Assessing the diet and trophic level of marine fauna in a fishing ground subject to discarding activity using stable isotopes

Abstract

Discarding practices have become a source of concern for the perennation of marine resources, prompting efforts of discard reduction around the globe. 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 increased trophic level (TL) of marine fauna, particularly in low TL consumers. Here, we explored this hypothesis by modeling the TL and assimilated diet of consumers living in fishing grounds subject to important discarding activity using stable isotope analysis. We found indications that benthic invertebrates and Chondrichthyes may depict a higher than expected TL, while other fish tend to depict similar to lower TL compared to global averages from the literature. Based on prior knowledge of discard consumption in the same area, stable isotope mixing models congruently revealed that discards may represent substantial portions of the assimilated diet of most benthic invertebrate macrofauna, cephalopods and Chondrichthyes. We highlight limitations and challenges of currently used diet assessment techniques to study discard consumption and stress that understanding their reintegration in marine food webs is crucial in the context of an ecosystem approach to fisheries management and to better understand the functioning of marine ecosystems subject to fishing.

Assessing discard consumption dynamic in shallow coastal environment using underwater video

“…Altogether, we can conclude that in shallow coastal fishing grounds, discards could be consumed by a large diversity of species, both in terms of taxonomic and trophic groups, including low trophic level benthic fish and omnivorous invertebrate macrofauna such as decapods (Froese and Pauly, 2021;Palomares and Pauly, 2021;Thompson et al, 2020). This supports previous research suggesting that the diversity of discard consumers may currently be underestimated (Guillen et al, 2018;Lejeune et al, 2022bLejeune et al, , 2022a. Whether such behaviors correspond to stable trophic strategies over time remains to be established, notably through repeated measures and an integrated use of multiple diet assessment techniques (e.g.…”

“…This study constitutes a first step towards a better understanding of facultative scavenging abilities across benthic fauna in the shallow fishing grounds of the Bay of Bourgneuf. Together with previous indications of discard ingestion and assimilation in the same fishing ground obtained via stable isotopes (Lejeune et al, 2022a) and DNA metabarcoding of gut contents (Lejeune et al, 2022b), our study suggest that many low trophic level benthic taxa (e.g. decapods, small fish, mollusks) may be important nodes to consider for the reintegration of discards within the marine food web.…”

“…Gut content metabarcoding analysis of marine fauna in fishing grounds of the Bay of Bourgneuf revealed that discard ingestion may be widespread across bentho-demersal consumers but more important in decapods (Lejeune et al, 2022b). Based on the analysis of muscle isotopic composition, a previous study in the same area also found that decapods depicted a high trophic level which may be explained by substantial reliance on fisheries discards (Lejeune et al, 2022a). But in the stable isotope study, evidence of discard consumption by fish was limited.…”