Spatial distribution and species diversity of Cephalopoda caught in ARSA bottom trawlsurveys were studied out during two different seasons (autumn and spring) in the Gulf ofCadiz (Spain) from 2000 to 2007. Species composition of cephalopod assemblages wasanalysed, using both clustering analyses and non-metric multidimensional scaling analysis(nMDS). Spatial distribution of the assemblages identified and abundance of the mainspecies were mapped with Surfer 8.0 software, using kriging as the geostatistical griddingmethod. A total of 35 cephalopod species belonging to 6 families were found at depthsbetween 20 and 700 m. In each season, Sepiolidae was the most abundant family in terms ofnumber of species, followed by Octopodidae. The species richness increased up to 100–120 mdepth, where it reached the maximum value. From 120 m, the species richness decreasedprogressively with depth. In spring, the species with the highest occurrence wasEledone moschata (34%) and in autumn it was Alloteuthis media(70%). In both seasons, the most abundant species in terms of weight wasOctopus vulgaris, while Alloteuthis media showed thehighest yields in terms of numbers. Most species showed wide bathymetric ranges,especially in autumn. Three different assemblages were found in both seasons during thetime 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 middleslope assemblage (320–700 m). The specific composition of these three assemblages wassimilar between spring and autumn and an overlap could be observed between them, mainly inthe two continental shelf groups: shelf and deep shelf/upper slope assemblages.Alloteuthis media and Alloteuthis subulata were themost abundant species in the shelf assemblage as well as in the deep shelf/upper slopeassemblage. In the middle slope assemblage, Illex coindetii was the mostabundant species. The assemblages and their spatial distributions could be largely relatedto a combination of physical and biological factors and their interactions.

The necessity to move from a traditional fishery management to an ecosystem approach is now acknowledged. Indicators are required to support the development of this approach. For example, community indicators have been proposed to assess the impact of fishing. We tested here the applicability of one such community indicator, the abundance-biomass comparison (ABC method), as a measure of the impact of bottom trawling (years of sampling: 2001 and 2003) on the benthic invertebrates – typically starfishes, crabs, squat lobsters, shrimps and large hard-shelled molluscs- of “Grande Vasière”, a major French fishing zone, in the Bay of Biscay. The ABC method is generally used as an impact indicator for different types of physical, biological and anthropogenic disturbances on benthic communities. This method is based on the assumption that increasing disturbance shifts communities from dominance by large-bodied species with low turnover rates toward dominance by small-bodied species with high turnover rates. At less disturbed areas the average biomass of individuals is greater than at more heavily disturbed areas. The ABC method measures this effect by comparing the ranked distributions of abundance and biomass within a given community. We applied the ABC method and compared the size structure and the species diversity at two areas exposed to moderate and high bottom trawl effort. Species diversity was lower in the most exploited area. The highly trawled area was dominated by opportunistic organisms, mainly one species of swimming crab Liocarcinus depurator, one species of squat lobster Munida rugosa and Norway lobsters Nephrops norvegicus, which are large-bodied species. Consequently, the results of the ABC method were inconsistent with the theoretical expectation for these particular macrofaunal communities and the measured levels of fishing intensity.