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

Abstract

A major source of bias and imprecision in acoustic abundance estimates of semi-demersal fish is detectability, defined here as the proportion of the true abundance of a target species within the ensonified volume (surface to bottom) that is detected by an echosounder and included in integration. In autumn 1996 and 1997, acoustic surveys for Atlantic cod (Gadus morhua L.) in inshore Placentia Bay, Newfoundland, Canada, indicated mean daytime densities (e.g. 0.016 fish m −2 in 1997) an order of magnitude higher than at night (0.001 fish m −2 ). A corresponding downward shift in vertical distribution at night was also observed. At the same site in 1996,in situ video censuses of cod from a submersible showed equivalent cod abundance by day (n=3) and night (n=1). Submersible observations indicated that at night cod were located nearer to bottom, and preferred rocky and boulder-strewn substrates and not open sandy bottoms (p<0.001). Acoustic densities measured from the submersible cruising 20m above bottom, and from the surface vessel, were similar. Submersible acoustic estimates and video census indices were positively 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 stock area confirmed the trend of higher acoustic density estimates during the day than at night. We advocate the inclusion of a time-dependent detectability coefficient in the scaling of acoustic backscatter to abundance for semi-demersal fish.