Impacts of Demographic Variation in Spawning Success on Reference Points for Fishery Management

Parametric relationships between recruitment and an index of parental stock size assume the latter is proportionally related to spawning potential (usually indexed by spawning biomass), irrespective of the demographic composition of adults. Recent empirical information, however, suggests that spawning by older, more experienced females is more successful than that by the young, small or inexperienced within a population. New models are proposed incorporating the proportion of the ith age class spawning for the jth time (PSP i,j ), from information contained in the maturity ogive, and experimental results relating the survival of eggs and larvae to the age, size or reproductive experience of adults. A series of spawning metrics (spawning stock biomass or SSB, egg production, hatched egg production, viable larval production) and associated recruitment-based fishing mortality reference points (F med , F crash = F extinction = Fτ ) and the F that allows at least one lifetime spawning per recruit) are contrasted for the Georges Bank cod stock. We conclude: (1) the time series of cod spawning intensity is significantly altered when hatched eggs or viable larvae are used as the metric, reflecting the importance of firstand second-time spawners in some years, and the increasing trend in F over time, (2) percent maximum spawning potential (%MSP) curves in relation to fishing mortality are steeper (e.g. result in lower %MSP for a given F), when the metric is hatched eggs and viable larvae rather than SSB or egg production per recruit, (3) lifetime expected numbers of spawnings per recruit are significantly reduced when the effects of spawning experience on egg hatching success are included, and (4) the median F estimated from 5,000 bootstrap realizations of the Beverton-Holt S-R curve for viable larvae (F=0.88) is much lower than that from SSB (F=1.40), with narrower confidence bounds. Our results suggest that traditional approaches to F-based reference points using SSB systematically overestimate the resiliency of stocks to fishing. This adds impetus to the need for adopting precautionary approaches to fisheries management. Additional laboratory studies of the life history of spawners in relation to the fate of eggs and larvae are clearly warranted.