Development and application of electrofishing with towed video as a new survey method for razor clams (Ensis spp.)

摘要

Electrofishing for razor clams (Ensis spp.) has gained increasing attention as a commercial harvesting technique due to its apparent efficiency and production of high quality product. In this paper, we report results from combining electrofishing with towed video as a novel method for assessing the density and size distribution of Ensis in shallow subtidal environments. Emergent razor clams could be readily identified and measured from the video recordings. Comparisons of video-based length frequency distributions with direct measurements of clams collected by divers confirmed that reconstructed shell lengths were sufficiently accurate for stock evaluation purposes. Forty tows were subsequently completed at four sites along the Ayr coast (Firth of Clyde, Scotland). Only E. siliqua were found at these locations with average densities between 0.3 and 0.8 m(-2). The majority of individuals were less than 200 mm in length. Comparing these results with hydraulic dredge surveys conducted in the early 2000s at one of these sites suggests possible declines in overall density and the abundance of larger animals. However, this conclusion should be interpreted cautiously because although electrofishing is thought to be highly efficient, the effects of varying towing speeds, site and environmental conditions on rates of Ensis emergence in the field are not yet fully understood.