Abstract
Cephalopods, including octopuses, squids, and cuttlefishes, are exploited by both bottom trawl and small-scale fisheries (SSF) in most of the Mediterranean areas. Bottom trawl fisheries regard cephalopods as a valuable bycatch, whereas for SSF, they are among the main target species. Cephalopods account for a relatively small proportion of the total landings in the Mediterranean. However, from an economic point of view, four cephalopods, Eledone cirrhosa, Eledone moschata, Octopus vulgaris, and Sepia officinalis, account for approximately 15% of the total landing value. Despite their economic importance, there are very few stock assessments of cephalopods in the Mediterranean because it is difficult to assess them by classical age-based methods, given their short life-cycles, and highly variable growth and recruitment. The production of E. cirrhosa, E. moschata, Illex coindettii, Loligo vulgaris, O. vulgaris, S. officinalis, and Todaropsis eblanae in the waters off the south of Sicily accounts for approximately 8% of the total Mediterranean yield of cephalopods. This study presents the first attempt to assess the state of these cephalopods in the Strait of Sicily by using surplus production models. Since species-wise landing statistics may be unreliable because of their morphological similarity, some octopuses (E. cirrhosa and E. moschata) and ommastrephid squids (I. coindetii and T. eblanae) were assessed combined. Landing data and abundance indices from trawl surveys were used to describe cephalopod stock dynamics through the Bayesian State Space Schaefer model (BSM) and Surplus Production model in Continuous Time (SPiCT) models. As survey data were not considered reliable indicators of their abundance, O. vulgaris, S. officinalis, and L. vulgaris stocks were assessed using the Catch-Maximum Sustainable Yield (CMSY) model. Overall, squid and cuttlefish stocks were observed to be in healthy conditions. However, assessments of octopus stocks indicated that their condition was critical or recovering. Here, we discuss the different stock statuses in the light of evolving fisheries and environmental factors in the area over time. Although cephalopods are not a priority in the current management system of Mediterranean fisheries, the importance of these species in the food web and their relevance for SSF underline their importance and their exploitation status should be periodically evaluated.
Highlights
Cephalopods, both as predators and prey, are key components of marine ecosystems (Boyle and Rodhouse, 2005)
To cope with these difficulties, some species were assessed as combined species: (i) E. cirrhosa and E. moschata were assessed as Eledone spp., and (ii) I. coindetii and T. eblanae were assessed together
The lack of contrast in the time series of catch and effort data, the short life cycle and cohort overlapping, and the resource intensive direct age estimation have been the main reasons for the limited cephalopod stock assessments to date (Pierce et al, 2010; Arkhipkin et al, 2021)
Summary
Cephalopods, both as predators and prey, are key components of marine ecosystems (Boyle and Rodhouse, 2005). In the last few decades, an increasing trend in cephalopod catch from commercial fisheries has been observed in some oceanic regions of the world, together with a progressive decline in groundfish stocks (Rodhouse et al, 2014; Arkhipkin et al, 2015, 2021; Hilborn et al, 2021) This trend has been attributed to several factors: increased biomass in response to global warming (Sauer et al, 2019), reduced competition for prey, and predation by depleted groundfish (Caddy and Rodhouse, 1998; Rodhouse et al, 2014; Doubleday et al, 2016). There is a general agreement on the impact of environmental factors on cephalopod growth and recruitment, and that these affect significant population dynamics and stock assessment parameters (e.g., Rodhouse et al, 2014)
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