Sea urchin removal to promote kelp recovery

What

Removing green sea urchins (Strongylocentrotus droebachiensis) from areas dominated by urchins has been demonstrated to initiate kelp forest recovery globally. In Norway, sugar kelp (Saccharina latissima) has recovered in wave sheltered areas, and tangle kelp (Laminaria hyperborea) has recovered in more wave exposed areas.  

 

Coastal graphic: Before

Coastal ecosystem before NbS have been implemented

Coastal graphic: After

Coastal ecosystem after NbS have been implemented

 

Where

This action constitutes a NbS in coastal ecosystems as a part of ecological restoration and closer-to-nature management. This NbS can also be applied in coastal constructed landscapes dominated by urchins, such as e.g. breakwater structures or other artificial seawalls. Within the Nordic countries, this NbS is relevant for Norway, Iceland and Greenland.  

Sea urchin barrens are marine deserts, which are devoid of other life. The blooming of sea urchins that graze kelps and transform the lush kelp forests into sea urchin barrens, are linked to overfishing and a human induced imbalance in the food webs through removal of important sea urchin predators such as wolffish and cod. This occurred at large spatial scale (9 800 km2) along the Norwegian coast, from mid-Norway to the Russian border, in the early 1970s. 

Why

In urban or constructed coastal ecosystems, promoting recovery or building new kelp forests will benefit local biodiversity and help to address societal challenges in the following ways: 

  • Recovered kelp forests serve as habitat, shelter, and nutrient source for multiple species and organisms, including commercial species, and provide a wide range of ecosystem services. Sea urchin removal that successfully promotes kelp recovery, also implies recovery of kelp forest ecosystem services. These include biodiversity enhancement through the kelp forests role as habitat (SDG 14). 

  • Sea urchin removal that initiates kelp recovery can be also used as a NbS for climate change mitigation and adaptation (SDG 13). This is because kelp stores carbon in the standing stock and sequesters carbon by exporting carbon to deep water. The kelps have high annual primary production rates. These equal the size of the standing stock, and about 10 kg kelp per m2 is lost and exported annually to other ecosystems. About 10% of the primary production (i.e. 10% of 1 kg carbon for L. hyperborea and S. latissima) is assumed sequestered.  

  • Kelp provides climate refuges for other species and can dampen the adverse impact of heat in heat waves. The kelps are known to dampen waves and hence can contribute to coastal protection and reduce disaster risk.  

  • Kelps’ high production rates provide food for commercial species. Hence, recovered kelp also contributes to food security (SDG 2).  

  • The uptake of nutrients by kelp contributes to cleaning coastal water (SDG 6) and can be used in water management.  

  • Recovery of lost kelp forests is important for job creation (fishery and tourism), recreation opportunities (fishing, snorkling, diving), and supporting coastal communities. Kelp itself is also used as raw material for a variety of uses. Hence, recovered kelp contributes to economic development (SDG 8). 

  • Restoration actions can be used in education and promotion of a highly needed ocean literacy, and contribute to human health and wellbeing (SDG3) by providing recreation opportunities as well as aesthetic experiences.

How

The objective of this NbS is to sufficiently reduce the abundance of sea urchins to allow kelp to recover from heavy urchin grazing pressure. In areas with high sea urchin recruitment, it is particularly important to repeat removing sea urchins to prevent new generations of urchins from blooming.  

Sea urchins can be removed in different ways, including destructive killing and harvesting. Several harvesting methods can be used, including handpicking by divers, traps (baited with seaweed), vacuum pumps and ROVs. There is an increasing interest in developing efficient sea urchin harvesting methods and utilizing the urchins as a resource for alternative products.  

When harvested, the urchins can be used as high-quality food because of the roe, or as raw material for soil conditioner products (high calcium, nitrogen, and micronutrient values), biomedicine and health food. Sea urchins are rich in vitamins, minerals, proteins, fatty acids, and polysaccharides, and possess anticancer, anticoagulant/antithrombotic, antimicrobial and antioxidant properties.  If the urchins are in good condition, they can be sent to the consumers directly. If harvested from urchin barrens, they are starving and need to be fed nutritional feed for the roe to be ready for market.  

Destructive removal of urchins can be done at small spatial scale using scuba and free divers to kill the urchins individually with a hammer (e.g. as demonstrated by Tarevoktere i Tromsø), or by spraying quicklime over a larger area, as done by NIVA and IMR in a large-scale, successful restoration project in Porsangerfjorden in 2013. Here, kelp recovery was initiated within 0.7 km2 area, with one week of treatment.  

Alternatively, or in combination with removing the urchins, introducing, or enhancing the presence of efficient sea urchin predators, such as wolffish or crabs, might be needed to control the urchin abundance. Providing suitable caves for the wolffish time-consuming care of eggs, combined with establishing no-take areas, have the potential to enhance the abundance of this important predator.  

