Maturation, fecundity, and spawning strategy of the thornback ray, Raja clavata: do reproductive characteristics vary regionally?

Abstract

In Portuguese waters, thornback ray Raja clavata spawns mainly between May and January, although females and males in spawning condition are found throughout the year. The maturation process can be divided into three main phases by using information on gonad weight, oviducal gland and uterus width in females and on gonad weight, clasper length and sperm duct width in males. Females attain length-at-first-maturity at 784 mm and males at 676 mm, at ages of 7.5 and 5.8 years, respectively. In females larger than length-at-first-maturity, a resting stage was identified characterized by low gonadosomatic index and well-developed oviducal glands and uteri. These results along with the low proportion of adult females that are effectively reproducing per month demonstrated that the thornback ray cannot be considered a continuous spawner as described in other studies. Fecundity was determinate with about 35 eggs released per batch. During the spawning season, a total of four batch episodes occur indicating that the total fecundity was approximately 136 eggs per female. Regional differences may exist in the reproductive strategy of the species, namely on the duration of the spawning season, length-at-first-maturity, and fecundity, which can be related to a more intense fishing pressure in northern European waters.