Is long-term persistence of harvested populations evidence of density dependence?

Most harvested marine animal populations persist over many generations after the arrival of fishing, attaining a new age-structure where the life span and lifetime egg production (egg per recruit) of an average female are truncated. In this paper, the average survival of Georges Bank sea scallops (Placopecten magellanicus) from egg to age of first reproduction is estimated For the population under both natural and harvested regimes. The hypothesis of density dependence is validated if a higher average survival rate of spawn is observed under exploitation. Average survival rate of eggs to age of recruitment is estimated by two methods: 1) For the present exploited population, it is estimated empirically by survey as the ratio of the average annual numbers of age-3 recruits divided by average eggs released per year. 2) For both unexploited and exploited cases, the average survival rate from egg to age of first reproduction is inferred under the assumption of stationarity This method relies on a relation, derived as an extension of Lotka's theory of demographics to stationary populations, that the average survival rate from egg to age of First reproduction equals the reciprocal of average lifetime egg production. Stationary age distributions are derived using the coefficients of natural and natural plus fishing mortality, respectively Average female lifetime egg production is obtained by multiplying population numbers-at-age by fecundities-at-age and summing over a full lifespan. Early life history survival is estimated as the reciprocal of lifetime egg production. Employing Methods (1) and (2), the estimates of average survival rate from egg to age 3 are 32 and 63 times greater under harvesting.