Blondes do it better? A comparative study on the morphometry and life-history traits of commercially important skates blonde ray Raja brachyura, thornback ray Raja clavata, and spotted ray Raja montagui, with management implications

Abstract

Due to data limitation and challenges with species-specific reporting, the management of European skate fish-eries currently lacks measures tailored to individual species. Given these caveats, detailed information about skate stocks at the species level is warranted. The morphometry and life-history parameters were determined for samples of 170 individuals (58 Raja brachyura, 92 R. clavata, 20 R. montagui) collected by observers in com-mercial fisheries, and two bottom trawl surveys (December 2020 - August 2021) in the Southern North Sea and Eastern English Channel. A combination of three morphometric measurements were considered and proven successful (3.846% misclassification, N = 78) in species discrimination for easily confusable species R. brachyura and R. montagui. Through the application of these morphometric descriptors in electronic catch monitoring via Artificial Intelligence (AI), the misidentification of easily confusable species may be reduced in future catch reporting from surveys, fisheries-dependent data, and market sampling. Additionally, the morphometric differ-ences between species may have implications for species' catchability, and gear modification to avoid bycatch of skates under the European Landing Obligation (LO). The length-weight relationships indicated a faster rate of increase in body mass per unit of size for R. montagui. Life-history trait parameter estimates were obtained for R. clavata and R. brachyura. Age was determined through digital processing of images from visually enhanced whole vertebral centra (0.01% crystal violet staining). Age reading precision was low (CV > 45%, PA < 60%). The von Bertalanffy and logistic growth models provided more accurate fits for the length-at-age data than the Gompertz model. Macroscopic maturity assessment revealed that male R. montagui matured at the smallest size (Lm = 57.5 cm), while female R. brachyura matured at the largest size (Lm = 87.6 cm). Maturity ogives were fitted for males (L50 = 65.4 cm) and females (L50 = 84.1) of R. clavata and females of R. brachyura (L50 = 82.7 cm), with statistically significant intersexual differences for R. clavata. The Legal Minimum Landing Size (LMLS) measures of smaller sizes may not offer adequate protection (to immature specimens) of the skate species in the present study, that mature at larger sizes. The life-history trait parameters for growth and maturity are discussed in relation to published results of earlier studies, and no major changes in the life-history parameter traits were observed in recent years. The interspecific differences in the morphometries and life-history traits of R. brachyura, R. clavata, and R. montagui highlight the need for species-specific data collection and management considerations of skate stocks in the Northeast Atlantic Region at the local and EU level.