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

Parametric relationships of recruitment to an index of parental stock size assume the latter is proportionally related to spawning potential. irrespec tive of the demographic composition of adults. Recent empirical information . however, suggests that reproduction by older and experienced females is m ore successful than by young and inexperienced females. New models are deve loped 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 demograp hy of spawners (based on experimental results). A series of spawning metric s [spawning-stock biomass (SSB), egg production, hatched egg production. vi able larval production] and associated recruitment-based fishing mortality reference points (F-med, F-crash, and the F that allows at least one lifeti me 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. reflect ing 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 r elation to egg viability was a more important factor contributing to discre pancies with SSB than was spawning experience in relation to hatching succe ss. Percent maximum spawning potential (%MSP) per recruit curves in relatio n 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 reduce d when the effects of spawning experience on egg hatching success are inclu ded, And finally. although point estimates of F-med and F-crash are similar using SSB and viable larval production, the median F-crash estimated from 5000 bootstrap realizations of the Beverton-Holt stock recruitment Curve fo r viable larvae is much lower than that from SSB. with narrower confidence bounds. Our results suggest that traditional approaches to F-based referenc e points using SSB systematically overestimate the resiliency of stocks to fishing. If age-at-spawning is at least partially heritable, then intense f ishing on younger ages may exert high levels of selection for early maturit y with negative impacts on net reproductive effort and trait diversity.