Comment: Recruitment paradigms for fish stocks

Over the last 20 years, there has been a substantial shift in the importance most fisheries scientists ascribe to recruitment overfishing; it was widely assumed (Gulland 1983) that most marine fishes could not be recruitment overfished, and the greatest emphasis in fisheries management was on yield per recruit. However, a large number of fish stock collapses that showed reduced recruitment at low spawning stocks led to a general reappraisal, so that Hilborn and Walters (1992) stated that ithe most important and generally most difficult problem in biological assessment of fisheries is the relationship between stock and recruitment.i Myers et al. (1994) used a large data base on many marine fishes to confirm that most stocks showed reduced recruitment at low spawning stock. Gilbert (1997) offers a revisionist interpretation of the Myers et al. (1994) data and appears to threaten to turn back the clock to a time when fishery managers believed they did not have to worry about maintaining spawning stock biomass. Is this truly a backwards step? I have to answer no; Gilbertis paper is a positive step in our ongoing scientific investigation of stock and recruitment because Gilbert highlights the chicken-or-the-egg dilemma in most spawner recruit analysis. Did low spawning stock lead to low recruitment, or did low recruitment lead to low spawning stock? The growing recognition of the importance of environmental changes in fish recruitment (Francis and Hare 1994) emphasizes the difficulty in interpreting the causes of declines in recruitment, and we are essentially revisiting the ThompsonnBurkenroad debate of 50 years ago (Thompson 1950; Burkenroad 1948; Skud 1975) with more data in hand. Gilbertis paper challenges those of us who have advocated maintaining spawning stock biomass as a prudent form of management to tighten up our arguments. We need to develop statistical analyses that determine the relative probability of competing hypotheses about the cause of recruitment declines. The analysis of Myers et al. (1994) did not consider an environmental change hypothesis; Gilbertis analysis is of limited utility because it relies on acceptance and rejection of null hypotheses, a statistical format that is of no utility for decision making. While Gilbert makes us face the alternative hypothesis that recruitment declined because of environmental conditions, he stops short of the next step and asking what is the prudent management action given the uncertainty regarding the cause of the recruitment decline. If we accept that both a recruitment overfishing hypothesis and an environmental change hypothesis are consistent with an individual stockis history, then what should a manager do if faced with a stock showing both reduced recruitment and reduced spawning stock: continue fishing on the assumption that the environment changed, or reduce fishing in an effort to rebuild spawning stock size? This is a problem in statistical decision theory that requires understanding the probabilities of competing hypotheses and a manageris analysis of risk. Clearly, the costs of not reducing fishing pressure at low abundance, if the true cause is recruitment overfishing, are grave: the stock will simply fail to recover until fishing pressure is reduced. The cost of reducing fishing pressure if the cause of the decline is environmental is simply short-term lost catch. These options are summarized in Table 1: the prudent form of management would seem to be to act as if recruitment overfishing were the cause. However, the choice of management option depends upon the probabilities assigned to the alternative hypotheses and the risk preference of the decision maker. The challenge to fisheries science is now to develop methods to determine the probabilities of alternative hypotheses and perform the risk analysis.