Uncommon biological patterns of a little known endemic Mediterranean skate, Raja polystigma (Risso, 1810)

Abstract

Rare and poorly studied species, like the endemic skates of the Mediterranean Sea, are considered prone to high rates of extinction and threat because of non-optimal reporting and sampling, which reduce the power of analyses. In this regard, the goal of this study was to establish some basic life-history parameters, unknown to date, of one of these endemics, Raja polystigma, caught as by-catch from experimental and commercial hauls in the Mediterranean Sea. Sexes were equally distributed with no major differences in sizes. The age and growth were assessed through annuli counts of vertebral centra from a sub-sample of 184 individuals. Among different growth models applied to the length at-age-data, the logistic function provided the best fitting curve (L∞: 691.49; k: 0.26; IP: 4.03 years). The oldest female and male were aged 11 (590 mm LT) and 8 years (521 mm TL), respectively. The estimated longevity was 10.6–15.4 years for females and 7.7–11.2 years for males. Females and males matured at about the same size (L50 506.1 mm TL and 488.1 mm TL, respectively), showing an uncommon pattern among Rajidae. A clear reproductive seasonality was observed during spring and summer. Depth influenced the distribution pattern of R. polystigma which appeared to complete its life cycle in coastal waters, with mature adults found exclusively in the shallows and immature specimens in the whole bathymetric range. Given its dependence on the coastal environment and its peculiar life-history features, measures to alleviate anthropic effects on this habitat are urgently needed.