Summing up - Symposium on reproductive and recruitment processes of exploited marine fish stocks

Abstract

Variability in recruitment results in variability in catch from year-to-year, which is a problem for fishermen, for the processing industry and for fisheries managers. It is commonplace to read how large the interannual variability in recruitment can be and how difficult it is to determine the causes. However for an ecologist, the astonishing feature of recruitment variability is how small it is, given that fish produce thousands or millions of eggs each and that survival is extremely unlikely and uncertain. Among the pelagic and demersal stocks assessed in the NE Atlantic the smallest year-class is generally within a factor of 10 of the biggest. It is not infrequent for interannual recruitment variability to be less than the variability of the spawning biomass of the stock (Brander, 2003). How is the variability damped out? What are the compensatory mechanisms? Density dependence is necessary for populations to remain within feasible bounds (Reddingius, 1971) but as Ed told us “A little density dependence, especially in the relatively long juvenile stage, can regulate recruitment”. Density dependence should not be invoked without good evidence in relation to processes like adult growth, however appealing it is to biologists or convenient for modellers trying to keep their models stable. Fisheries assessment and management, at least in the North Atlantic, has focussed on year-to-year tracking of stock biomass in order to set annual catch limits. Questions about the role of recruitment were therefore also principally directed at annual prediction and the understanding of processes needed in order to do this. The biological book-keeping was done using sequential population analysis which gives estimates of both annual recruitment and spawning stock biomass (SSB). The latter is assumed to represent spawning output of the stock, but as many of the papers in this symposium show, fecundity, maturity, spawning frequency and egg quality, are not constant. The relevance of the symposium is therefore obvious, since strategies for sustainable management, using precautionary reference levels of SSB and fishing mortality, are based on the relationship between SSB and recruitment. We need to move on to a longer term view which includes a wider range of information about the biological state of fish, their environment and their interaction with the rest of the marine ecosystem. Ed once more: “’Solving the recruitment problem’ is no longer the holy grail of fishery science. Appreciating recruitment variability, explaining probable causes, considering implications for management, and understanding it in the context of broader variability in marine ecosystems are worthy goals.”