Scanning sonar as a supplement to conventional acoustic fish stock assessment in Atlantic Canada

Pelagic fish stock assessment by conventional dual or split beam acoustic echosounding can be supplemented by scanning or multi-beam sonar observations yielding improved spatial and volumetric water column coverage. Inaccuracies in conventional acoustic assessment arising from sparsely distributed fish aggregations, aggregation in the unsampled very near surface, or survey ship avoidance by mobile pelagics can be examined and potentially quantified. Applications are illustrated from field deployment of a SIMRAD MS 900 scanning sonar in offshore Nova Scotia. Volume fraction estimates of widely dispersed herring schools on German Bank show a close relationship to volume backscattering strength as a function of depth. This implies that even fairly simplistic interpretation of high spatial coverage scanning or multi-beam sonar data can provide a useful check on the validity of high precision narrow vertical beam measurements when fish schools are widely separated and infrequently encountered by the latter type instrumentation. A transect through spatially extensive and homogeneous herring shoals in the upper Bay of Fundy is shown to delineate systematic vessel avoidance. Caution however is advised in precise quantitative applications. Off-vertical acoustic measurements are significantly affected by anomalous acoustic propagation especially at shallow depths for horizontal ranges greater than 100-200 m. Also scanning type sonars ensonify fish at shallow aspect angles where acoustic cross sections differ from those appropriate to vertical beam interpretation.