Abstract
Raja clavata is the most widespread and landed skate species in the Mediterranean Basin. Despite its diffusion and economic importance, several aspects of its life history, such as age and growth, are poorly understood. This study evaluated the species’ growth in the South Adriatic Sea (Central Mediterranean Sea) and for the first time attempted an age validation through a tagging experiment. Thin sectioning of vertebral centra proved to be a more accurate preparation method in terms of age estimation precision and reproducibility than whole vertebral centrum staining (cobalt nitrate and ammonium sulfide technique). Marginal analysis showed a clear seasonal pattern, confirming the hypothesis of a single annulus deposition per year. A total of 291 vertebral centra were sampled and used for age estimation purposes. The oldest female was estimated to be 12 years old [total length (TL) = 89 cm], while the oldest male was aged 8 years (TL = 79.9 cm). Females were also found to be characterized by a slightly wider longevity range (ωL = 11.5, ωU = 16.8 years) than males (ωL = 7.8, ωU = 11.2 years). The von Bertalanffy growth curve fit the age and length data more accurately than the Gompertz and logistic models. Eighty-three thornback rays were tagged and released, of which two were recaptured. In both recaptured specimens, oxytetracycline marks were clearly visible. The band deposition after oxytetracycline injection and growth during the freedom period (about 1 year) were consistent with the age estimation method and criteria used and with the obtained growth results. Thus, the analysis of the vertebral centra extracted from the two recaptured specimens confirmed the hypothesis of the deposition of a single annulus per year and in general the age estimation criteria used in this study.
Highlights
Elasmobranchs represent a considerable proportion of bycatch landings in the Mediterranean Basin (Food and Agriculture Organization [FAO], 2016) accounting for 2% of small-scale fishery and 0.8% of bottom trawl total landings in the South Adriatic Sea (Maiorano et al, 2019)
The following data were collected from the collected specimens: total length (TL; in centimeters) to the nearest 0.1 cm, total weight (TW) to the nearest 0.1 g, sex, and maturity stages (AAVV, 2017; Follesa et al, 2019a)
The accuracy of age reading linked to each hard structure and preparation method represents the first step to establishing a protocol for age estimations (Carbonara and Follesa, 2019; Vitale et al, 2019), deepening in this perspective the influence of each processing technique on the data quality and its cost and time requirements (International Council for the Exploration of the Sea [ICES], 2011)
Summary
Elasmobranchs represent a considerable proportion of bycatch landings in the Mediterranean Basin (Food and Agriculture Organization [FAO], 2016) accounting for 2% of small-scale fishery and 0.8% of bottom trawl total landings in the South Adriatic Sea (Maiorano et al, 2019). Age Validation of the Thornback Ray in Central Mediterranean (Food and Agriculture Organization [FAO], 2018; International Council for the Exploration of the Sea [ICES], 2018), accounting for up to 38% of the total elasmobranch landings in Italian fisheries (Food and Agriculture Organization [FAO], 2016). Living in a wide bathymetric range (KrstulovicŠifner et al, 2008; Marongiu et al, 2017; Follesa et al, 2019b), the thornback ray is accessible to different fishing métiers (International Council for the Exploration of the Sea [ICES], 2018). Several studies have shown that k-selected species with low intrinsic population regeneration potential are the most impacted by harvesting (Stevens et al, 2000; Ferretti et al, 2010; Dulvy et al, 2014)
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