Biology of two species of elasmobranch batoid fishes, the skates Raja miraletus and the Mediterranean endemism Raja polystigma

Abstract

[eng] Elasmobranchs, and especially rays and skates are considered globally one of the most vulnerable group of marine fishes due to their vital characteristics such as slow growth, late maturation or low fecundity. One of their biggest threats is fishing exploitation although they are usually captured as by-catch. In the Mediterranean Sea, elasmobranchs' populations have experienced a decline. However, their assessment is scarce since fisheries monitoring prioritizes commercial target species. Very few studies are found about Raja miraletus and R. polystigma, the latter being an endemic skate in the Mediterranean Sea. Information published so far does not provide complete knowledge on the main biological traits of these two species. Therefore, the aim of this work is to provide updated and accurate information about basic biological parameters, ecology and population status of these two species. The data used in this work has been collected during MEDITS surveys and biological samplings of the commercial catches. Here we show total length-weight (total and eviscerated) relationships for females and males of both species, as well as a size distribution of both populations. We also set up a methodology to prepare and count bands in vertebral sections and provide a key relating total length and the number of growth bands present in these sections as a first approach to the ageing. Regarding reproduction, the sex-ratio, gonadosomatic index, spawning season, length at first maturity and potential fecundity were determined, and maturity scales are provided by sex for each species. The evolution throughout the year of condition indexes such as Le Cren's index and the hepatosomatic index, and the digestive potential was assessed for both species. Moreover, the diet of both skates was studied through the analysis of the stomach content and the calculation of the frequency of appearance, percentage number, percentage volume and the index of relative importance of prey. The geographic distribution of both species was mapped throughout the time series of MEDITS surveys, whereas their bathymetric distribution was also modeled. Finally, a first assessment of population status of R. polystigma and R. miraletus is also provided.