Stock delineation of pink snapper and tailor from Western Australia by analysis of stable isotope and strontium/calcium ratios in otolith carbonate

The O-18/O-16 and C-13/C-12 ratios in the otolith carbonate of pink snapper Pagrus auratus and tailor (bluefish) Pomatomus saltatrix, from several loc ations along the Western Australian coast, indicated that pink snapper stoc ks are location specific but that tailor stocks are less so. The hypersalin e Shark Bay, on the coast of Western Australia, generated strongly characte ristic isotopic signatures which serve as natural tags. Otolith carbonate f rom pink snapper from normal oceanic waters just north of Shark Bay showed no evidence that the fish had been in hypersaline water. Similarly, pink sn apper from the hypersaline bay showed no evidence of having spent time at n ormal oceanic salinity. By contrast, some tailor from oceanic waters showed evidence of having spent considerable time in the bay, and some fish from the bay had oceanic signatures. This suggested that tailor were more migrat ory than snapper. The similarity in the distribution of the isotopic signat ures (from oceanic to hypersaline) in otolith carbonate from tailor from oc eanic waters north of Shark Bay (Koks Island), and from those within Shark Bay, indicated a single stock in this region (in contrast to pink snapper). Moreover, tailor from coastal south-western Australia and from the Shark B ay area could be considered seperately for some management purposes. For pi nk snapper stocks from oceanic waters, oxygen isotope signatures were clear ly related to water temperature although the temperature relationship was o bscured for fish within Shark Bay because of the strength of the signal gen erated by the hypersaline water. For tailor the temperature relationship wa s not obvious, probably because migrations of tailor smeared the temperatur e effect, and the hypersaline Shark Bay waters dominated, and, possibly, at the southern extemity of the range, the freshwater in some estuaries influ enced the isotopic signatures of the otolith carbonate. Strontium/calcium r atios in pink snapper otoliths also indicated a separation of stocks, but f or tailor overlap of signatures again suggested migratory behaviour. (C) 19 99 The Fisheries Society of the British isles.