Trophic ecology of Illex coindetii to understand population dynamics and improve fisheries management.

As mesopredators, squids have a pivotal position in the marine food web, yet their ontogenetic trophic ecology remains poorly understood. This study investigates the trophic ecology of the broadtail shortfin squid, Illex coindetii (Vèrany 1839), in three regions (South Bay of Biscay, Gulf of Cadiz, Western Mediterranean). Using stable isotope analysis (δ15N and δ13C) of muscles and eye lens, we tested four hypotheses: (1) δ15N and (2) δ13C increase with size, (3) eye lens niche area increases through the squid’s life, and (4) muscle and eye lens isotopic signatures follow a 1:1 relationship. Our results showed significant regional variability. As expected, muscle δ15N increased with size; however, this rise was moderate, suggesting a similar trophic position throughout its life. In contrast, muscle δ13C did not increase with size, suggesting little reliance on benthic prey despite the onset of nyctemeral migrations. Eye lens analysis also contradicted our hypothesis with distinct regional trophic niche patterns. The Bay of Biscay and the Mediterranean niche areas displayed progressive diminution, indicating a diet specialization through I. coindetii life, while the Gulf of Cadiz showed a distinct regional pattern reflecting regional trophic plasticity. Finally, muscle consistently displayed higher isotopic values than the eye lenses' outermost layer, reflecting different integration times and rejecting the 1:1 relationship. These findings highlight the trophic plasticity of Illex coindetii and support region-specific fisheries management plans.