Trophic relationships among cephalopod species along the water column inferred from stomach contents and stable isotope analyses

Abstract

It is well known that cephalopods play a key role in the marine food webs, either as voracious predators or important prey of a large set of predators. In this study we investigated the trophic relationships among cephalopod species taken along the water column by means of stomach content and stable isotope analyses. With the main aim of determining if there are fluxes of matter between nectobenthic and pelagic domains mediated by cephalopods, we analysed different aspects such as diet composition, niche breadth, diet overlap and day-night feeding rhythms from samplings conducted in the western Mediterranean. Samples were collected on the shelf (200 m depth, bathymetric stratum 1) and slope (600-900 m, bathymetric stratum 2) during summer and autumn surveys. At the shelf bathymetric stratum, sampling was carried out at: 1) near surface (SUR1) from 0-60 m; 2) in the benthic boundary layer (BBL1), less than 50 m above the bottom; and 3) on the bottom (BOT1). At the slope bathymetric stratum, sampling was performed at: 1) near surface (SUR2) from 0-80 m; 2) in the 400-600 m deep scattering layers (DSL); and 3) on the bottom (BOT2). For comparative purposes, a few hauls were also performed near the bottom in this stratum (BBL2). In all cases, SUR, BBL and DSL samplings were performed using a mid-water trawl, while the BOT samplings were obtained using a bottom trawl. The stomachs of all cephalopod individuals caught in these samplings were analyzed, with the only exception of a few cases where random samples were taken owing to the large amount of available material. Whenever possible, a sample of three individuals per species was collected for stable isotope analyses. A total of 1286 stomachs from 26 cephalopod species belonging to 12 Families were analyzed. Although the percentage of empty stomachs was slightly higher in autumn (48%) than in summer (41%), this pattern was not general for all species. Significant diet overlap (Schoener index>0.6) was only found for a reduced number of species (Abralia veranyi vs Histioteuthis reversa vs Sepietta oweniana; and Heteroteuthis dispar vs Rondeletiola minor). Levins’ standardized niche breadth was lowest for Loligo forbesi (0.022) and highest for Pteroctopus tetracirrhus (0.720); for all other species this index ranged from 0.118 to 0.639. In most species diet composition changed with season, and, in general, prey diversity was higher in summer than in autumn