The Reproductive Biology of Deep-Sea Elasmobranchs and Batoids from Chatham Rise and the Sub-Antarctic Region of New Zealand
The reproductive biology of thirteen poorly studied deep-sea elasmobranch species, on Chatham Rise and the Sub-Antarctic region of New Zealand, was assessed. The study species are all commonly caught as bycatch in commercial fisheries and include: three viviparous species (Centroselachus crepidater, Centrophorus squamosus, Deania calcea), five deep-sea catsharks (Apristurus spp.), and five deep-sea batoid species. However, due to a lack of knowledge on their general biology, ecology, and taxonomy – the impact of fishing on these species is unknown. A species’ resilience to fishing pressure depends on its biological productivity and susceptibility to capture. Accurate assessment of maturity is critical to understanding productivity and the effects of fishing pressure on fished stocks. Maturity is commonly assessed macroscopically, using a visual assessment that lacks precision and relies on subjective judgement. The wide array of macroscopic maturity assessment keys, used internationally, employ various sets of characteristics to define the same reproductive processes, which can lead to errors and inconsistencies in maturity assessment and parameter estimates (e.g. length-at-maturity), making direct comparisons between studies difficult. Objective reproductive measurements (oviducal gland size, follicle size, uterus width, follicle number and gonad weight) were used to assess the validity and quality of the macroscopic maturity staging key used in New Zealand, towards determining the onset of maturity and accurately distinguishing between macroscopic stages. The results showed that no single measurement gave a clear-cut indicator of maturity and some fish classified as ‘maturing’ were very likely ‘mature’. Uterus width, follicle size and gonadosomatic index values were found to be the most useful attributes in determining the onset of maturity. Uterus width and follicle size were also useful in determining differences between different macroscopic stages, whilst gonadosomatic index values were useful in distinguishing between reproductive strategies. Histological observations, with a particular focus on sperm storage, were also used to inform the quality of macroscopic maturity assignment. Sperm storage was observed for the first time in Centroselachus crepidater, Centrophorus squamosus and Brochiraja asperula. This study successfully highlighted problems in the macroscopic maturity assessment key currently used in New Zealand and proposes an improved, more objective macroscopic staging key. The improved key aims: 1) to assist in distinguishing between maturity stages, particularly between stage 2 (maturing), stage 2 (resting) and stage 6 (post-partum) females, by examining the same key reproductive structures across all macroscopic stages, and 2) to provide more representative maturity data for use in fisheries and demographic models, for more robust assessment of the impacts of fishing pressure on poorly studied deep-sea chondrichthyans.