Correction Factors Derived from Acoustic Tag Data for a Juvenile Southern Bluefin Tuna Abundance Index in SouthernWestern Australia

Abstract

Juvenile southern bluefin tuna (SBT, Thunnus maccoyii), migrate down the coast of Western Australia reaching the southern coast at age-1. In these waters an acoustic survey for SBT schools was initiated to generate a fisheries-independent abundance index. A decline in this abundance index led to an acoustic tagging and monitoring project to determine if a change in migration route or timing could explain the decline. Five years of acoustic monitoring revealed interannual differences in key factors that could impact the abundance index. Acoustic tag data were used to demonstrate that (i) a high proportion of fish (similar to 70%) may be too shallow for detection in the acoustic survey, and that interannual variation in (ii) inshore-offshore fraction (similar to 30-70% each year) and (iii) residence time (12-37 days) will impact calculation of an index. These factors should be included in estimating an abundance index for SBT, together with a correction for (iv) the fraction of juvenile SBT that migrate to southern Western Australia. Collectively, these results illustrate how electronic tagging data can be used to improve understanding of abundance patterns necessary for sustainable management of this exploited species.