High trophic specialization structures the epi- to bathypelagic fish community in the Bay of Biscay

The deep-pelagic ecosystem is characterized by significant environmental gradients, particularly in food resources. The absence of primary production below the epipelagic zone leads to a decrease in food resources with depth. Two opposite feeding strategies have been described for this community in response to this decline in food resources: stochasticity, with species adopting opportunistic feeding strategies with a generalist diet, and determinism, with species segregating and specializing to mitigate strong interspecific competition through niche partitioning. To test these aspects, we analyzed the isotopic niches of 16 fish species using stable isotope analysis of carbon and nitrogen carried out on muscle samples. The data were collected in canyons of the Bay of Biscay between 25 and 1335 m. Our primary objective was to identify isotopic niche segregation or overlap and determine whether species sharing similar isotopic niches show depth-based segregation by grouping them into trophic guilds and comparing their depth distribution with trawl data. We then used null model comparisons to test whether competition resulted in smaller values of isotopic niche size and overlap within each depth assemblage compared to those obtained by chance. We found that several species with similar isotopic niches were segregated based on depth. The comparison with null models showed that competition drove species to reduce niche size and specialize to avoid strong interspecific competition in the epi- to bathypelagic layers. Utilizing isotopic diversity indices weighted by biomass, our calculation showed significant divergence within the community, indicating that species with the highest biomass had extreme isotopic values. The high degree of specialization of species raises concerns about their vulnerability to various pressures, including climate change and exploitation. At the community level, this vulnerability is also a concern in maintaining the integrity of ecological processes.