To ensure long-term success it is important to maintain the sea urchin abundance at a low level. This implies either to introduce or enhance the presence of sea urchin predators such as crabs (edible crab and/or red king crab) or wolffish, or to repeat harvesting/killing actions of the sea urchins. As red king crab is an alien species for Norway, we do not recommend to actively introduce the species into an area it has not become established.  

Outcomes

The direct ecological effects of a recovered kelp forest include: 

  • Increased primary and secondary production from the kelp, the epiphytes/macroalgae living on the kelp plants, and of the associated fauna living on all parts of the kelp (holdfast, stipe and lamina), 

  • Increased biodiversity related to increased number of species and abundance of flora and fauna closely associated to the kelp plants, as well as to the larger niches created by the three-dimensional kelp forest,  

  • Support of more complex food webs compared to the scavenger dominated ecosystem in the barrens, with more trophic levels, 

  • Recovery of the lost ecological functions that the kelp forest provides, including their role in exporting kelp material to other ecosystems.  

Restoring an urchin barren of the size of a soccer field (i.e. about 7000 m2) provide a gain of about 70 000 kelp plants, 70 t kelp biomass and a similar amount of yearly kelp biomass production, storing of 22 t CO2, and approximately 700 million small invertebrates.  

ATTENTION

To promote kelp recovery, the harvesting of sea urchins need to be done systematically and, in a science-based manner. This includes the following considerations: 

Substrate type: 

  • To promote kelp recovery, it is the urchins on rocky substrate that need to be removed, as kelp forests need a stable, rocky substrate to thrive.
  • Smooth bedrock makes it easier to remove the urchins, both by divers, quicklime and ROVs
  • Stony bottoms where sea urchins can hide is a greater challenge for efficient removal.
  • Nearby nursery habitats for sea urchins, such as maerl beds and stony substrate, need special attention as possible sources for re-colonisation of the restored kelp forest by new urchin generations.
  • Soft sediment beneath a rocky shoreline, can function as a barrier for the surrounding sea urchins. 

Depth and terrain:

  • The main depth interval for the occurrence of kelp forests is 0 to about 20 m below LAT (lowest astronomical tidal level), hence the focus for sea urchin removal should not be deeper than 20 m. The green sea urchin can live down to more than 100 m depth. 
  • Sea urchin removal should be started at the shallowest area, where sea urchin density is high, and kelp colonisation and growth are rapid.
  • Flat or gently sloping areas will receive more light for photosynthesis per area cleaned for sea urchins, than steep slopes, hence such areas should be prioritised for restoration. 

Distance to nearest kelp forest and size of the restoration area: 

  • Kelp is known to recover even at long distances from the nearest mother population (as demonstrated by the case studies at Vega and in Porsangerfjord). However, strategic sampling of urchins at barrens close to existing kelp forests, would probably facilitate a more resilient recovered forest because of its continuity with the existing forest, and presence of natural urchin predators. This may make it easier to restore a sufficiently large area.
  • If sufficiently large area, the new kelp forest has the potential to house a sufficient diversity and numbers of urchin predators, to be able to maintain itself on long-term. 

Predator enhancement: 

  • Predators can control the sea urchin populations and allow kelp recovery. The abundance of native predators can be enhanced through fishing bans, and predators lost/reduced could be reintroduced or enhanced through breeding. Efficient urchin predators include wolffish and crabs. 
  • Crabs and wolffish need shelter, in particular wolffish when they protect their eggs, hence providing houses for these predators might be needed to build up the predator’s abundance to the level needed to control the urchins. Recovered kelp will provide additional shelter for these predators. 

Maintenance and establishment of no-take areas: 

  • Sea urchin abundance need to be monitored, and removal actions repeated when needed, probably every one or two months.
  • Protection of urchin predators in form of no-take areas, are needed to build their abundance and secure their role in controlling the urchins. 
  • The ecological impact of predator measures needs to be tested, monitored and modified if needed. 

Potential ecological, economic and social restraints: 

  •  There is a conflict of interest between the commercial sea urchin fishermen’s need for sea urchins as a resource, and the low urchin abundance needed to promote kelp recovery. Areas with low urchin density will give low returns for the fishermen and hence be of low interest.
  • Competition between turf algae and kelp may arise in the southern part of the potential restoration area in Norway, slowing down kelp recovery. Barrens formed by long-spined sea urchin (Gracilechinus acutus) in Western Norway, may transform into a turf algae dominated community instead of kelp forest.
  • Several hazards are relevant for the divers and snorklers harvesting or removing the urchins. Awareness of the risks, training and adherence to safety protocols, are needed to reduce the likelihood of incidents.
  • Today, there are no laws regulating sea urchin harvesting in Norway, but regulation exist in Iceland. However, this might change if the activity increases. Deploying artificial reefs/houses for wolffish need permits.
  • Lack of strategic management integrating the proposed approaches will reduce the chances of successful large-scale restoration of grazed kelp forests. 

