Research Document 1998/68

An assessment of biases associated with inshore acoustic cod surveys

By G. Rose

Abstract

We assess biases in survey methods for demersal fishes in coastal environments by comparing visual observations from submersible transects to surface-made acoustic density estimates of Atlantic cod, in Placentia Bay, Newfoundland, during fall 1996. Daytime acoustic density estimates were significantly greater than those made at night. A strong nighttime preference for close proximity to the bottom and rocky substrates likely reduced the probability of distinguishing fish from the bottom acoustically, thereby lowering acoustic densities at night. Repeated passes of acoustic transects suggested no active boat avoidance by the cod. Acoustic density estimates were correlated with the assumed 'ground truth' density measurements made by visual transects (Spearman's r=1.0, p<0.01, n=3). Submersible transects showed a higher density of redfish (Sebastes spp.) than estimated from acoustic surveys, but there was no evidence that cod density estimates were biased by the presence of redfish. In general, our results suggest that acoustics can provide a useful and accurate means of estimating inshore cod density provided that diel movements are considered.

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