Seasonality of diet overlap among small pelagic fish in the waters southwest of the UK

Small pelagic fish (SPF) play an important ecological role by facilitating energy transfer in marine ecosystems, from lower to higher trophic levels. The Celtic Seas ecoregion is home to several economically important SPF that spend some or all their life in these waters. In recent decades, major changes in the relative abundances of the 6 main SPF in the region (anchovy, herring, horse mackerel, mackerel, sardine, and sprat) have been observed. Changes in the region may impact SPF, and previous studies in neighbouring seas have highlighted possible diet overlap of these species; therefore, it is important to understand the changes in diet overlap and trophic relationships, particularly across different seasons. Consequently, we investigated the seasonal diet composition of these 6 SPF in the Celtic Seas ecoregion, identified diet overlap, and determined species selectivity towards prey size. Combining historical and new observations on fish-stomach contents, diet-overlap indices were computed to quantify potential seasonal intra- and inter-specific overlap. Diet overlaps among species changed between seasons; in particular, sprat and anchovy changed significantly between spring and autumn, and mackerel between spring, summer, and winter. SPF species primarily consumed calanoid copepods, with herring and horse mackerel having more selective diets compared to mackerel, anchovy, and sardine. All species fed at similar trophic levels between seasons. To our knowledge, this study is the first to show seasonal variability in diet overlap among the 6 SPF in the Celtic Seas ecoregion. These results can improve the understanding of feeding dynamics for SPF in ecosystem model outputs.