Revisiting sardine recruitment hypotheses: Egg‐production‐based survival index improves understanding of recruitment mechanisms of fish under climate variability

Proportionality between spawning stock biomass (SSB) and annual total egg production (TEP) has been an inherent assumption in spawner–recruitment models for fisheries management and studies on recruitment mechanisms in fish. However, recent evidence of density dependence in egg production has led to a need to reconsider the validity of recruitment per spawning stock biomass (RPS), which is a SSB‐based survival index from hatching to recruitment, in recruitment studies. Here we revisit sardine recruitment hypotheses based on recruitment per egg production (RPE), which is a TEP‐based survival index, by reanalyzing published data for the Pacific stock of Japanese sardine (Sardinops melanostictus) in the Kuroshio Current system at a multidecadal scale. By adopting RPE, we detected statistically significant relationships of the survival from hatching to recruitment to large‐scale ocean climate anomaly, ambient temperature in the nursery grounds and growth rate during the early life stages. None of these relationships were detected by RPS. We propose the adoption of TEP‐based survival indices rather than SSB‐based ones in future studies on recruitment mechanisms of fish in relation to environmental and biological factors. Developing egg‐production‐based survival indices has the potential to improve understanding of recruitment mechanisms of fish under climate variability.