A MATRIX MODEL FOR SHORT-TERM DYNAMICS OF SEEDED POPULATIONS OF SEA SCALLOPS

Abstract

Recently, there has been increasing interest in releasing (seeding) bivalves onto the seabed for purposes of aquaculture or population enhancement. The success of such enterprises has varied greatly and is related to the interactions between mortality, dispersal, and growth of the organisms. In this paper, we have constructed a stage-based matrix model for short-term population dynamics of seeded sea scallops (Placopecten magellanicus). Our goals were to predict scallop survival to commercial size and to de- termine the relative contributions of predation, dispersal, and growth to loss of scallops. Competing risk theory was used to account for predation by crabs and by sea stars, and for correlations between dispersal and predation (both of which depend on encounters with predators). Density dependence (in the form of predator functional responses), seasonal variation, and a simple spatial structure were also incorporated into the model. The model was parameterized from the results of small-scale experiments and tested against indepen- dently observed population trajectories. Uncertainty analysis was used to determine the effect of parameter sampling error on model output. Sensitivity analysis indicated that variables affecting predation by crabs were important and that variables affecting inter- mediate-sized scallops (e.g., large juveniles) were more important than those affecting other size classes. Using perturbation analysis, we ranked alternative management scenarios for increasing final scallop survival from the most effective to the least effective as follows: reducing predator densities, increasing size of seeded scallops, changing the initial density of seeded scallops, increasing the dimensions of the site, and changing the season of seeding. Inclusion of seasonal variability in predator densities (thereby converting the deterministic model into a stochastic model) did not greatly change final scallop survival.