A Method of Surveying Scallop Populations From a Submersible

Abstract

A system is described for continuously recording density changes across a population of benthic organisms from a submersible. This system was tested in the Northumberland Strait over a population of scallops (Placopecten magellanicus). A continuous record of scallops visible from the viewport of the submersible was registered on a chart recorder, concurrently with a series of exposures by an externally mounted automatic camera. Transects up to 4 km long were completed in a single dive.A distance trace provided by a towed odometer wheel allowed visual counts to be expressed as linear densities (numbers seen per unit distance travelled) independently of submersible speed. An estimate of the width of visual field at a known height off bottom enabled linear densities to be converted into population densities (numbers seen per unit area of sea floor).Scallop distribution in the area was markedly contagious. Areas of high scallop density (up to 4.8 scallops/m²), of the order of 0.5 km across, on sand, were separated by stretches of mud where scallop density was below 0.1/m².