How much do we know?

The recovery of kelp when sea urchin density is sufficiently reduced in urchin barrens is very well documented in Norway, as described in the case studies below. In all case studies, the kelp did not need to be actively introduced to the restoration sites. The kelps were able to recolonize the sites by dispersal of spores from even long-distance mother kelp plants. If this is the case in urchin barrens in Iceland and Greenland, needs to be tested. 

Costs  

The cost will depend on the harvesting or removal strategy. 

  • If an urchin ranching industry exists, the industry will pay the harvesting costs. Verbeek et al. (2021) estimates that a fishermen can harvest 1 ton of urchins a day using 150 traps, with about 20% of the urchins being larger than 4 cm in test size. Assuming a mean weight of the large and small urchins equal to 60 and 30 g, this amounts to 30 000 individuals caught per day. Furthermore, assuming an urchin density of 30 per m2, this implies that 1000 m2 can be harvested per day.  

  • Miller & Shears (2023) found that culling sea urchins was 2–4 times faster than collecting and diving 1.5 to 3 times faster than freediving. Effectiveness was lower in large-scale compared to small-scale removals, but sufficient to promote kelp recovery. Estimated time per area using culling was about 50–60 hours per ha.  

Location: Tromsøya, Troms county, Norway

Which ecosystem type: Kelp forest 

Title/ name of the NbS: Tarevoktere (guardians of the kelp) 

Summary: A voluntary initiative to preserve and restore kelp forests in Norway, mainly focused on the removal of invasive sea-urchins. A small-scale ongoing experiment with hammering the urchins, as well as harvesting urchins with traps and/or by divers picking the urchins manually.

Relevant links: https://www.tarevoktere.org/ 

Location: Troms County, Norway 

Which ecosystem type(s): Kelp forest 

Title/name of the NbS: Restoration of kelp forests 

Summary: Using sea urchin removal as a tool to connect travellers with nature conservation, and science with the rest of society, facilitating regenerative travelling.  

Relevant links: Wild Lab Projects 

Carlsson PM, Christie HC (2019) Regrowth of kelp after removal of sea urchins (Strongylocentrotus droebachiensis). NIVA report: 7431-2019.  

Christie H, Gundersen H, Rinde E, Norderhaug KM, Fagerli CW, Bekkby T, Gitmark JK, Jorgensen N, Pedersen T (2019) Can multitrophic interactions and ocean warming influence large-scale kelp recovery? Ecology and Evolution.  

Blicher M (2010) Structure and dynamics of marine macrozoobenthos in Greenland and the link to environmental drivers. PhD thesis Greenland Climate Research Centre & Department of Biology, University of Copenhagen. Greenland Institute of Natural Resources, 126 p.  

Eger A, Marzinelli EM, Baes R, Blain C, Blamey L, Carnell P, Choi CG, Hessing-Lewis M, Kim KY, Lorda J, Moore P, Nakamura Y, Pontier O, Smale D, Steinberg P, Verges A (2021) The Economic Value of Fisheries, Blue Carbon, and Nutrient Cycling in Global Marine Forest. EcoEvoRxiv.  

Hjorleifsson E, Kassa O, Gunnarsson K (1995) Grazing of kelp by green sea urchins in Eyjdafjordu, North Iceland. In: Skjoldal HR, Hopkins C, Erikstad KK, Leinass HP (eds) Ecology of fjords and coastal waters. Elsevier, Amsterdam, 593−597.  

Hynes S, Chen W, Vondolia K, Armstrong C, O'Connor E (2021) Valuing the ecosystem service benefits from kelp forest restoration: A choice experiment from Norway. Ecological Economics 179:106833.  

Leinaas HP, Christie H (1996) Effects of removing sea urchins (Strongylocentrotus droebachiensis): stability of the barren state and succession of kelp forest recovery in the east Atlantic. Oecologia 105:524-536. 

Matheson K, Gagnon P (2021) Growth and feeding resilience of green sea urchin (Strongylocentrotus droebachiensis) to visible-light quantity and quality. Marine Biology 168:179. 

Miller KI, Shears NT (2023) The efficiency and effectiveness of different sea urchin removal methods for kelp forest restoration. Restoration Ecology 31:e13754. 

Sibiya A, Jeyavani J, Sivakamavalli J, Ravi C, Divya M, Vaseeharan B (2021) Bioactive compounds from various types of sea urchin and their therapeutic effects — A review. Regional Studies in Marine Science 44:101760  

Strand HK, Christie H, Fagerli CW, Mengede M, Moy F (2020) Optimizing the use of quicklime (CaO) for sea urchin management — a lab and field study. Ecological Engineering: X:100018.  

Verbeek J, Louro I, Christie H, Carlsson PM, Matsson S, Renaud PE (2021) Restoring Norway's underwater forests. A strategy to recover kelp ecosystems from urchin barrens. SeaForester, NIVA & Akvaplan-niva, Report, 2021.