Toward Climate-Informed Fisheries Assessment: Integrating Oceanographic Variables into Frigate Tuna Stock Assessment in the Southeast Atlantic Ocean

Frigate Tuna (Auxis thazard) is a top management priority for the International Commission for the Conservation of Atlantic Tunas (ICCAT). Yet, no stock assessment for the Southeast Atlantic has integrated catch data with ocean- and climate-standardized abundance indices to produce climate-informed stock status estimates. This study presents the first such assessment, using annual purse-seine catch and effort data from the ICCAT Task I and Task II databases (1991-2023) and five oceanographic variables from the National Oceanic and Atmospheric Administration (NOAA). Catch-per-unit effort (CPUE) was standardized using a Generalized Additive Model (GAM), and stock status was evaluated using the Stochastic Surplus Production Model in Continuous Time (SPiCT). The Task II-derived CPUE index (S1) outperformed the Task I-derived alternative (S2) in GAM's cross-validation (mean Root Mean Square Error = 0. 028, Mean Absolute Error = 0. 020), penalized likelihood fit (lower Restricted Maximum Likelihood - REML and scale estimates), and SPiCT's residual diagnostics (normality and homoscedasticity). Significant drivers of relative abundance included year, latitude, sea surface temperature, chlorophyll a concentration, and ocean surface wind, with abundance increasing northward under favorable conditions (peaking around 4°-6°S). SPiCT results indicate that biomass exceeds the target reference point median biomass to biomass at maximum sustainable yield (B/BMSY) = 1.8 and that fishing mortality remains below the fishing mortality at maximum sustainable yield (F/FMSY = 0.2). High-stock resilience (median intrinsic growth rate (r) = 0. 65) was confirmed across four sensitivity scenarios. Retrospective analyses (Mohn's rho, ρ: 0.15 to 0.20), Ljung-Box and Shapiro-Wilk tests confirmed negligible bias and stable model performance. Although current exploitation is sustainable, the SPiCT assessment indicated that yield-maximising strategies may compromise long-term sustainability; thus, precautionary management is recommended. This assessment is also the first scientific evaluation of ICCAT Task II purse-seine data on small tunas in the Southeast Atlantic, providing a quantitative baseline for future regional assessments.