Exploratory Simulation of English Sole Recruitment Mechanisms

We constructed a computer simulation model to test several hypotheses about year‐class formation of English sole Parophrys vetulus off the coast of Oregon. Rather than use the model to generate best‐fit estimators of future recruitment, we sought to construct a model that can serve as a framework for studying mechanisms that may not always be dominant in their effect. The model was constructed from available information on the speciesˈ life history and coastal ocean environment. We investigated effects on year‐class strength of relationships between (1) fecundity and temperature, (2) egg‐hatch success and temperature, and (3) advective loss of larvae and wind. Hypotheses were translated into functional relationships and evaluated based on the correspondence between year‐class strength estimated by cohort analysis and simulated with the model. The model was driven by time series of fishery and physical data. The final model accounted for 53% of the variance during 1960–1970 and emphasized the integration of several components consequential to recruitment: stock size and structure, fecundity, spawning seasonality, egg hatching success, and advection. These results weakened the concept of critical period because events before spawning and during the full pelagic stage were also very important. The variability in time of spawning was particularly consequential to year‐class success. Spawning generally was adapted to periods of optimal temperatures for egg development and favorable larval advection; it was partly cued by increasing shelf temperatures coincident with onshore drift. Also, time of spawning may be adapted to periods of enhanced feeding success because it is likely that the major prey ( Oikopleura spp.) of English sole larvae are swept into the coastal zone during downwelling.