Biological parameters and ecology of the Thornback Ray (Raja clavata Linnaeus, 1758) in the Balearic Islands, western Mediterranean

Abstract

The thornback ray, Raja clavata Linnaeus, 1758, is the most commonly landed batoid species in bottom trawl fisheries of the Balearic Islands. Its commercial importance has led to unsustainable exploitation in recent decades, potentially threatening population stability. This study aims to estimate key biological and ecological parameters of this species in the Balearic Islands, including bathymetric distribution, reproduction, growth, and diet. Biological sampling was carried out with 776 individuals (11–95 cm length) obtained from MEDITS scientific surveys (2021–2024) and commercial catches (2022–2024). Young-of-theyear, juvenile, and adult populations were not bathymetrically segregated. Four growth models (two-parameter and three-parameter von Bertalanffy, Logistic, and Gompertz models) were fitted to age data derived from readings of vertebrae growth bands. The estimated L50 values were 77.4 cm for females and 63.4 cm for males, while the corresponding A50 values were 11 years and 8 years, respectively. The presence of active males and females, as well as the year-round occurrence of egg capsules, suggests a continuous reproductive cycle for the species. Stomach content analysis of 928 individuals revealed a marked ontogenetic dietary shift from small crustaceans to fish, occurring earlier in males (~50 cm TL) than in females (~60 cm TL); this shift preceded sexual maturation with no associated bathymetric segregation. A better understanding of the species’ biology and ecology in the Balearic Islands will help improve stock assessments and support effective management and conservation measures.

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