Optimal Environmental Window of the Recruitment of Thread Herring <scp> <i>Opisthonema libertate</i> </scp> in the Southern Gulf of California

ABSTRACT The recruitment processes in small pelagic populations are an important topic for assessment and management. Different hypotheses have been proposed to explain how some environmental variables like upwelling and turbulence affect it. The main hypothesis is the optimal environmental window (OEW) hypothesis: 5–6‐m/s wind speed as an OEW. However, this generalization may not apply to all species. In this study, we proposed that OEW should be determined according to an optimal recruitment level. For this, data for Ekman transport components and wind speed were used to calculate the upwelling (UI) and turbulence (TI) indices in the southern Gulf of California (SGC). Recruits (Rec) and spawners (SP) annual data of Opisthonema libertate were obtained from the 2021 official stock assessment. A stepwise procedure was used to fit a generalized additive model (GAM) to Rec data with UI, TI, and SP as covariates. Based on an 11‐step algorithm, we determined the OEW of recruitment for this species using partial effects analysis from GAMs. The relationships between the predictors and Rec were nonlinear (dome‐shaped). The OEW of recruitment for this species using the proposed approach was 5.29–22.72 m 3 /s per 100 m of coastline for UI and 79.56–106.67 m 3 /s 3 for TI. These results showed that O. libertate recruitment in the SGC is maximized under moderate upwelling and turbulence conditions, and that the proposed approach to determine the OEW is functional when the relationship between predictor variables and recruitment is nonlinear.