California Sea Grant College Program

The population growth rates of benthic broadcast-spawning species may be limited at low densities due to extreme reductions in fertilization rates. I investigated the influence of aggregation characteristics on the low-density population dynamics by incorporating aggregation-influenced fecundity estimates into a demographic matrix model for both pink (Haliotis corrugata) and red abalone (H. rufescens). I measured aggregation-level characteristics in populations representing a broad range of densities and spatial dispersions in order to characterize the relationships between population density, nearest-neighbor distances, and aggregation sizes.

Average aggregation sizes were strongly correlated with population density density, such that lower-density populations contained smaller aggregations. The relationship between nearest-neighbor distances and population density was nonlinear, such that distances increased rapidly as the population density decreased below a threshold level. The effect of aggregation size on fecundity translates into a rapid decline in population growth below a threshold average aggregation size. The magnitude of the depensation effect may be amplified when additional factors influencing fertilization success (i.e. nearest neighbor distances) are considered.

The results of a four-year study of a pink abalone population near San Diego, California, indicate that the population is partially recovered as defined by the California Department of Fish and Game Abalone Recovery and Management Plan. However, the

average population growth rate estimate was 1.070 yr-1 for high fertilization success conditions, and 0.902 yr-1 for low fertilization success conditions. Based on the average nearest-neighbor distances measured in this population (>5 meters), the expected fertilization success rate is low. This finding suggests that, in the absence of an external larval supply, this population may be declining.

This work provides important baseline information on aggregation characteristics that was previously unavailable for the California populations of pink and red abalone, and wavy turban snails (Megastraea undosa). The matrix models illustrate how the aggregation-level characteristics may be incorporated into a formal population model in order to predict the critical aggregation size for that population. The results may be used to review the recovery of abalone stocks and to analyze the effectiveness of future management and recovery schemes.