Age determination of commercial fishes contributes to fisheries assessment to ensure sustainable harvest. Visual interpretation of fish otoliths (ear bones) has been the primary method to determine fish age due to the relative ease of obtaining otoliths and minimal equipment requirements. However, age determination of some fish species can be particularly challenging due to indistinct growth bands and structural complexities. Here, we focused on eastern school whiting (Sillago flindersi), which are caught within commercial trawl fisheries in eastern Australia encompassing NSW, Victoria, Tasmania and South Australia. We applied Sensitive High Resolution Ion Microprobe (SHRIMP) analysis to determine oxygen isotopes (δ18Ootolith) from the core to the edge of otoliths of fish obtained from commercial catch across their spatial distribution. We applied a genus-specific oxygen isotope fractionation model to estimate water temperature profiles that fish had experienced during their lifetime. Visual age assessments of otoliths were corroborated with δ18Ootolith profiles, revealing alignment of opaque bands with distinct peaks in δ18Ootolith, indicative of winter formation of opaque zones. This technique provides a valuable validation of fish age readings, and may be suitable for other fish species whose otoliths may be difficult to interpret based on visual analysis alone.