Application of SHRIMP for high temporal resolution otolith thermometry for eastern school whiting (Sillago flindersi)

Analyses of oxygen isotope profiles in calcium carbonate accretionary structures, such as fish otoliths, provide temporal and spatial insights into the thermal histories of individual fish, as their deposition directly reflects ambient water temperatures. We applied Sensitive High Resolution Ion Microprobe (SHRIMP) analysis to quantify oxygen isotope ratios within otoliths (δ 18 O otolith ) of eastern school whiting ( Sillago flindersi ), sampled from different locations across their spatial distribution in south-eastern Australia. Compared to conventional acid digestion isotope ratio mass spectrometry and some other ion probe techniques, SHRIMP offers enhanced spatial resolution and analytical precision, with a small ion beam size that enables finer scale δ 18 O otolith measurements along otolith growth profiles. This fine-scale resolution allowed us to detect seasonal and ontogenetic variation in δ 18 O otolith values along the otolith growth profile from the core (representing the larval and juvenile life stage of a fish) to the edge of the otolith (representing the time of capture). SHRIMP analysis revealed minor fluctuations in δ 18 O otolith values with a much greater temporal resolution than can be achieved through milling or whole otolith analysis, capturing fine-scale changes in water temperature experienced over a fish’s lifetime. Water temperatures derived from δ 18 O otolith values approximated sea bottom temperature data for capture locations, with a tendency to underestimate expected ambient temperature. Peaks in δ 18 O otolith values coincided with annular increments on the otolith, indicating that opaque zones were formed during cooler water temperatures of winter, reinforcing the suitability of SHRIMP analysis for validation of age estimates from fish otoliths. Understanding the thermal preferences of fish, their distribution patterns and individual environmental histories can contribute to robust fisheries management approaches under current and future climate scenarios, as well as ongoing harvesting pressure.