Does sea otter predation enhance abalone recruitment via aggregation? Initial assessment of red abalone settlement in the Monterey Bay

Background/Question/Methods

Early recruitment is difficult to measure in marine organisms that reproduce via larval dispersal; nonetheless, understanding this process is crucial for important fisheries or populations close to collapse. For example, abalone species in central and southern California were virtually extirpated due to disease and over-fishing. However, little is known about abalone settlement rates over space and time. Abundances of young-of-the-year abalone in recruitment modules deployed by the California Department of Fish and Wildlife have been sporadic in northern California, exhibiting boom and bust years, despite relatively high abalone densities (0.45 abalone/m²). In 2012-2013, we conducted the first assessments of newly settled abalone recruits in central California, where abalone are less dense (0.2 abalone/m²) but highly aggregated due to sea otter predation. We surveyed coralline covered cobble samples in 2012 and 2013 by sieving newly settled communities into jars and morphologically identifying newly settled red abalone (Haliotis rufescens). We evaluated 29-33 cobbles/year from Hopkins Marine Station and are evaluating 33 samples from each of two neighboring marine sites from 2012, all three sites being within two adjacent state marine reserves: Lovers Point and Pacific Grove Marine Gardens (southern Monterey Bay).

Results/Conclusions

Our assessments of newly settled red abalone resulted in 16 and 13 abalone in 2012 and 2013, respectively. In comparison, using an identical methodology, only 3 newly settled red abalone were found in the larger 2012 northern California assessment (6 sites with 80 cobbles/site). Thus, during a season of poor recruitment in northern California, red abalone settlement was higher in central California. Relatively high settlement over the past two years (~0.5 abalone/cobble) is similar to that of northern California high settlement years, e.g. 2013. This may indicate consistent red abalone settlement in central California despite low densities. We hypothesize that this is due to the aggregation of adult abalone in rock crevices at Hopkins (and possibly elsewhere) as a result of sea otter predation, though this should be tested over longer time frames. By evaluating samples from two additional southern Monterey Bay sites from 2012, we will test the generality of our findings on regional red abalone settlement. If aggregation is playing a role in the success of this low density species in the Monterey Bay, aggregation in crevices may be an important ecological process and potential tool for population recovery and management.