Description
Predation is an important process that has both direct and indirect effects on species and communities. Many prey species utilize refuge habitat and exhibit gregarious behavior to reduce the risk of predation, and these strategies may be particularly advantageous to early life stages that are highly susceptible to predators. In this study, I worked with the California spiny lobster (Panulirus interruptus Randall) to explore patterns in habitat utilization and sheltering behavior on southern California rocky reefs, and to determine how these associations and behaviors influence survival of subadult (5.0 - 6.0 cm carapace length, CL) lobsters. The California spiny lobster is a gregarious, shelter-dwelling species whose distribution, abundance, and survival may be closely tied to habitat structure at a variety of scales, particularly for subadult lobsters that are vulnerable to a variety of predators. It is also an economically and ecologically valuable species that is among those targeted for protection in California's recently established South Coast Region marine protected areas (MPAs). Many ecological processes are tied to species-habitat interactions, and MPAs may have little impact on populations if appropriate and complex habitat is lacking within an MPA. Therefore, an important component of my study was to provide baseline information on spiny lobster behavior and habitat associations that may be used in future efforts to determine whether MPAs are positively affecting P. interruptus populations, and if behavioral shifts occur due to changing conditions within MPAs. In part one of my study, I assessed habitat characteristics and lobster behavior (habitat associations and shelter use) within and outside of four of the South Coast Region MPAs within southern California's heavily fished coastal waters. To link lobster presence and behavior to benthic habitat features at the regional level, and to determine if these linkages vary among locations, I conducted SCUBA-based transect surveys of rocky reefs within and outside of each MPA. I recorded substrate composition, relief, and dominant vegetation cover to characterize habitat, as well as the number, size, and shelter type utilized by all lobsters encountered. I found that substrate composition and algal assemblage varied across the region and that each MPA had distinct habitat characteristics. As a result, the odds of finding lobsters were related to different habitat variables within each MPA, and no single benthic feature reliably predicted the occurrence of lobsters across the entire study region. Lobster density varied only slightly across MPAs, and lobsters were generally gregarious across the study region. However, lobster sheltering behavior varied among MPAs, which may be due to variation in habitat parameters and biotic factors (e.g., predation risk and intraspecific interactions) across the southern California region. In part two of my study, I determined the impact of lobster behavior on survival for subadult California spiny lobsters. Working in the La Jolla State Marine Reserve (SMR), I conducted tethering experiments using standardized artificial shelters to examine (1) how shelter size influences relative survival for solitary subadult lobsters, and (2) how the size of conspecifics affects survival of grouped subadult lobsters. I also conducted a laboratory-based shelter selection experiment to examine how the presence of potential predators impacts subadult shelter choice, and used data from my transect surveys to determine whether behaviors in naturally occurring shelters corresponded to optimal antipredator strategies and to lab-based shelter preferences. Proportional survival was highest for subadults tethered to shelters with reduced aperture heights, indicating that shelters more closely scaled to body size offer more protection from predators. This corresponded to a weak but significant relationship between lobster body size and shelter height on naturally occurring rocky reefs. Being grouped with adult vs. subadult conspecifics also promoted lobster survival, which corresponded to results from my laboratory experiments, in which subadults selected shelters with adult conspecifics in the presence of predators, and trends on naturally occurring rocky reefs where subadult lobsters more often shelter with adult vs. subadult conspecifics. Overall, my results indicate that antipredator strategies of California spiny lobsters are similar to those of other spiny lobster species, but that lobsters associate with different habitat types across the locations I surveyed. My study provides a more comprehensive examination of California spiny lobster behavior than has previously been available, and provides baseline information on behaviors and habitat associations that rarely are available when evaluating MPA effectiveness.
