An examination of the population structure of the eastern Australian salmon (Arripis trutta) stock in southeastern Australia

Abstract

The population structure of the eastern Australian salmon Arripis trutta stock in the waters of southeastern (SE) Australia was examined using information provided by historical as well as current data sources. An extensive tag-recapture program and aging study undertaken during the 1960s demonstrated widespread mixing of the A. trutta population in SE Australian waters and established a robust model of general movement of fish from Tasmania north to Victoria and NSW with the approach of sexual maturity at ~four years of age. However, this work also hypothesized that the portion of the stock at Flinders Island in Tasmanian waters was resident and did not undergo this northward migration. Otolith chemistry analyses were therefore used as a tool in a ‘weight of evidence’ approach to further examine the population structure of the A. trutta stock in SE Australia. Samples of five year old A. trutta for analysis of otolith chemistry were collected over seven weeks from two sites (10 per site) within each of four locations: northern NSW, southern NSW, Victoria and Tasmania. The cores and edges of otoliths were analyzed using laser ablation inunductively coupled plasma mass spectrometry. Univariate analyses did not find spatial differences for any of the elements Li, Na, Mg, Mn, Ba or Sr between locations. Multivariate analyses however, did find differences between the multi-element ‘fingerprints’ of fish from Tasmania compared to each of the other locations (which were similar). This difference was driven by a group of fish collected from Flinders Island in north-eastern Tasmanian waters. The fish collected at this site were also significantly smaller at five years of age than fish from all other sites, indicating reduced growth rates. The lack of consequential and definitive differences in otolith chemistry data combined with the highly migratory nature of A. trutta in this region demonstrated by tagging studies confirm that the most likely stock structure model for A. trutta in SE Australia is of a single well mixed biological stock spanning Tasmania, Victoria and NSW with fish moving north from Tasmania to mainland Australia with the approach of sexual maturity. However, the reduced growth rates and distinct elemental signature for A. trutta from Flinders Island highlights the need for further work to examine the preexisting hypothesis of a potential resident sub-population there.