Distribution and population structure of the fish Helicolenus dactylopterus dactylopterus (Delaroche, 1809) in the Central Mediterranean (Southern Tyrrhenian Sea)

Abstract

The blue‐mouth, Helicolenus dactylopterus dactylopterus (Delaroche, 1809) (Pisces: Scorpaenidae), is a scorpionfish widespread in the whole Mediterranean basin, where it plays an important ecological role in deep‐sea fish communities. The depauperation of this large‐size sedentary and slow‐growing fish can be an index of overexploitation. This species is found throughout all Italian seas; the juveniles are mainly located around 150–300 m depth, whereas the adult specimens are spread over a wider depth range from 200 m to as deep as 1000 m. The abundance in number and weight, the spatial distribution, the population structure, the growth and the recruitment pattern of H. dactylopterus were analysed in this study. The present paper integrates data from 11 bottom trawl surveys carried out in the Southern Tyrrhenian Sea (Central Mediterranean) along Calabrian and Sicilian coasts, within the framework of the MEDITS Project. A total of 1412 specimens of H. dactylopterus weighting 20.051 kg were caught. The species appeared in 40% of the 296 hauls analysed, throughout the whole depth range surveyed. The highest values of frequency of occurrence (>67%) were obtained in the slope, whereas in the shelf they fell to around 16%. On the continental shelf the species was found between 106 and 196 m depth, and in the slope between 400 and 645 m. The highest mean density index (N·km −2 ) value was obtained in the 100–200 m depth stratum, whereas the highest mean biomass index (kg·km −2 ) value was recorded in the deeper 200–500 m stratum. The length distribution of the specimens ranged between 2.5 and 24.5 cm. From the Von Bertalanffy growth model the following parameters for the whole population were obtained: L ∞ : 30 cm; K: 0.16 year −1 ; t 0 : −0.02. The sampled population is composed mainly of the young‐of‐the‐year fraction, which can reach up to 98% of the whole catch. Recruits (age 0+) and juveniles (age up 4 years) are limited to waters shallower than 500 m, whereas adults are present below this depth.

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