Dispersal of tagged blacklip abalone, Haliotis rubra: Implications for stock assessment

Dispersal will cause tag disappearance during abalone tagging studies when the area in which abalone were tagged is the same as the area subsequently searched. The degree of dispersal will depend upon the magnitude of the movements of tagged individuals. Despite the potential for dispersal to bias estimates of natural mortality it is mostly ignored. We describe a method we developed to estimate dispersal from movements of tagged blacktip abalone, Haliotis rubra, from fished and unfished experimental plots. Although the movements of tagged abalone were small, dispersal contributed 40-60% of total tag disappearance. Indeed, dispersal had at least as much effect on over-all estimates of tag disappearance as did the combination of tag loss and observer error. Substantial contributions from dispersal may explain why estimates of natural mortality from tagging studies often seem larger than anticipated. The magnitude of movements varied with habitat quality and was affected by fishing. Preferred habitat occurred on medium-to-high relief reef that accounted for only 37% of the study site but contained 60% of the total population. Prefishing dispersal rates were smaller in areas with more preferred habitat, and the removal of 40% of the population during experimental fishing caused a decrease in the proportion of abalone dispersing from the fished plots. This study demonstrates that substantial dispersal will occur despite only small movements resulting from heavy fishing pressure within complex reef structure.