Population structure and life history of orange roughy (Hoplostethus atlanticus) in the SW Pacific: inferences from otolith chemistry

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.