Aging and life history traits of the longnose spiny dogfish in the Mediterranean Sea: New insights into conservation and management needs

Aging chondrichthyans has been characterized as a slow and difficult process, but at the same time it is considered of fundamental importance for the holistic management and conservation of their populations. This study aims to assess the age and growth of the longnose spurdog S. blainville, to correlate the results to the species' biological features and to compare them with those reported in previous studies that used different aging methods. By counting the growth bands on the enamel surface of the dorsal fin spines, age was estimated in 569 out of 810 individuals that were taken as by-catch from commercial fisheries in the Mediterranean Sea during an 8-year period. Spine morphometrics revealed sexual dimorphism as far as spine length, spine base width and spine weight are concerned. The between-sex differences were also reflected in the length-weight relationships, in the gonadosomatic (GSI) and hepatosomatic (HSI) indices, and in the condition factor (K). All three somatic indices differed significantly among maturity stages, seasons or age classes. The reproducibility of the age readings was considered high based on the estimated CV and APE precision indices. Age bias plots also indicated no significant intra- and small inter-reader variation. The estimated VBGF parameters were: L∞ = 1097.3 mm, k = 0.03 yr−1 and t0 = −5.58 yrs for females, and L∞ = 665.5 mm, k = 0.08 yr−1 and t0 = −3.35 yrs for males. Likelihood ratio tests showed that all growth parameters were statistically significantly different between sexes (χ2 = 38.26, df = 3, P < 0.001). Females reached higher longevity (28 yrs) than males (22 yrs) and attained maturity at higher age and larger size (T50 = 17 yrs and L50 = 568.1 mm) than males did (T50 = 11.3 yrs and L50 = 460.6 mm). S. blainville seems to be a long-lived shark of slow growth and late maturity, a combination of traits indicating a species of great vulnerability to fishing pressure that can only sustain a low harvest rate.