Some morphological and biological aspects of longnosed skate, Dipturus oxyrinchus (Elasmobranchii: Rajiformes: Rajidae), in Syrian marine waters (eastern Mediterranean)

Abstract

Background. Dipturus oxyrinchus (Linnaeus, 1758) is a common skate in Syrian elasmobranch fishery. This species is listed as an Endangered cartilaginous species on the International Union for the Conservation of Nature (IUCN) Red List. Although heavily exploited, until now, no study  has been performed on this skate along the Syrian coast in the Levantine  basin. This study focuses on the determination of the reproductive period, size at maturity and fecundity of D. oxyrhinchus. Materials and methods. Total length and total weight were determined for 249 specimens of the longnosed skate, captured off Syrian coast. Skates collected were measured and weighed according to standard procedures. Based on the acquired measurements the following relations were determined: disc width–total weight (DW–TW), total weight–total length (TW–TL), total weight–disc width (TW–DW), and total length–disc width (TL–DW). Median size (TL50) at maturity was estimated according to commonly accepted methods. Hepatosomatic index (HSI) and gonadosomatic index (GSI) were calculated for males and females. The proportion of the occurrence of developmental stages were noted seasonally. Ovarian fecundity was calculated based on ripe follicles in both ovaries of mature females. Fecundity, expressed as annual egg production per female. Tests for significance (P < 0.05) were performed by using ANOVA, Student t-test, and the chi-square test. In the relations TW–TL and TW–DW, comparisons of curves were carried out by using ANCOVA Results. The observed sex ratio was 58.2% ÷ 40.2% (female ÷ male). Mean body measurements of females (n = 14) were slightly higher than males (n = 8). The total length of females ranged from 34.1 to 100.0 cm and males from 34.5 to 81.6 cm. The total length–total weight, disc width–total weight, and total length–disc width relations were TW = 0.0009TL3.35 and TW = 0.0033DW3.34and DW = 0.6894TL + 0.281, respectively, for all sexes combined. Median size (TL50) at maturity were estimated as TL50 = 79.5 cm for females (n = 149) and TL50 = 70.9 cm for males (n = 100). This species had a continuous reproductive cycle. Fecundity was estimated between 50 and 100 egg cases per year. Conclusion. Information on the reproduction of D. oxyrinchus may allow the decision makers to set appropriate catch limits based on biological reference points, and serves as a baseline in the event of increasing exploitation rates. Especially, this information showed that a possible nursery area maybe established in the Syrian marine waters.

Keywords

• maturity
• reproductive cycle
• fecundity
• Syrian co