Spatio-temporal distribution and abundance of the lesser spotted dogfish (Scyliorhinus canicula) in the Celtic Seas Ecoregion varies by sex; and the extent of sperm storage in the female chrondrichthyans

Abstract

Most chondrichthyans occupy higher trophic levels in their selected habitats, and as such, provide essential ecosystem services, such as removal of weak or sick prey items from the ecosystem, population control of prey species and helping to maintain biodiversity and a healthy ecosystem. However, over the last three decades elasmobranch populations have decreased markedly, with some species populations, such as the oceanic white tip (Carcharhinus longimanus; Poey, 1861) decreasing by as much as 90%. Factors contributing to this decrease are loss of natural habitat, incidental capture in non-targeted fisheries (bycatch) and over-exploitation in targeted and illegal fisheries. Understanding a species’ distribution, biology and particularly, its reproductive modes, is a key component for efficient management. The storage of spermatozoa is utilised by many terrestrial and marine species including many species of elasmobranchs. To assess the prevalence of such storage amongst elasmobranchs, the oviducal glands from 29 species of elasmobranchs from the northeast Atlantic and Northern Indian Ocean were examined for the presence of spermatozoa. Spermatozoa were observed in six species. Some instances suggested incidental occurrence due to the location of the spermatozoa, likely due to recent copulation prior to capture. Spermatozoa were most commonly found in specimens from the Family Triakidae. Spermatozoa were observed in mature, pregnant and post-partum females, suggesting that spermatozoa are stored over an extended period, with upwards of 50% of the samples for some species containing stored spermatozoa. Understanding a species’ distribution and spatial range is key to effective management. The CSE is a large, highly productive region located in the north-eastern Atlantic and is a key area for European fisheries. The lesser spotted dogfish (Scyliorhinus canicula) is a common, ecologically important small demersal-dwelling catshark that occurs in the northeast Atlantic, north African coast and Mediterranean Sea and is frequently bycaught in fisheries. For this reason, this species was chosen to examine the temporal changes in distribution and abundance, using fisheries data from independent fishery data collected within the CSE. Species distribution and occurrence was analysed using the Vector Autoregressive Spatio-Temporal (VAST) package. Two separate models, one sex-aggregated and a sex-specific model with four maturity categories (juvenile male, juvenile female, adult male and adult female) were investigated. From the sex aggregated analysis, notable increases in density and biomass were observed beginning in 2000. Similar increases were observed in the sex-specific model; however, adult female density and biomass were notably lower and increased less than adult males. A similar sex specific trend was observed in length frequencies, with a larger decrease in size of large mature females and an increase in the size of large mature males. Correlation of occurrence for each maturity category was analysed and it was found that juveniles tend to occur together in equal ratios. For females, mature females tended to occur more inshore than juvenile females. A similar trend was observed for juvenile and adult males. Mature males dominated inshore areas when compared to females resulting in a sex ratio of 3:1. From the analyse of the sex-aggregated model, an increase in population size was observed, driven markedly by increases in the abundance of males, and large differences in sex-specific distributions between males and females post maturation. Additional research is required to further understand the distribution and population dynamics of lesser spotted dogfish, in particular the factors driving the male dominated increases in population size.