Population dynamics of Strongylocentrotus -dominated sea urchin barrens on the Palos Verdes Peninsula

Abstract

Sea urchin barrens are the result of shifts from kelp-dominated to sea urchin-dominated states on rocky reefs; these expanses of reef support little to no macroalgal cover and high densities of sea urchins. As kelp forests support diverse and abundant communities of fishes and invertebrates, urchin barren states may reduce local abundances of species targeted by commercial and/or recreational fisheries. The demographic impacts of a lack of primary macroalgal food sources on Strongylocentrotus populations was investigated using metrics of fitness and fecundity from collected red ( S. franciscanus ) and purple ( S. purpuratus ) urchins. From August, 2009 to July, 2011, ten diver surveys were completed to assess the fish, invertebrate, and algae densities at four urchin barren and two kelp forest sites. Red (n=1005) and purple (n=1070) urchins were collected and test diameter recorded. A subsample was collected to determine the relationship between test diameter and weight. Growth curves were constructed and used to estimate biomass for all reefs. Results were analyzed via ANOVA; urchin barrens were found to support significantly higher densities per 100m? and lower gonad-free weight, test diameter, and gonadosomatic indices. Biomass of S. franciscanus was found to be significantly higher within barrens, while no significant difference in biomass of S. purpuratus was found between states. These results imply that Strongylocentrotus populations within urchin barrens are not as fit as similar populations within kelp forests and must make significant sacrifices in terms of fitness and fecundity to sustain such high densities.