal of waste from the fish farm. Non-parametric multivariate regressions of environmental variables suggested that tidal current speed, distance and sediment grain size (< 0.25 mm) together explained > 45 % of the observed spatial variations, and therefore could serve as main predictors for the impact. Our results suggest that stable isotopes and fatty acid profiles of both abiotic and biotic components can serve as practical tools for assessing the spatial impact of open-sea-cage fish farms o n marine assemblages.