California Sea Grant College Program

It is well documented that sea urchins can have vast impacts on kelp forest community structure as a result of kelp grazing. Despite the ecological importance of sea urchins, direct field studies on the relative effects urchin predators have on shaping urchin populations are rare for southern California. I conducted surveys at three kelp forest sites near San Diego, CA, including heavily fished and marine reserve sites, to measure sea urchin size, abundance, and habitat use as well as the abundance of potential sea urchin predators. I also examined whether purple sea urchin (Strongylocentrotus purpuratus) proportional mortality varied with urchin density, time of day, and algal cover in the La Jolla Ecological Reserve, where densities of potential predators such as sheephead and spiny lobsters are high. Transect surveys showed urchin behavioral changes among the three sites with urchins tending to be more cryptic inside the reserve as compared to sites adjacent to the reserve and in heavily fished areas. Fished species, including spiny lobsters and red urchins were found to be less abundant in non-reserve sites, while unfished purple urchins were more abundant in non-reserve sites possibly due to reduced predation pressure in these areas. Predator densities and mean sizes of predators were highest inside the reserve and lowest in the heavily fished site. In the experiment, urchin proportional mortality decreased with increasing urchin density. This trend was stronger in open plots than in plots covered with algae, in which proportional mortality was more variable. Substantial nighttime predation was observed and was not influenced by the presence of algal cover. Examining whether urchin mortality from predation is density-dependent and how habitat complexity influences this relationship is imperative because behavioral changes and increases in urchin populations can have vast ecological and economic consequences in kelp forest communities.