The effect of sea urchins as biogenic structures on the local abundance of a temperate reef fish

Although widely distributed on rocky reefs at Santa Catalina Island (southern California, USA), the small, substrate-oriented fish, Lythrypnus dalli, often reaches its highest densities in areas of extreme vertical relief where the sea urchin, Centrostephanus coronatus, is common. Through surveys and manipulative field experiments, we examined the extent to which the availability of C. coronatus influenced the distribution and abundance of L. dalli. Adult L. dalli were strongly associated with C. coronatus, but not with two shorter-spined urchins, Strongylocentrotus franciscanus and S. purpuratus. Variation in fish abundance among C. coronatus was affected by three features: the amount of space between the body of the urchin and its rocky shelter, the location of occupied shelters, and urchin size. Field manipulations of C. coronatus density indicated that spatial patterns of L. dalli abundance are causally linked to the presence of urchins. Urchins also had a significant effect on recruitment, migration, and survival of L. dalli. These processes were greatly enhanced where urchins were present. Structural substitutions (models) produced about half the effect of live urchins indicating that biological, as well as structural, attributes of urchins may be important to fish. Our results suggest that habitat selection at the time of settlement, small-scale migration of fish after settlement, and differential survival relative to the presence of sea urchins contribute to the formation and maintenance of the association between L. dalli and C. coronatus. By altering the availability of refuges, sea urchins appear to have a direct and positive effect on the local abundance of this temperate reef fish.