IN SEARCH OF THRESHOLDS FOR RECRUITMENT OVERFISHING

In this study we consider the problem of estimating, for management pu rposes, a minimum biomass reference level at which recruitment to a fi sh stock is seriously reduced. We take an empirical-comparative approa ch to the problem by examining observations on a wide range of fish st ocks. Eight methods for estimating spawning stock biomass thresholds f or recruitment overfishing are investigated. Their behaviour is tested using stock and recruitment data for 72 finfish populations, each wit h at least 20 years of data. We considered three classes of thresholds defined by: (1) the stock size corresponding to 50% of the maximum pr edicted average recruitment; (2) the minimum stock size that would pro duce a good year class when environmental conditions are favourable; a nd (3) the stock size corresponding to 20% of various estimates of vir gin stock size. The estimators of the first type are generally prefera ble because they are easily understood, relatively robust if only data at low stock sizes are available, and almost always result in higher levels of recruitment above the threshold.