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Distribution and population structure of thornback ray, Raja clavata, in the northern and central Adriatic Sea

Population structure and distribution of the Thornback ray, Raja clavata, L. 1758, in the northern and central Adriatic Sea were described based on data from bottom trawl surveys conducted in the area between 1996 and 2006 (MEDITS). R. clavata was caught at depths between 23 and 270 m, and the highest catches were recorded at depth stratum 50-100 m. Sex ratio was close to 1:1. Length at first sexual maturity for both sexes was calculated. Length-weight relationships were obtained for males, females and for the whole sample. Parameters of the length-weight ratio for the whole sample were: a = 0.0011, b = 3.4081. The positive allometry for both sexes was observed. The recent state of the population was compared with existing data from 1948-49 (« Hvar» expedition). This comparison revealed that since « Hvar» expedition significant changes occurred in distribution of R. clavata in the Adriatic Sea. While during « Hvar» expedition this species was distributed on the whole continental shelf, during MEDITS it was found only in restricted areas and with much lower abundances. The Thornback ray was very abundant during “ Hvar” expedition with the average biomass index 65.25 km/km2 and it was at second place in fish catches during 1948-49, while during MEDITS the average biomass index of R. clavata was 4.58 kg/km2. Besides, significant changes occurred in the demographic composition of the population. Described changes are most probably a result of the high fishing intensity to which the species has been exposed in the Adriatic Sea for the last 60 years. Part of the problem is in the biology of this species (e.g. slow growth, large body size, low fecundity) that enhances the vulnerability of R. clavata to the fishing pressure. This concerns almost mainly bottom trawl nets - the fishing tool most frequently used for fishing of this species.

Raja clavata; fish; Adriatic; distribution; population structure

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