Population structure and life history of orange roughy (Hoplostethus atlanticus) in the SW Pacific: inferences from otolith chemistry
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We examined site differences in the elemental composition of the primordium and ontogenetic variability of Sr in otoliths of fish from Australia and New Zealand and, as an out-group, the North Atlantic. Differences among sites in primordium composition are slight, but significant for all five elements assayed (Sr, Pb, Cu, Zn and Hg), but principally reflect differences between the North Atlantic and SW Pacific specimens, do not replicate for independent samples in the SW Pacific and constitute a poor “natural tag” in roughy, with <25% of fish successfully assigned to source location. However, mean Sr weight-fractions at the primordium showed similar latitudinal variation across sites in Australia, New Zealand and the Tasman Sea, indicating both spatially structured populations and a common structuring process across the region. Comparisons of ontogenetic variability of Sr in otoliths from juveniles and young adults within and between sites in the SW Pacific strongly support the hypothesis that variability in this element is site-specific and environmentally sensitive, although the environmental factors involved are not obvious. The otolith analysis confirms previous suggestions that juvenile and adult Hoplostethus atlanticus are relatively sedentary, but also indicates that the population sub-structuring by age within sites is more complex and there are likely to be more spawning areas in Australian waters than previously thought. More broadly, although single point analysis of otolith composition at the primordium resolves a population structure in roughy, alone it is not precise enough to test hypotheses about the processes causing this structure. Ontogenetic variability in Sr provides better resolution of spatial structure, even in a relatively homogenous marine environment like the deep ocean, and also provides insight into behavioural and ecological factors. Ontogenetic analyses of Sr in otoliths are expensive to obtain, require more effort in specimen preparation than single point analyses, and are difficult to compare statistically, but the increased information they yield warrants their broader consideration in marine species.
KeywordsPIXE Nursery Area Discriminant Function Analysis Stock Structure Orange Roughy
We thank staff of the Micro-Beam Laboratory at CSIRO Division of Minerals (Clayton, Victoria) for technical advice and assistance with WD-EPMA analyses, staff at the Heavy Ion Analytical Facility at CSIRO Exploration and Geoscience (North Ryde, NSW) for technical advice and assistance with PIXE analyses, and R. Bailey, K. Evans, C. Mackinnon and D. Mills for their assistance in preparing otoliths. We particularly thank the skippers and crews of commercial fishing vessels for their assistance in obtaining samples of fish for otoliths, as well as D. Evans of Western Australia Department of Fisheries, M. Scott of Aust. Fisheries Management Authority, the Tasmanian Department of Primary Industry and Fisheries, the Central Aging Facility (MAFRI), and the National Institute of Water and Atmospheric Research (NZ). Finally we thank N. Bax, R. Ward and two anonymous referees for constructive comments on the MS. This work was supported by grants from the Australian Fisheries Research and Development Corporation and the Australian Fisheries Management Authority, as well as by CSIRO.
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