A systematic framework for assessing conservation effectiveness of migratory marine species: Integrating biomass, habitat quality, and functional traits

The global expansion of Marine Protected Areas (MPAs) under the "30×30" target necessitates rigorous effectiveness assessments. Traditional evaluations for migratory marine species often rely on static biomass metrics, which fail to capture spatiotemporal mismatches between dynamic migration patterns and static reserve boundaries, deceptively masking structural population crises. Addressing these gaps, we developed a systematic framework integrating biomass, habitat quality, and functional traits. We validated this using the small yellow croaker (Larimichthys polyactis) in the Yellow and East China Seas, employing machine learning, kernel density estimation, circuit theory, and population dynamics models on data spanning 1995 to 2024. Results uncovered a distinct "resource recovery paradox" where quantity uncoupled from quality. Concurrent with enhanced conservation and fishery-management measures, biomass status and habitat quality showed a marked rebound from the 2010s to the 2020s. Year-round Ecosystem-based MPAs and Gear-restricted Fishery Conservation Zones generally outperformed Seasonal Fishery-based MPAs (S-MPAs), particularly in spring. However, this quantitative recovery concealed intrinsic population degradation. Despite a partial recovery in body length, critical functional traits, including fecundity and trophic level, declined continuously, indicating a stock dominated by vulnerable recruitments. Furthermore, severe seasonal asymmetry persists. Current measures appeared more effective in sheltering spring and summer spawning stocks than in protecting deep-water overwintering grounds and migration corridors. Consequently, biological gains associated with summer closed seasons may be weakened by subsequent winter fishing pressure, causing continued winter biomass declines within S-MPAs. We recommend shifting toward dynamic ocean management to restore full life-cycle connectivity and implementing trait-based strategies to rebuild population resilience. This adaptable framework provides a scientific paradigm for optimizing global MPA networks.