Sex‐Disaggregated Condition and Reproductive Investment in Sardina pilchardus Reveal a Decadal Reversal and Non‐Stationary Climate Responses in the Southern Alboran Sea

ABSTRACT Small pelagic fish are highly sensitive to environmental variability, yet sex‐disaggregated assessments of condition and reproductive investment remain rare in Mediterranean fisheries science and absent for the Southern Alboran Sea (SAS). Here, we report a decadal reversal in sex‐specific somatic condition in Sardina pilchardus and show that this reversal is mechanistically explained by the asymmetric reproductive costs imposed by warming‐induced changes in spawning phenology. Using 20,383 individuals sampled across two contrasting periods (1999–2003 and 2012–2023) and 10 environmental predictors, we applied generalized additive models (GAMs) to quantify environmental drivers of relative condition factor (Kn) and gonadosomatic index (GSI) and to project future biological states under SSP245 and SSP585 for 2050 and 2100. Females exceeded males in Kn during 1999–2003 but fell consistently below them by 2012–2023 (sex × period interaction: p < 0.001, ΔAIC = 228). Concurrently, the explanatory power of environmental drivers over Kn eroded substantially between periods (seasonality: −40%; SST: −39%), and this erosion was larger in females than males, constituting a sex‐differentiated form of ecological non‐stationarity not previously quantified for this species or region. SST and seasonality dominated both indicators but with opposing effects: Kn increased monotonically with SST, whereas GSI declined sharply above 16°C–20°C, revealing a physiological trade‐off invisible to biomass‐based monitoring. NPP exhibited a negative threshold effect on GSI, consistent with a phenological mismatch between peak productivity and the thermal spawning window. Projections indicate Kn declining 18%–59% and male GSI falling up to 46% under SSP585 by 2100, whereas female GSI remains comparatively stable. These findings expose multi‐level hidden stress that standard stock assessments cannot detect and establish a formal case for sex‐disaggregated monitoring as a prerequisite for climate‐resilient management of small pelagic fisheries.