An estimator of episodic mortality in bivalves with an application to sea scallops (Placopecten magellanicus)

In marine invertebrate populations natural mortality events tend to be episodic in nature. The impact of these episodic mortality events can be estimated from the decline in catch rate of live animals over time obtained through successive population sampling. However, the high variability of catch rates makes them relatively insensitive indicators of mortality. For bivalve mollusks another means to estimate mortality is to assess the change in abundance of recently dead animals, called clappers, which are gaping shells still hinged together. We present a change in ratio estimator and its associated variance of the proportion of animals dying due to an episodic mortality event. This estimator is more sensitive to mortality events and may be preferable to a traditional estimator based upon changes in catch rates of live animals. When applied to a scallop population in Narragansett Bay, RI, the change in ratio estimator indicated that a 72% reduction in the population occurred after an intense mortality event associated with an epizootic of a rickettsia-like organism.