Comparative rates of growth of the Port Jackson shark throughout its southern Australian range

Abstract

Abstract. Port Jackson sharks are distributed throughout southern Australia, with evidence suggesting that potential subpopulations exist. If subpopulations are evident, then phenotypic variation among groups should result in differences in life-history parameters. The present study tested for patterns of spatial variability of life-history parameters among regional Port Jackson shark populations. Rates of growth from Port Jackson sharks caught in the gulf waters of South Australia were calculated on the basis of counts of vertebral increments. Growth parameters were obtained by fitting the length-at-age data to von Bertalanffy and Gompertz growth functions. While the derived growth curves fit the length-atage data well (r 2 ranged from 0.87 to 0.91), parameters showed considerable differences between the two functions, with the von Bertalanffy function providing the more realistic estimates of growth (combined sexes: k ¼ 0.081 year À1 , L N ¼ 1232 mm total length and t 0 ¼ À1.937 years). Life-history parameters for South Australian Port Jackson sharks were collated with the available data for the species, facilitating comparisons among regional populations. Growth curves among populations varied significantly; however, considerable overlap in the length ranges of size at birth and sizes at maturity among populations were evident. Overall, the data presented here do not provide definitive support for the presence of subpopulations across the distribution of the Port Jackson shark, suggesting that molecular analysis maybe required to directly test for structuring.

Port Jackson Shark Growth Is Sensitive to Temperature Change

“…Gulf St Vincent is an inverse estuary that is largely sheltered from oceanic waves. Demographic data and tagging indicates that Port Jackson sharks inhabiting Gulf St Vincent and the adjacent Spencer Gulf form discrete subpopulations (Rodda and Svane, 2007;Izzo and Rodda, 2012).…”

“…Shark total length (in mm) and sex were recorded, and a section (n = 8-12) of vertebrae collected from under the first dorsal fin, were prepared for age determination using standard protocols (Izzo and Rodda, 2012). Vertebral centrum diameter was measured (in mm).…”

“…Growth increments (paired opaque and translucent bands) were counted, and widths measured under a dissecting microscope with transmitted light and image analysis software. Port Jackson shark vertebrae possess clear growth increments, and chemical marking has been used to verify that growth increments form annually (Tovar-Ávila et al, 2009;Izzo and Rodda, 2012), enabling the temporal resolution of shark growth to be validated.…”

“…Port Jackson sharks are commonly encountered as a bycatch species throughout their southern Australian distribution (Powter and Gladstone, 2008a;Jones et al, 2008;Tovar-Ávila et al, 2010;Izzo and Rodda, 2012), however, the species is seldom retained, is highly robust, and not prone to post-capture mortality (Frick et al, 2010). This study assumed negligible fishing induced mortality in the Port Jackson shark population in Gulf St Vincent, and hence deviations in population growth were primarily attributed to environmental factors.…”

“…We also interpreted this as confirmation of the absence of an agebias in the sample data (Morrongiello and Thresher, 2015). Yet we acknowledge that the samples were predominately small to medium (juvenile/immature) individuals, as large adults are infrequently encountered in Gulf St Vincent (Izzo and Rodda, 2012). This impeded our ability to explore relative growth responses across all Port Jackson shark life history stages through the omission of potential growth phenotypes; and therefore, future studies should seek to access all life stages to assess the potential for differing growth responses throughout development (Morrongiello et al, 2012).…”