Age, growth, sexual maturity and reproduction of the thornback ray, Raja clavata (L.), of the Gulf of Gabès (south-central Mediterranean Sea)

Abstract

The thornback ray Raja clavata is one of the most abundant bycatch species of trawl fishing in the waters of the Gulf of Gabès, but little is known about its life history in this area. The present study provides the first data on age and growth of this species in Tunisian waters, as well as additional data on its reproduction. The age of individuals was estimated by counting growth bands on central vertebral sections. Marginal increment and edge analysis supported the hypothesis of annual deposition of growth bands, with the opaque band formed during autumn. The parameters estimated from von Bertalanffy and Gompertz growth functions showed differences between sexes. The oldest thornback ray female was aged 15 years (104 cm TL), whereas the oldest male was 12 years (89 cm TL). Variations in the gonadosomatic and hepatosomatic indices of both sexes and the presence of vitellogenic activity in mature females during the year indicated that the species cannot be considered to be a continuous spawner. Males were found to mature between 55 and 75 cm TL, whereas females matured between 72.4 and 85 cm TL. The TL at which 50% of the population reached maturity was 67.44 and 81.42 cm for males and females, respectively. Males and female thornback rays mature, respectively, at approximately 5.3 and 7 years old.

Key words:

• Age
• growth
• reproductive parameters
• Raja clavata
• Gulf of Gabès
• Mediterranean Sea

Acknowledgements

We thank the staff of the department of sclerochronology at IRD/IFREMER, Brest, France, for providing technical support. We also wish to extend our thanks to the anonymous reviewers for comments that greatly improved the submitted manuscript.

Editorial responsibility: Keiichi Sato