Predators of the destructive sea urchin grazer Strongylocentrotus droebachiensis on the Norwegian coast

Summary

: In central Norway, populations of the green sea urchin Strongylocentrotus droe- bachiensis are collapsing, but the factors controlling its population density have not yet been elucidated. Through field sampling, we identified several sea urchin predators and investigated their predation rates on recently settled S. droebachiensis in laboratory experiments. Tethering experiments in kelp forest and on barren ground study sites in the area where sea urchin popula- tions are collapsing confirmed predation by some of the predators tested in laboratory experi- ments. The edible crab Cancer pagurus was the most efficient sea urchin predator, and it was more abundant at kelp forest sites than on barren grounds. Stocks of C. pagurus have increased dramatically in central Norway since the 1990s, and predation by C. pagurus may contribute to the decline in sea urchin densities, allowing kelp recovery and conferring resilience of the new kelp forest state.