KBWOT 2012: the use of an acoustic technique in mapping beds of razor clams (Ensis sp.)

Samenvatting

For the survey of shellfish in the Dutch coastal zone (WOT Ensis), a fixed stratified sampling grid is used. Stratification is based on expectation of occurrence, for which previously observations by Spisula fishermen were used. Spisula subtruncata has largely disappeared and was replaced by Ensis sp. However, the stratified sampling grid is still mainly based on expected occurrence of Spisula. The quality of the data would be improved with an entirely independent basis for the stratification. An improved accuracy of stratified sampling grids will increase the efficiency of the WOT surveys and will also increase the confidence level of stock assessments. This will benefit management of shellfish stocks and fishery and will also enhance the reliability of environmental impact assessment studies. Acoustic techniques are increasingly applied for seafloor mapping and optimum allocation techniques for stock assessments. In the framework of the Belgian Science Policy project EnSIS multibeam technology was successfully used to find an acoustic signal representative of dense Ensis sp. aggregations in Belgian waters. In this study however, only relatively few sampling stations were present in the areas with the acoustic signal; a plea was held for more ground-truthing enabling applications in regular monitoring of Ensis sp. or other benthic species that form dense aggregations. Within the KBWOT Fisheries programme of 2012 we further studied the possibility to apply multibeam technology in the annual WOT stock assessment of Ensis sp. in cooperation with the Royal Belgian Institute of Natural Sciences – Management Unit North Sea Mathematical Models (RBINS-MUMM), that also performed the EnSIS project in cooperation with other institutes such as IMARES. The aim of this project was fourfold: 1. study the ability to discern different types of seafloor (e.g. mud, sand, gravel, shellfish beds infaunal and epifaunal) using a high frequency multibeam acoustic sounding system; 2. assess the applicability of multibeam for stratified sampling in the coastal zone: will it optimize the sampling strategy and enhance efficiency? 3. determine what is needed to develop this innovative technique within IMARES and to apply it for stock assessments of shellfish and possibly other benthic communities (expertise, software, etc); 4. determine with whom to cooperate in future regarding availability of multibeam equipment and analysis techniques.