Barium variation in Pagrus auratus (Sparidae) otoliths: A potential indicator of migration between an embayment and ocean waters in south-eastern Australia

Chronological variation in otolith chemistry can be used to reconstruct migration histories of fish. The use of otolith chemistry to study migration, however, requires knowledge of relationships between the chemical properties of the water and elemental incorporation into otoliths, and how water chemistry varies in space and time. We explored the potential for otolith chemistry of snapper, Pagrus auratus, to provide information on movement history between a large semi-enclosed bay, Port Phillip, and coastal waters in south-eastern Australia. Comparisons of water chemistry across two years demonstrated that ambient barium (Ba) levels in Port Phillip Bay were approximately double those in coastal waters (11 μg L⁻¹ versus 6 μg L⁻¹). Ba levels in otolith margins of wild juvenile snapper were highly positively correlated with ambient levels across 17 sampling locations, and levels in otolith margins of adult snapper collected from Port Phillip Bay were approximately double those of snapper collected in coastal waters. Mean partition coefficients for Ba (D_Ba) were similar for juvenile (0.43) and adult (0.46) otoliths, suggesting that otolith Ba incorporation relative to ambient levels was similar across life-stages. Low Ba variation across otoliths from adult snapper maintained in tanks for three years indicated that annual temperature and/or growth cycles did not strongly influence otolith Ba variation. We concluded that chronological Ba variation in snapper otoliths would be a reliable proxy for life-history exposure to variable ambient Ba. We used water chemistry data and Ba levels across otoliths of ocean resident snapper to estimate otolith Ba levels indicative of residence in Port Phillip Bay (>10 μg g⁻¹) or coastal waters (<6 μg g⁻¹). Peaks in Ba exceeding 10 μg g⁻¹ were common across otoliths of snapper collected in Port Phillip Bay and a nearby coastal region. The location of strong Ba peaks within otoliths was consistent with residence in Port Phillip Bay during the spring/summer when snapper move into the Bay from coastal waters to spawn. Our results for snapper support the use of otolith Ba as a proxy for ambient levels throughout the life-history, however, confident interpretation of migration history from otolith Ba chronologies will most likely require matching time series of ambient Ba in the water bodies of interest.