Cephalopod assemblages, abundance and species distribution in the Gulf of Cadiz (SW Spain)

Abstract

Spatial distribution and species diversity of Cephalopoda caught in ARSA bottom trawl surveys were studied out during two different seasons (autumn and spring) in the Gulf of Cadiz (Spain) from 2000 to 2007. Species composition of cephalopod assemblages was analysed, using both clustering analyses and non-metric multidimensional scaling analysis (nMDS). Spatial distribution of the assemblages identified and abundance of the main species were mapped with Surfer 8.0 software, using kriging as the geostatistical gridding method. A total of 35 cephalopod species belonging to 6 families were found at depths between 20 and 700 m. In each season, Sepiolidae was the most abundant family in terms of number of species, followed by Octopodidae. The species richness increased up to 100-120 m depth, where it reached the maximum value. From 120 m, the species richness decreased progressively with depth. In spring, the species with the highest occurrence was Eledone moschata (34%) and in autumn it was Alloteuthis media (70%). In both seasons, the most abundant species in terms of weight was Octopus vulgaris, while Alloteuthis media showed the highest yields in terms of numbers. Most species showed wide bathymetric ranges, especially in autumn. Three different assemblages were found in both seasons during the time period analysed (although 2003 and 2006 were not included in the cluster analysis): shelf assemblage (20-160 m), deep shelf/upper slope assemblage (100-350 m) and middle slope assemblage (320-700 m). The specific composition of these three assemblages was similar between spring and autumn and an overlap could be observed between them, mainly in the two continental shelf groups: shelf and deep shelf/upper slope assemblages. Alloteuthis media and Alloteuthis subulata were the most abundant species in the shelf assemblage as well as in the deep shelf/upper slope assemblage. In the middle slope assemblage, Illex coindetii was the most abundant species. The assemblages and their spatial distributions could be largely related to a combination of physical and biological factors and their interactions.