Emerging from Hjort's Shadow

Abstract

Early in the 20 th century, Johan Hjort developed compelling arguments and hypotheses to explain recruitment variability that became dominant for more than 75 years. A cautious emergence from Hjort's shadow began late in the 20 th century. Hjort's "Critical Period" hypothesis, i.e., failure of first-feeding larvae to find food, and a second hypothesis, "Aberrant Drift" of eggs and larvae, were proposed to explain causes of recruitment variability. Tests of the Critical Period hypothesis became an obsession, although support for it was inconsistent and equivocal. Single-minded research on the Critical Period hypothesis gave way to realization that recruitment variability was the outcome of complex trophodynamic and physical processes acting over many temporal and spatial scales and throughout pre-recruit life. A complex mix of main effects and interacting factors can easily generate order-ofmagnitude variability in recruitment via small effects on mortality and growth rates during the abundant egg and larval stages, or via cumulative effects during the pre-recruit juvenile stage. New considerations of compensatory mechanisms that can dampen variability and stabilize recruitment emerged. A little density dependence, especially in the relatively long juvenile stage, can regulate recruitment. Multidisciplinary programs instituted in the 1990s and successful development of statistical models and coupled bio-physical models, offered new insights into mechanisms generating recruitment variability. Despite progress, forecasting recruitment remains a formidable challenge. "Solving the recruitment problem" is no longer the Holy Grail of fishery science. Appreciating recruitment variability, explaining its probable causes, considering implications for management, and understanding it in the context of broader variability in marine ecosystems, are all worthy goals.