Current low densities of the northern abalone Haliotis kamtschatkana may be affecting the fertilization success of this endangered broadcast spawner, thus preventing its populations from recovering to historical levels. This study attempted to determine whether the northern abalone were significantly aggregated during the period just before spawning because this may compensate in part for their low densities. We used scuba to map the spatial distribution, sex, and spawning condition of tagged abalone within grids at three different sites close to Bamfield Marine Sciences Center on the west coast of Canada. Underwater tagging methods were then used to monitor individuals over a 3-wk period during the 2009 spawning season. We found that the populations at all sites had nearest neighbor R ratios significantly less than 1.0, which indicates an aggregated distribution. Within the range of densities observed for our 3 sites (0.12-0.64 adults/m2), the mean distances to the nearest neighbor (1) of either sex, (2) of the opposite sex, and (3) of the opposite sex with ripe gonads were always less than 1.00 m except in one case. Individual abalone aggregated independently of sex; therefore, the probability of finding both a ripe male and a ripe female within an aggregation increased linearly with density. We estimated that the northern abalone populations observed were sufficiently aggregated to make successful fertilization more likely at low densities. This is the first study to map abalone sex and degree of gonad development that allow the analysis of nearest neighbor measurements with respect to gender and spawning condition.
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Vol. 30 • No. 3