Life history of deepwater chondrichthyans

As fisheries move into deeper waters the ability of deepwater chondrichthyans to sustain fishing is relatively unknown, although the limited information to date suggests most are inherently vulnerable to exploitation. Knowledge of deepwater chondrichthyans life history traits is required to enable effective management of deepwater chondrichthyans, as life history traits are important indicators of the productivity of species and consequently, species abilities to tolerate fishing pressure. This dissertation aimed to further knowledge of the life histories of deepwater chondrichthyans by assessing patterns in their traits and studying the life history of species taken as bycatch in a deepwater trawl fishery within the Great Barrier Reef Marine Park. It also addressed the issue of the inability to age many deepwater chondrichthyans through investigation of a novel approach to ageing, that is, by using near infrared spectroscopy.\ud \ud The association of chondrichthyan life history traits and habitat was examined among the three main marine habitats of shelf, pelagic and deepwater, and also within the deepwater habitat. Life history traits were obtained from 226 populations of chondrichthyans across the three habitats. These traits were analysed for patterns using a linear mixed effects model that controlled for the correlation among species due to their common evolutionary history. Life history differences were identified between chondrichthyans from shelf, pelagic and deepwater habitats, and within the deep habitat down the continental slope. Deepwater species as a group, had lower growth rates, later age at maturity and higher longevity than both shelf and pelagic groups of species. In the deep habitat, with increasing depth, most species matured later, lived longer, had smaller litters and bred less frequently. The group of deepwater species also had a smaller body size than the groups of shelf and pelagic species, which indicates body size is not appropriate as a predictor of vulnerability in most deepwater chondrichthyans. The visual interaction hypothesis offers a potential explanation for these findings, and it is apparent habitat influences the trade-offs in allocation of energy for survival and reproduction. In general, deepwater chondrichthyans are more vulnerable to exploitation than shelf and pelagic species and this vulnerability markedly increases with increasing depth. The life history traits of the deepwater chondrichthyans as a group are unique from the shelf and pelagic groups of species, and reflect adaptations driven by both mortality and resource limitations of their habitat.\ud \ud The chondrichthyan bycatch from the deepwater eastern king prawn fishery at the Swain Reefs, in the southern Great Barrier Reef Marine Park, was sampled onboard two commercial prawn trawlers. In all, 1533 individuals were collected from 11 deepwater chondrichthyan species, with a skate, Dipturus polyommata, most dominant by number at 50% of the bycatch. A dogfish, Squalus megalops and a catshark, Asymbolus pallidus were the other dominant species accounting for 28% and 15% of the bycatch, respectively. They were followed by a gummy shark, Mustelus walkeri which accounted for 3% of the bycatch. The remainder of the species were captured infrequently, and included an angelshark (Squatinidae), stingarees (Urolophidae) and a shortnose chimaerid (Chimaeridae). Preliminary biological data was obtained from all these infrequently caught species. Dipturus polyommata, S. megalops and M. walkeri all had reliable ageing structures and were taken in sufficient numbers to enable detailed life history studies. A fourth deepwater species (a dogfish, Squalus montalbani) that occurs in the Great Barrier Reef Marine Park but which was collected from New South Wales, was provided to this study and was also investigated for age, growth and reproduction life history information.\ud \ud Dipturus polyommata was a small skate that was the shortest lived of the four species, with an observed maximum longevity of 10 years. It also had the fastest growth, with a von Bertalanffy growth completion parameter of k = 0.208 year⁻¹ for combined sexes, and it matured at the earliest age, with males reaching maturity at 4.0 years and females at 5.1 years. Mustelus walkeri was a moderately sized shark that lived longer than D. polyommata, with the oldest male 9 years and oldest female 16 years. It also grew more slowly with a combined sexes k = 0.033 year⁻¹ , and matured later with males estimated to mature at 7–10 years and females at 10– 14 years. The two dogfish species, S. megalops and S. montalbani were small and moderately sized dogfish, respectively. They had the slowest growth rates of all four species; with S. megalops combined sexes k = 0.003 year⁻¹ and S. montalbani combined sexes k = 0.007 year⁻¹. These dogfish were also longer lived than D. polyommata and M. walkeri with maximum observed ages for males and females of 18 and 25 years for S. megalops, and 28 and 27 years for S. montalbani. Further, they were the oldest at maturity with S. megalops males mature at 12.6 years and females at 19.1 years. Squalus montalbani males were mature at 21.8 years and females at 26 years.\ud \ud All viviparous species had low fecundity with litter sizes of 5–7 for M. walkeri, 2–3 for S. megalops and 9–16 for S. montalbani. The oviparous D. polyommata had an average ovarian fecundity of 8 follicles which was low compared to other small–medium sized Dipturus species. The age, growth and reproduction of these four species corroborated the life history pattern analyses. Assuming that fishing mortality was the same among species, it is likely that the shallowest dwelling D. polyommata was the most productive, followed by the slightly deeper occurring M. walkeri and then the two deeper dwelling dogfish. Squalus megalops was likely more productive than the deepest occurring S. montalbani.\ud \ud Reliable age information is essential to accurately predict the ability of chondrichthyan species to sustain exploitation, yet many deepwater chondrichthyans cannot be aged as they do not have reliable age structures. A novel approach to ageing, that is, using near infrared spectroscopy for ageing (NIRS), was first trialled on two coastal species, Sphyrna mokarran and Carcharhinus sorrah, both of which been previously age validated. NIRS successfully predicted their ages up to 10 years of age for both species, with the correlations between the known ages of the vertebrae and their near infrared spectra strong at R² values of 0.89 and 0.84 for S. mokarran and C. sorrah, respectively. The NIRS approach was then applied to three species of deepwater sharks using their dorsal fin spines, vertebrae and fin clips. Ages were successfully estimated for the two dogfish, S. megalops and S. montalbani, and NIRS spectra were correlated with body size in the catshark, Asymbolus pallidus. Correlations between estimated-ages of the dogfish dorsal fin spines and their NIRS spectra were good, with S. megalops R² = 0.82 and S. montalbani R² = 0.73. NIRS spectra from S. megalops vertebrae and fin clips that have no visible growth bands were correlated with estimated-ages, with R² = 0.89 and 0.76, respectively.\ud \ud This study demonstrated that the NIRS approach to ageing was feasible for age estimation of sharks. The NIRS ageing approach is rapid, which is a major advantage of the method, and could enable large numbers of sharks to be aged quickly. This offers the fisheries management benefit of improving the reliability of age information for stock and risk assessments. NIRS also has the capacity to non-lethally estimate ages from fin spines and fin clips, and thus could significantly reduce the numbers of sharks that need to be lethally sampled for ageing studies. This NIRS approach is the first chemical assay approach investigated in shark ageing that is simple, rapid and cost-effective. The detection of ageing materials by NIRS in poorly calcified deepwater shark vertebrae that have no visible bands, could potentially enable ageing of this group of sharks that are vulnerable to exploitation.\ud \ud This ageing and life history research has improved the understanding of deepwater chondrichthyans. This group of chondrichthyans have a unique suite of life history traits that render them more vulnerable to fishing pressure than their counterparts in shelf and pelagic waters, with their vulnerability increasing with depth. This knowledge is vitally important to fisheries management because the deeper the fishing, the less capacity the chondrichthyans have to recover. The findings of this dissertation can be used to facilitate more effective management and conservation strategies needed to ensure this group of chondrichthyans are sustainably fished and not placed at risk of extinction