STOCK DISCRIMINATION OF PILCHARDS SARDINOPS-SAGAX BY STABLE-ISOTOPE RATIO ANALYSIS OF OTOLITH CARBONATE

Measurement of the ratios of the stable isotopes O-18:O-16 and C-13:C- 12, by standard mass spectrometric techniques, in the otolith carbonat e of samples of pilchards Sardinops sagax from coastal waters adjacent to the 4 main fishing ports of southwestern Australia (Fremantle, Alb any, Bremer Bay and Esperance) provided location-specific signatures w hich showed that Fremantle fish could be regarded as a separate stock from those taken at the other 3 locations, and fish from Esperance con stituted a different stock from those fish taken al Albany and Bremer Bay. The isotope signatures for fish from the different locations pers isted through time, and it could be concluded that they comprised sepa rate stocks for most of the purposes of fisheries management. Values f or ratios of stable oxygen isotopes in pilchard otolith carbonate indi cated that they were probably deposited close to equilibrium and there fore reflected ambient water temperatures as governed by latitude and movements of the Leeuwin Current. This suggested that the measurement of ratios of stable oxygen isotopes can be a valuable method for the d iscrimination of management units of adult fish where the range of dis tribution of a species covers waters of different temperatures.