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 (TL 50) 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 = 0.0009TL 3.35 and TW = 0.0033DW 3.34 and DW = 0.6894TL + 0.281, respectively, for all sexes combined. Median size (TL 50) at maturity were estimated as TL 50 = 79.5 cm for females (n = 149) and TL 50 = 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.

Batoid Abundances, Spatial Distribution, and Life History Traits in the Strait of Sicily (Central Mediterranean Sea): Bridging a Knowledge Gap through Three Decades of Survey

“…Concerning the biological aspects of D. oxyrinchus , the maximum size appears to be smaller than that reported by Serena et al [ 79 ]; however, this is similar to other Mediterranean areas [ 31 , 87 ]. The positive allometry indicated by the b parameter of the LWR, estimated in the present study, was also comparable to other areas [ 73 , 88 , 89 ]. On the contrary, the L 50 was higher and lower when compared, respectively, to male and female specimens from the Gulf of Gabes [ 31 ], North Aegean Sea [ 88 ] and Syrian waters [ 89 ], while it was lower when compared to those from Sardinian waters [ 87 ].…”

“…The positive allometry indicated by the b parameter of the LWR, estimated in the present study, was also comparable to other areas [ 73 , 88 , 89 ]. On the contrary, the L 50 was higher and lower when compared, respectively, to male and female specimens from the Gulf of Gabes [ 31 ], North Aegean Sea [ 88 ] and Syrian waters [ 89 ], while it was lower when compared to those from Sardinian waters [ 87 ]. Strangely, the L 50 estimates, resulted lower for females than males; this probably maybe due to low representativity of the data about males.The sex ratio was quite unbalanced towards females, as reported in Syrian waters [ 89 ], whereas, in other areas, it was quite balanced (e.g., Tunisia [ 31 ]; North Aegean Sea [ 88 ]).…”

“…On the contrary, the L 50 was higher and lower when compared, respectively, to male and female specimens from the Gulf of Gabes [ 31 ], North Aegean Sea [ 88 ] and Syrian waters [ 89 ], while it was lower when compared to those from Sardinian waters [ 87 ]. Strangely, the L 50 estimates, resulted lower for females than males; this probably maybe due to low representativity of the data about males.The sex ratio was quite unbalanced towards females, as reported in Syrian waters [ 89 ], whereas, in other areas, it was quite balanced (e.g., Tunisia [ 31 ]; North Aegean Sea [ 88 ]). Estimates of L∞ in the the South of Sicily were quite similar to the other Mediterranean areas, with the exception of the Aegean Sea [ 88 ] ( Table S3 ).…”

Growth Characteristics of Long-Nosed Skate Dipturus oxyrinchus (Linnaeus, 1758) Inhabiting the Northeastern Mediterranean Sea

“…The overall length ranges recorded in our study were found to be smaller than those reported by Alkusairy and Saad [ 11 ] for the same species in Syrian waters (34.1–100 cm for females and 34.5–81.6 cm for males). Yigin and Işmen [ 8 ] reported a total length of 14.9–100 cm in females in Saros Bay (the North Aegean Sea), while Bellodi et al [ 12 ] reported these values as 15.2–86.5 cm in males and 10.4–117.5 cm in females in Sardinian waters.…”

“…Long-nosed skates have been studied satisfactorily by researchers during recent years in other parts of the Mediterranean Sea in terms of age, growth, distribution, systematics, length–weight relationships (LWR) and feeding habits. However, no data on growth parameters are available for this species in the Northeastern Mediterranean Sea [ 8 , 9 , 10 , 11 , 12 ]. The presence of juveniles and mature males of the long-nosed skate was previously reported by Başusta and Başusta [ 13 ] from the same region.…”