Analysis of stable isotope ratios to investigate stock structure of red emperor and Rankin cod in northern Western Australia

The ratios of stable isotopes O-18/O-12 and C-13/C-12, in sagittal otolith carbonate from two tropical demersal teleosts, red emperor Lutjanus sebae a nd Rankin cod Epinephelis multinotatus, from several locations in northern Western Australia. differed between sites. On a broad scale, fish from the four locations, Shark Bay, Ningaloo, Pilbara, and Broome had stable isotope values that were sufficiently different to indicate separate stocks, and i t is appropriate to manage these populations of the two species independent ly in these areas. On a smaller scale, there may be limited mixing of these species between the Pilbara trawl fishery and the trap and line fisheries operating out of Onslow and Broome. Values of stable oxygen isotopes were s trongly related to sea surface temperature, although there were some sites in shallow water where low values of stable oxygen isotopes indicated that fish were living in warm water. The use of stable oxygen and carbon isotope values is a valuable, cost effective method of determining the degree of m ixing of fish stocks.