The importance of detectability to acoustic surveys of semi-demersal fish

A major source of bias and imprecision in acoustic abundance estimates of s emidemersal fish is detectability, defined here as the proportion of the tr ue abundance of a target species within the ensonified volume (surface to b ottom) that is detected by an echosounder and included in integration. In a utumn 1996 and 1997, acoustic surveys for Atlantic cod (Gadus morphua L.) i n inshore Placentia Bay, Newfoundland, Canada, indicated mean daytime densi ties (e.g. 0.016 fish m(-2) in 1997) an order of magnitude higher than at n ight (0.001 fish m(-2)). A corresponding downward shift in vertical distrib ution at night was also observed. At the same site in 1996, in situ video c ensuses of cod from a submersible showed equivalent cod abundance by day (n 3) and night (n=1). Submersible observations indicated that at night cod w ere located nearer to bottom, and preferred rocky and boulder-strewn substr ates and not open sandy bottoms (p<0.001). Acoustic densities measured from the submersible cruising 20 m above bottom, and from the surface vessel, w ere similar. Submersible acoustic estimates and video census indices were p ositively associated during daytime. The sole night-time acoustic estimate was near zero while the corresponding video index was the highest recorded. We conclude that diel change in acoustic density resulted from variations in detectability caused by cod vertical movements and habitat preferences. A broad-scale springtime inshore and offshore survey of cod in the same sto ck area confirmed the trend of higher acoustic density estimates during the day than at night. We advocate the inclusion of a time-dependent detectabi lity coefficient in the scaling of acoustic backscatter to abundance for se mi-demersal fish. (C) 1999 International Council for the Exploration of the Sea.