Otolith oxygen isotopes inform dispersal and movement patterns for eastern school (Sillago flindersi) and stout (Sillago robusta) whiting along the east coast of Australia

Understanding stock structure and connectivity of harvested marine fishes is crucial for informed management and conservation efforts. We analysed stable oxygen isotopes in fish otolith carbonate to infer dispersal and movement patterns of stout whiting (Sillago robusta) and eastern school whiting (Sillago flindersi) in southeastern Australia. Adult fish were collected from commercial catches across seven locations spanning 8° latitude for stout whiting and 16 locations distributed over 15° latitude for eastern school whiting. Sagittal otoliths were sectioned, polished and micromilled to produce a carbonate sample from the juvenile ‘core’ and adult ‘edge’ portion of each otolith. Samples were analysed for oxygen isotope ratios (δ¹⁸O_otolith) using isotope ratio mass spectrometry. Oxygen isotope ratios in stout whiting otoliths were significantly influenced by the interaction between capture location and life stage, with temperature preferences between life stages more pronounced with increasing latitude. Variation in δ¹⁸O_otolith values among adult eastern school whiting sampled from different locations indicated limited connectivity between northern and southern extremities of the species distribution, but considerable mixing across adjacent latitudes. Additionally, significant variation in δ¹⁸O_otolith values between male and female eastern school whiting suggested sex-biased dispersal may be occurring, with males dispersing further than females. For each species, low core δ¹⁸O_otolith values implied movement from warmer juvenile nursery habitats to cooler waters as adults, which may involve inshore-offshore ontogenetic migrations and/or southerly longshore migrations using prevailing currents. The intricate dynamics of connectivity and ontogenetic movements of stout and eastern school whiting across multiple jurisdictions highlight the importance for co-operative and collaborative management efforts.