Identifying trends and drivers of fish growth in commercial species is important for ongoing sustainable management, but there is a critical shortage of long-term datasets in marine systems. Using otolith (ear bone) sclerochronology and mixed-effects modeling, we reconstructed nearly four decades (37 yr) of growth across four oceanographically diverse regions in an iconic fishery species, snapper (Chrysophrys auratus). Growth was then related to environmental factors (sea surface temperature, chlorophyll-a, and Southern Oscillation Index) and population performance indicators (recruitment and commercial catch). Across the decades, growth rates declined in the two most productive fishery regions. Chlorophylla (a measure of primary productivity) was the best predictor of growth for all regions, but direction and magnitude of the relationships varied, indicating regional-specific differences in intra-specific competition. Sea surface temperature was positively correlated with fish growth, but negatively correlated after temperature reached optimum thermal maxima, which suggests individuals in warmer regions may be under thermal stress. Growth also decreased at the extremes of the Southern Oscillation Index, indicating fish growth is impeded in significant climatic events. Contrasting relationships between growth, catch, and recruitment indicated regional-specific density-dependent effects, with growth positively correlated with population size in one region but negatively correlated in another. Our results indicate that under future ocean warming and increased frequency of extreme climate events, fish growth and fisheries productivity are likely to be affected. Furthermore, the interactive effects of extrinsic factors also indicated that stressors on fisheries should be managed collectively. We show that otolith chronologies are an effective method to assess long-term trends and drivers of growth in fishery species. Such informed ecological predictions will help shape the sustainable management of fisheries under future changing climates.

Metabolic rate underpins our understanding of how species survive, reproduce and interact with their environment, but can be difficult to measure in wild fish. Stable carbon isotopes (δ 13 C) in ear stones (otoliths) of fish may reflect lifetime metabolic signatures but experimental validation is required to advance our understanding of the relationship. To this end, we reared juvenile Australasian snapper (Chrysophrys auratus), an iconic fishery species, at different temperatures and used intermittent-flow respirometry to calculate standard metabolic rate (SMR), maximum metabolic rate (MMR) and absolute aerobic scope (AAS). Subsequently, we analysed δ 13 C and oxygen isotopes (δ 18 O) in otoliths using isotope-ratio mass spectrometry. We found that under increasing temperatures, δ 13 C and δ 18 O significantly decreased, while SMR and MMR significantly increased. Negative logarithmic relationships were found between δ 13 C in otoliths and both SMR and MMR, while exponential decay curves were observed between proportions of metabolically sourced carbon in otoliths (M oto) and both measured and theoretical SMR. We show that basal energy for subsistence living and activity metabolism, both core components of field metabolic rates, contribute towards incorporation of δ 13 C into otoliths and support the use of δ 13 C as a metabolic proxy in field settings. The functional shapes of the logarithmic and exponential decay curves indicated that physiological thresholds regulate relationships between δ 13 C and metabolic rates due to upper thresholds of M oto. Here, we present quantitative experimental evidence to support the development of an otolithbased metabolic proxy, which could be a powerful tool in reconstructing lifetime biological trends in wild fish.

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