Oocyte dynamics and reproductive strategy of Aphanopus carbo in the NE Atlantic—Implications for fisheries management

Black scabbardfish is a deep water species of high commercial interest in the NE Atlantic; however the life history and stock structure is poorly understood. For this study, specimens were collected from commercial trawls off west of the British Isles and longliners from the Madeira Islands, between September 2008 and May 2010. Geographical differences in the reproductive state of scabbardfish were evident, supporting the theory of a north-south migration. Only immature specimens were found in the northern area, while in Madeira all maturity stages were observed, with the peak spawning in October–November. Consistently, the gonadosomatic index (GSI) showed an increase throughout the year, reaching a maximum in October and November for males and females, respectively; while for the northern area the GSI values had low variability. Oocyte development was described and characterized for each maturity stage. Histology revealed that the oocytes have a group-synchronous development and it was concluded that black scabbardfish was a total spawner and had determinate fecundity. The geographical quasi-complete separation of the immature and mature individuals necessitated the use of a novel bias-reduction GLM in the estimation of length at maturity (LC50%). Estimated LC50% for both sexes was significantly higher when data from both areas were combined (females = 1156 mm, males = 1098 mm) than using the Madeira dataset (females = 1110 mm, males = 1010 mm). The results highlight large scale dispersal in this species which needs to be treated as a highly migratory species and be managed as a single population.