Stock discrimination of Southern Garfish (Hyporhamphus melanochir) by stable isotope ratio analysis of otolith aragonite
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Stable isotope analysis (δ18O and δ13C) of otolith aragonite was used to infer ontogenetic patterns of movement, delineate sub-population structure, and determine the mixing of Southern Garfish (Hyporhamphus melanochir) in South Australian waters. Adult garfish from the 2+ age class were collected from three sites within each of six regions along the South Australian coast: Northern Gulf St. Vincent, South West Gulf St. Vincent, Kangaroo Island, Northern Spencer Gulf, South West Spencer Gulf and the West Coast of Eyre Peninsula. Significant spatial variation was detected in mean δ18O and δ13C values among sites within regions and as a function of fish age. Salinity, rather than sea surface temperature, appeared to account for regional variation in δ18O, whereas variation in diet and metabolic processes were suggested to account for observed differences in δ13C. Otolith oxygen and carbon stable isotope values indicated that the South Australian Southern Garfish fishery is comprised of multiple, regional, population components that persist through time. Both the West Coast and Northern Spencer Gulf regions constitute distinct population components that exhibit little inter-regional mixing and therefore may be considered as discrete management units. The South West Spencer Gulf region may also be considered a separate population component, however, its level of connectivity with Gulf St. Vincent is uncertain. With the exception of juvenile garfish from Kangaroo Island, there was no clear regional separation within Gulf St. Vincent.
KeywordsPopulation structure Stable isotopes Hemiramphidae Otolith South Australia
We thank Matt Lloyd, Bruce Jackson, David Schmarr and Jason Earl for valuable field and laboratory assistance. Benjamin Walther (University of Adelaide) provided advice on micromilling and associated laboratory procedures. Thanks also to Tim Ward and two anonymous referees for providing constructive reviews on earlier drafts of the manuscript. This work was funded by the Fisheries Research and Development Corporation (Project no. 2007/029), and logistical support was provided by the South Australian Research and Development Institute.
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