Age and growth determination of three sympatric wobbegong sharks: How reliable is growth band periodicity in Orectolobidae?

Abstract

The determination of age for large, harvested species such as chondrichthyans is important to the estimation of growth and other key life history parameters such as natural mortality, age-at-maturity, longevity, and recruitment. Vertebrae from 760 wobbegongs (275 Orectolobus ornatus, 232 O. maculatus, and 253 O. halei) were collected between June 2003 and December 2007 at seven locations in eastern Australia (Queensland and New South Wales) to estimate growth parameters for these species. A multi-model inference (MMI) information theoretical approach including four candidate models, with back-calculated estimates of length in earlier life stages to account for limited numbers of pup and juvenile wobbegongs, was used to determine the most appropriate growth model for each species. The models that combined observed and back-calculated lengths-at-age did not provide a better fit than the model using observed lengths-at-age data only. Taking into account biologically meaningful estimations of L∞ and k, the models with the best fit to the data were the logistic growth function for O. ornatus and O. halei, and the von Bertalanffy growth model for O. maculatus. Using these models, growth parameters obtained were: 999, 1630 and 2128mm total length for L∞ and 0.19, 0.09 and 0.20 for k, while the maximum number of growth bands was 20, 22, and 27, for O. ornatus, O. maculatus, and O. halei, respectively. All three species were monomorphic, with similar growth rates for males and females. Verification and validation undertaken using edge and marginal increment analyses, as well as chemical marking of captive and wild wobbegongs, suggested that growth band deposition in orectolobids is more likely to be linked to somatic growth than seasonality. This study is the first to use chemically marked wild Orectolobiformes to investigate growth band deposition rate. Five orectolobid species have now been shown not to deposit growth bands following a synchronous annual pattern, in contrast to that inferred for most other chondrichthyan species. The growth parameters estimated in this study are crucial for stock assessments and for demographic analyses to assess the sustainability of commercial harvests.