Lack of sea urchin settlement may explain kelp forest recovery in overgrazed areas in Norway

Abstract

In this study, we investigated whether failure of sea urchin recruitment can explain an ongoing phase shift from overgrazed, barren grounds to kelp forests along the coast of mid- Norway. Settlement of sea urchins on artificial substrates deployed in mid-Norway and North Norway was compared on barren grounds and in kelp forests. Settlement varied in time and space, and was low in mid-Norway—the area where kelp is recovering. Settlement did not differ between kelp forest and barren grounds. Densities of adult and juvenile sea urchins were estimated and gonad index (GI) was measured in order to relate sea urchin settlement to local population structure and the reproductive potential of the sea urchin populations. Sea urchin density was generally lower in mid-Norway than in North Norway, but reproductive potential did not differ significantly between the areas. Increasing water temperature is considered a critical factor for sea urchin larval development and may be partly to blame for the recent sea urchin recruitment failure in mid-Norway. Lack of sea urchin recruitment may explain the recovery of kelp in mid-Norway.