A COMPARISON OF 2 UNDERWATER CENSUS METHODS FOR ESTIMATING THE ABUNDANCE OF THE COMMERCIALLY IMPORTANT BLACKLIP ABALONE, HALIOTIS-RUBRA

The blacklip abalone, Haliotis rubra, fishery is the most commercially important in Victoria, Australia. Two underwater census methods have been used to survey the status of its stocks. Collection rates from ti med searches and counts from radial transects were used to estimate ab alone abundance at various locations along the Victoria coast. Radial transect surveys provided precise estimates of abalone abundance; thes e did not vary significantly between research divers under controlled conditions. During stock monitoring, there were no significant differe nces among nine of twelve research divers' estimates of abalone abunda nce from 1339 radial transacts sampled at 60 sites during 1992-94. Mon te Carlo simulations, used to estimate the probability of detecting an nual changes in abalone abundance with each method, provided evidence that radial transects have the potential to detect smaller annual chan ges in abalone abundance than do timed searches. We suggest that radia l transects provide a credible alternative to timed searches and are l ess subject to potential biases that affect the latter.