Recruitment-driven fish production in two regions where fish biomass has drastically declined

Abstract

Abstract Catches have remained relatively high in the Gulf of Thailand and the Bohai Sea, China, despite severe biomass declines (around 95%) evidenced by fishery-independent surveys. Such high production at very low stock sizes is not predicted by simple-surplus production theory, but can be explained by age-structured models that predict high recruitment rates even when biomass per recruit (BPR) has been drastically reduced. Recruitment rates can be reconstructed by estimating changes over time in biomass and BPR, for alternative assumptions about survey catchability, growth, and mortality rates. These reconstructions indicate that likely severe decreases in BPR, due to high fishing mortality rates, imply that total recruitment rates have likely been relatively stable over time, with catch consisting largely of new recruits making up most of the low biomass. These considerations may apply to numerous areas in east and southeast Asia where most of the catch of coastal trawlers is used to produce animal feeds, notably for aquaculture.