Frontiers in Marine Science (Nov 2015)
Life-history traits of the long-nosed skate Dipturus oxyrinchus, from the central western Mediterranean Sea
Abstract
Skates are often considered vulnerable to overfishing because of their peculiar life cycle characteristics rendering them susceptible to overexploitation. In this regard, life-history traits of a by-catch species, the long-nosed skate Dipturus oxyrinchus (Linnaeus, 1758) from Sardinian waters, were investigated. The age, growth and reproduction were studied using 706 specimens sampled during experimental and commercial hauls (depths 76-671 metres). Females (10.4-117.5 cm in total length, TL) attained larger sizes than males (14.5-99.5 cm TL). To evaluate the growth, a subsample of 130 individuals (76 females and 54 males) was employed. The age was estimated through the annuli counts on sectioned vertebral centra. A high ageing reproducibility and precision was obtained (Index of Average Percent Error=6.56; %CV=8.9). Four models were applied on length at age data: the von Bertalanffy (VBGF), the Exponential, the Gompertz and the Logistic functions. According to the Akaike’s Information Criterion, the Gompertz model (L∞=127.5±4.9 cm; k=0.14±0.009; inflection point=3.97 y-1±0.9) provided the best fitting curve, showing a higher growth rate and a lower L∞ than what obtained with the VBGF (L∞=144.4±11.5 cm; k=0.08±0.001; t0=-1.09±0.23). The oldest female and male were 17 (115.5 cm TL) and 15 years (96 cm TL), respectively. Lengths at maturity were 103.5 cm (14 years) for females and 91 cm (12.1 years) for males corresponding to 88 and 91.5% of the maximum observed length, respectively. The monthly evolution of maturity stages highlighted an extended reproductive cycle with spawning females and active males almost over the year, as confirmed by the Gonado-somatic Index. Ovarian fecundity reached a maximum of 26 yolked follicles. Given the relative abundance in Sardinian seas, we hope that these results will prove useful for the implementation of basic management measures in order to ensure the sustainability of catches of this species, particularly sensitive to fishing pressure, in the Mediterranean Sea.
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