Life history traits of dogfish Squalus chloroculus and Deania calcea in southern Australia

© 2014 Dr. Bastien Edouard André RochowskiDeepwater dogfish species are commonly caught as bycatch and byproduct of deep-water fisheries in Australia but our knowledge of the dogfishes in deep-sea environments and their basic biological aspects is limited. Chondrichthyans show a diversity of life history strategies, so it is hard to predict the biological parameters of species that have not yet been studied and the effects that fisheries will have on these species. What we do know is cause for concern. Innate biological characteristics of slow growth, late maturation, long life spans, and low fecundity in chondricthyans do not support extensive exploitation. Furthermore, deep sea environments have low productivity, so that deepwater dogfish are likely to have especially slow reproduction and growth. Australian stocks of dogfish species on the continental slope have been heavily exploited, so an investigation of their biological characteristics is crucial. This study is a quantitative analysis of life history traits of two dogfish species from the continental slope off southern Australia: Squalus chloroculus and Deania calcea. The study of the reproductive cycle of the endemic S. chloroculus revealed that the female population has a triennial and continuous reproductive cycle, which is different from most shark species from the upper slope habitat. Males did not show any seasonality, which means they are in breeding condition all year round. Sexual dimorphism was clearly observable from the length-mass relationship and females reach maturity at a larger size than males. Also, though females mature by 799 mm total length (TL), maternity was not reach until 883 mm TL and only 33% contribute to recruitment each year and give birth between September and December. The litter size for the species ranged from 4 to 15 with a 1:1 embryo sex ratio and the size at birth was estimated to be 250 mm TL. The globally distributed brier shark, D. calcea, has a non seasonal and asynchronuous reproductive cycle, with an extensive resting period between pregnancies. This is charac- teristic of mid-slope species. As for S. chloroculus, the length-mass relationship and size at maturity differ between males and females in southern Australia. However it was not possible to determine the length of the reproductive cycle and therefore size at maternity. A maximum litter size of 10 embryos was found in southern Australia and this does not vary much with maternal size. When compared with populations in northern latitudes, D. calcea appear to mature at a smaller size in central and southern latitudes, with the exception of New Zealand. Deepwater chondrichthyans differ from shelf species by their intrinsic productivity. Ex- tended longevity is present among shelf, upper and mid-slope species, but deepwater chon- drichthyans tend to have a slower growth rate and reach maturity at an older age compared with shelf species, so that they contribute to recruitment later in their life cycle and thus more slowly. Furthermore they have extended reproductive cycle and inter-pregnancy pe- riod, together with small litter sizes which produce larger neonates, and suggests lower natural mortality. The dorsal spine structure of both species proved to conform to that of the few species where spine structure and growth has been well studied: only two dentine layers are present with the inner dentine layer providing the clearest and greatest number of growth bands. The estimated growth and age-based parameters indicate that both S. chloroculus and D. calcea are long lived (∼ 25 years) and that females do not reach maturity before ∼ 14 − 15 years old. Finally, growth rates for both species are relatively low, especially for females. As for the length-based parameters, geographical variability exists between female populations of D. calcea, such that males reach maturity at a younger age and females at an older age in south eastern Australia and New Zealand than on the continental slope off southern Australia. Improved understanding of the reproductive output, growth and mortality of these species provides a basis for better fishery management and grounds for estimating the sustain- ability of current harvest level for D. calcea. The life history parameters determined for both species suggest they have very low biological productivity. There is already evidence of depletion of the stocks by fishing, so that to rebuild their depleted populations, the deepwater fisheries of this region will require careful management