Long-term changes in distribution and demographic composition of thornback ray,Raja clavata, in the northern and central Adriatic Sea

Abstract

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 within the scope of the ‘MEDITS’ project. R. clavata was caught at depths between 23 and 270 m; highest abundance and density indices were recorded at a depth stratum 50–100 m. The 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 the whole sample. Calculated parameters of the length–weight ratio showed a positive allometry for both sexes. The recent state of the population was compared with data from the 1948–49 ‘Hvar’ expedition. The comparison revealed that significant changes have occurred in distribution and abundance of R. clavata in the Adriatic Sea since the time of the ‘Hvar’ expedition. During the 1948–49 ‘Hvar’ expedition R. clavata was distributed throughout the entire continental shelf; during the 1996–2006 MEDITS expedition the species was found only in restricted areas and with much lower abundances. Significant changes also occurred in the demographic composition of the population. All described changes indicate an over-fishing of the thornback ray in the Adriatic Sea, a result of the high fishing intensity to which the species has been exposed for the past 60 years.