Linking inconsistencies in trophic level of marine fauna to fisheries discard consumption

Discarding practices have become a serious source of concern for the perennation of marine resources, motivating discard bans implementation around the world. However, little is known about the fate of discards in marine environments. Discarding may provide food for various marine consumers, potentially affecting food web structure and stability. Yet, quantifying reliance upon discards is difficult because identity and frequency of discards may change according to multiple factors, and most previously used diet assessment techniques do not allow to assume consistency of feeding strategies over time. One currently untested hypothesis is that significant contribution of discards over time should reflect in an increased trophic level (TL), particularly in low TL consumers. Here, we test this hypothesis by modelling the TL and assimilated diet of consumers living in fishing grounds subject to important discarding using stable isotopes. We found that all sampled invertebrates, Chondrichthyes and some Actinopterygii depicted a higher than expected TL, which could be linked to potential discard consumption. We stress that understanding fisheries discards reintegration in marine food webs is crucial in the context of an ecosystem approach to fisheries management, to better understand the functioning of marine ecosystems subject to fishing and anticipate the potential impacts of discard bans.