Shifting control of recruitment of walleye pollock Theragra chalcogramma after a major climatic and ecosystem change

The critical stage in the life history of walleye pollock Theragra chalcogramma at which recruitment is largely determined has shifted in the Gulf of Alaska population. This change follows a major environmental regime-shift in the late 1970s and subsequent dominance of the ecosystem by long-lived predatory flatfishes and cod. An exploratory life table of data on stage-specific abundances was constructed to analyze decadal-scale changes in population demographics. Prior to the mid-1980s, recruitment was correlated with larval mortality, which was largely influenced by environmental conditions. After the mid-1980s, the larval mortality/recruitment relationship eroded, and there was a trend for greater juvenile mortality coinciding with an increase in the abundance of predatory flatfishes and cod. Control of recruitment appears to have shifted in recent years from environmental effects on larvae to biological control of juveniles. Ecosystem change and regime status need to be considered when evaluating mechanisms underlying fish-population dynamics.