Movement patterns of barramundi Lates calcarifer, inferred from Sr-87/Sr-86 and Sr/Ca ratios in otoliths, indicate non-participation in spawning

The migration patterns of the large catadromous fish, barramundi Lates calcarifer in southern Papua New Guinea were examined by analysing Sr-87/Sr-86 and Sr/Ca ratios by MC-LA-ICPMS and LA-ICPMS, respectively. Individual migration histories between coastal and riverine habitats could be inferred from the Sr-87/Sr-86 ratios. These were used to calibrate the Sr/Ca ratios from a larger sample of fish and identify periods when the fish were resident in freshwater. The largest freshwater population of barramundi occurs in the middle reaches of the Fly River. We found that most adult barramundi from this population do not migrate annually to spawn on the coast. Fish vary in the age that they first enter freshwater, and most have arrived by sexual maturity (4 yr). We estimate that only half the adult fish in the Fly and nearby Kikori rivers have ever migrated back to the coast during their lifetime. This implies that many barramundi do not participate in spawning. Fish examined from the spawning ground were mostly marine residents, and some had spent short periods in rivers adjacent to their spawning ground. Tagging and scale chemistry studies from northern Australia have also found little evidence of movement of freshwater resident barramundi back to the sea. This implies that the phenomenon of non-participation in spawning may be widespread in this species. Our results suggest that freshwater fisheries for barramundi may have less effect on the spawning population than those in coastal regions and thus be able to sustainably remove a greater proportion of the population.