Impacts of demographic variation in spawning characteristics on reference points for fishery management

Abstract

Parametric relationships of recruitment to an index of parental stock size assume the latter is proportionally related to spawning potential, irrespective of the demographic composition of adults. Recent empirical information, however, suggests that reproduction by older and experienced females is more successful than by young and inexperienced females. New models are developed incorporating the proportion of each age group spawning for the first, second, etc., time (from information contained in the maturity ogive) and differences in the survival of eggs and larvae in relation to the demography of spawners (based on experimental results). A series of spawning metrics [spawning-stock biomass (SSB), egg production, hatched egg production, viable larval production] and associated recruitment-based fishing mortality reference points (Fmed, Fcrash, and the F that allows at least one lifetime spawning per recruit) are contrasted for the Georges Bank Atlantic cod (Gadus morhua) stock. The time-series of spawning intensity is significantly altered when hatched eggs or viable larvae are used as the metric, reflecting the contribution of first- and second-time spawners in some years, and the increasing trend in F. The effect of reduced average maternal size in relation to egg viability was a more important factor contributing to discrepancies with SSB than was spawning experience in relation to hatching success. Percent maximum spawning potential (%MSP) per recruit curves in relation to F are steeper (i.e. result in lower values of %MSP for a given F) for hatched eggs and viable larvae than for SSB or egg production per recruit. Lifetime expected numbers of spawnings per recruit are significantly reduced when the effects of spawning experience on egg hatching success are included. And finally, although point estimates of Fmed and Fcrah are similar using SSB and viable larval production, the median Fcrash estimated from 5000 bootstrap realizations of the Beverton-Holt stock recruitment curve for viable larvae is much lower than that from SSB, 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. If age-at-spawning is at least partially heritable, then intense fishing on younger ages may exert high levels of selection for early maturity with negative impacts on net reproductive effort and trait diversity.