Movements, growth rates and strong sexual segregation in critically endangered tope sharks Galeorhinus galeus in the Northeast Atlantic.

Abstract

The tope (Galeorhinus galeus, Linnaeus, 1758) is a critically endangered shark, which, like numerous elasmobranchs, faces severe global decline. There are, however, substantial disparities between this species' global conservation status and those of some local populations, with the Northeast Atlantic representing a relative stronghold for this species. However, several areas of uncertainty, particularly regarding individual movement patterns, currently hamper region-specific conservation efforts. Therefore, utilising capture-mark-recapture tagging data, collected predominantly by recreational anglers in Ireland - but with recaptures throughout the Northeast Atlantic and Mediterranean - we investigated regional population structure, spatial segregation and individual movement patterns, and estimated sex-specific individual growth rates. This revealed a marked pattern of sexual segregation, with females residing further south than males overall, and with an Irish-specific trend towards male-dominated catches in Atlantic regions versus more even sex ratios in the Irish and Celtic Seas. Recapture timings and locations suggest that female movements are more strongly driven by seasonal water temperatures changes, being broadly in line with the north-south migratory paradigm. Spatiotemporal overlap of mature individuals suggests that the North Channel, Irish Sea and northern Celtic Sea may constitute a key mating area. Female sharks may utilise southerly regions during gestation, with the presence of young-of-the-year indicating that the Irish Sea and neighbouring regions subsequently act as parturition/nursery areas. Our results demonstrate the value of such long-term programmes, in this case facilitated by citizen science, in identifying broad-scale movement patterns in wide-ranging species and specific regions of interest for further study and/or implementation of targeted conservation measures.