Otolith chemistry of juvenile snapper Pagrus auratus in Victorian waters: natural chemical tags and their temporal variation

Abstract

Determining contributions of juveniles from different areas to adult populations is difficult using traditional methods such as manual tagging and recapture. Environmental variation can, however, produce natural chemical tags in the otoliths of juvenile fish. Spatial variation in the otolith chemistry (Ba, Sr, Mn) of age 0+ snapper collected from inlets along the coast of Victoria, south-eastern Australia, enabled characterisation of otolith chemical tags for specific areas. In particular, a tag based largely on high Ba levels was specific to snapper from the largest juvenile area and most important fishery, Port Phillip Bay. This tag enabled accurate discrimination between snapper from this bay (98 and 85%) and other Victorian nursery areas in the 2000 and 2001 year classes, respectively. Differences in classification accuracy between these year classes were due to lower Ba levels in otoliths from some sites within Port Phillip Bay in 2001. Variation in otolith chemical tags between adjacent sampling months during the recruitment period, although significant, did not confound spatial discrimination between Port Phillip Bay and other inlets. Likewise, variation in otolith chemical tags between adjacent year classes, while greater than between months, did not greatly confound spatial discrimination. Comparisons across 5 year classes over a 9 yr period for Port Phillip Bay, however, showed substantial differences in the elemental tag for this Bay. Differences in otolith chemistry between year classes were driven by variation in Ba and Sr levels, whereas between-month differences were due to Mn variation. Future classifications of adults to juvenile nursery areas will require chemical tags characterised from juveniles of the same year class(es) as the adults being classified. Small-scale (mo) temporal mismatches between the sampling of juvenile and adult otoliths of the same year class should not influence the accuracy with which adults are classified to juvenile origins.