SCALES OF VARIABILITY IN LARVAL SETTLEMENT WITHIN THE CHANNEL ISLANDS NATIONAL MARINE SANCTUARY AND ALONG THE MAINLAND COAST

We explored variability in the rates of sea urchins (Strongylocentrotus franciscanus, S. purpuratus), and rock crabs (Cancer spp.) across different spatial (hundreds of meters to 90 km) and temporal (biweekly to decadal) scales in nearshore waters off Santa Cruz, Santa Rosa, and Anacapa islands and along the Santa Barbara mainland coast. Here, we refer to settlement as a best estimate of the abundance of small stages of invertebrates recently settled from the plankton. Settlement was measured using two types of collectors: wood-handled scrub brushes and tuffy scrub pads attached 1 to 2 m off the bottom either on buoyed mooring lines at stations inside and outside of two marine protected areas (MPAs) at the islands or suspended from piers, one of which is located inside an MPA at Anacapa Island. Benthic recruitment of urchins was also measured at the Anacapa Island station. Larval varied dramatically on a scale of hundreds of meters within and across MPA boundaries. For both red and purple urchins, average at the Santa Cruz and Santa Rosa island sites in 2006 was an order of magnitude higher than average at mainland sites in 2006 and also higher than the historical mainland average (based on a 16-year time series). Comparisons of relative to benthic recruitment at the Anacapa Island station were suggestive of a link for red urchins, but no relationship was evident for purple urchins. Although more data are needed, our results suggest that local and regional scale variability in larval could have important implications for population dynamics, fishery management, MPAs and sampling design. For example, areas with lower rates may take longer to recover depleted populations or be less suitable for restoration efforts based on natural replenishment than areas with greater settlement. Settlement data could provide a fishery-independent measure of stock health and a means for evaluating the role of larval supply in regulating adult populations of economically and ecologically valuable benthic invertebrates.