Sea urchin densities were reduced on four occasions between October 1988 and June 1989, and the long-term effects on algae and sea urchins at this reduced density was measured until October 1993. Sea urchins were removed repeatedly from 1988 - 1989 to ensure that their density remained low during one recruitment period for the main algal species in the area. Sea urchin densities and algal cover were estimated using SCUBA at 2, 5 and 10 m depth along a 10-15m wide vertical transect on the south sides of both the experimental and control skerries.
Severe reductions in urchin density can facilitate kelp forest reestablishment. Additionally, while different kelp species may interact competitively, they can also facilitate the growth of other kelp species by preventing urchin overgrazing.
Reduced sea urchin densities initiated luxuriant kelp growth, while more moderate reductions allowed establishment of opportunistic algae, but no kelps. At first the substrate was colonized by filamentous algae, but within few weeks they were outcompeted by the fast growing kelp Laminaria saccharina. After 3-4 years of the removal experiment, the slower-growing, long-lived kelp L. hyperborea became increasingly dominant. Increased food availability after reductions in sea urchin density led to increased individual growth of the remaining sea urchins, however, the population density did not increase. Possibly, early establishment of a dense kelp stand may prevent sea urchins from re-establishing barrens.
Nature of Disturbance:
Researchers had also planned a supplementary caging experiment with replicates on several skerries, but this was spoiled by a mass mortality of S. droebachiensis in summer 1991. The mass mortality of urchins in 1991 also resulted in an algal bloom in the control site.