Restoration Project

University of Oslo - Vega Island

Restoration Objective:

The general goals of this research were to study aspects of community stability of a sea urchin-dominated barren state and the transition of the system towards a kelp-forest-dominated state. Specifically, it aimed to elucidate: (1) primary responses of algae to reduced grazing pressure, and successional processes involved in the development towards a L. hyperborea kelp forest, and (2) the organism and population responses of sea urchins to improved availability of macroalgal food-plants. In addition, variability in the mass mortality gave valuable information about the reduction in sea urchin density needed to initiate kelp recovery.

Site Selection Criteria:

An area was selected 15km shorewards from the outer kelp forest, in a group of small skerries.

Cause Of Decline:

In northern Norway, the sea urchin Strongylocentrotus droebachiensis has caused kelp forest destruction along several hundred kilometres of coastline during the last 15-20 years. S. droebachiensis is widely distributed in the North Atlantic and the Northeast Pacific and is responsible for extensive overgrazing of kelps world-wide. Kelp forest destruction off Nova Scotia was first reported around 1970. The dominant kelp in Norway, Laminaria hyperborea, is slow-growing and long lived (15-20 years) , compared to kelps exposed to grazing by S. droebachiensis in North America which have life spans of 2-4 years.

Key Reasons For Decline:


Scientific Paper

Effects of removing sea urchins (Strongylocentrotus droebachiensis): Stability of the barren state and succession of kelp forest recovery in the east Atlantic

H.P. Leinaas, H. Christie, , Oecologia, Vol. 105.


University of Oslo

Site Observations:

Observation Date

1st Oct 1988 – 1st Jul 1993

Action Summary:

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.

Lessons Learned:

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.

Project Outcomes:

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.

Key Reasons For Decline:


Area of Restoration (In Square Metres)


Indicator Data:


Ending Value:

Starting Value:

Adult Kelp Density

/ m2
/ m2
Cost Currency:USD