ICES CM 2010/O:12

Offshore wind farms and their potential for shellfish aquaculture and restocking Claus Stenberg1, Mads Christoffersen1, Patrizio Mariani1, Carsten Krog2, Per Dolmer1 Marie Maar3 & Ditte Tørring4Dolmer , Marie Maar & Ditte Tørring

Wind farms in European waters have become relatively common and are quickly expanding. In Denmark 11 offshore wind farms are in operation and an additional 23 new installations are being considered to be operational by 2025. This study aims to investigate whether shellfish production can be combined with the industry in Danish waters. Three of the worlds largest offshore wind farms (Horns Rev 1, Anholt & Nysted) are used as cases.offshore wind farms (Horns Rev 1, Anholt & Nysted) are used as cases.

Wind FarmsWind farms (WF) in Danish watersare among the largest in the world(80‐110 turbines, 25‐80 km² in area,figure 1).WF are placed in shallow waters,relatively close to harbors.To prevent sediment erosion large

Species and production potential Blue mussels (Mytilus edulis): Long‐line concept with pelagicproduction.Individual biomass was estimated using a Dynamic EnergyBudget (DEB) model (Maar et al., 2009). The model estimatedyearly biomass production, and total harvest pr meter of cultureto be 25‐35 % higher at Horns Rev 1 compared to Anholt andNysted (table 2).I d ll O&M f bi l d i iboulders are placed around the

monopole turbines.The distances between the turbinesare app. 500 meters.The increasing number of WF, theirvolume and spatial placement callsfor multiple uses of the large areas.One promising co‐use is mari‐culture.

However, operation & maintenance(O&M) of the turbines have highpriority and involves small boats

In order to allow O&M of turbines mussel production unitscovering 25% of the total WF area. The production potentialwas estimated to be between 15‐25 % higher at Horns Rev 1compared to the other two sites (table 2).

European lobster (Homarus gammarus): Sea ranching conceptwith yearly release of 5‐6 cm juvenile lobsters.

Nysted WF with salinities below 10 ‰ was below the tolerancelimit for lobster.

Catch size was set to 25 cm total size, The period estimated toreach this size 25 cm was app. 5½ years (table 2).

Th j ti t h t bi id d th lpriority and involves small boats,larger barges and ship cranes. (table1). This challenges design, pos‐sibilities and management of mari‐culture in aWF.

The scour projection at each turbine was considered the onlysuitable habitat for lobster in a WF. The total density was set to25 individuals pr turbine.

The production potential at aWF was estimated to be app. 300‐400 kg (table 2).

Figure 1. Map showing running, and proposal for future WF in Denmark  

Table 1. O&M in a typical WF Table 2. Production and harvest potential.

Yearly production Total harvest Total harvest pr WF 25 cm TL  Total harvest pr WFkg mˉ² yearˉ¹ kg mˉ¹ tons years tons

Horns Rev 6,5 15 2100 5,3 0,28Anholt 4,8 11 1925 5,7 0,37Nysted 5,2 12 1680 ‐ ‐

Blue mussels European lobster

EnvironmentThe three WF sites are located along a marked salinity gradient Horns Rev 1 (surface 32/bottom33), Anholt (23/29) and Nysted (13/16).WF are generally placed in areas with strong winds. Strong winds which lead to high waves andhamper O&M. The maximum significant wave height for O&M is 1.2 m. Wind speeds >8 m sˉ¹occur about 40‐50% of the time inWF (figure 2).

Observations and high resolution models of the atmospheric boundary layer have shown strongeffects of turbines in generating turbulent wakes where wind velocity is reduced and turbulencemotion increases. This effect could have great importance for the suitability of WF as sites for

Economical profitabilityIn order to evaluate the economical sustainability a ”back of the envelope” calculation was performedto estimate breakeven price. Initial investment, life span and yearly running expenses for blue musselswas provided by Møhlenberg et al (2008) while for lobsters it was estimated to be 16350€, 8 years and4100€ respectively.

The breakeven price for blue mussels was app. 1.5 € kgˉ¹ in a traditional long‐line solution, and app. 2€ kgˉ¹ in a Smart Farm solution. European lobster was app. 15.5 € kgˉ¹ in a sea ranching solution.

Conclusions & future perspectivesBlue mussels: Mariculture is possible in WF. However, present design of WF, O&M and roughwind/wave conditions provide challenges.

aquaculture activities. Production is feasible in Anholt, while environmental conditions are too rough on Horn Rev 1, andsalinity too low at Nysted for production of high quality mussels for human consumption. 25% of thearea in aWF is likely to be available. Bottom culture is an alternative that increases the production areaand is in less conflict with O&M in theWF

Large scale production would probably reduce operating costs which was not included in the evaluationof the economic sustainability.

European lobster: The habitat in a WF covers a small proportion. An increase in suitable habitat couldbe combined with innovative cable protection or artificial lobster reefs within theWF.

Production is feasible in Horns Rev 1 and Anholt, while salinity is too low at NystedSea ranching in WF will in present conditions only be small scale. However, the economical analysisindicates that it could be profitable.

1National Institute of Aquatic Resources, Charlottenlund Castle, Jægersborg Alle 1, DK-2900 Charlottenlund, Denmark

3The National Environmental Research Institute, Aarhus University, Frederiksborgvej

2Krog Consult ApS, Skæringvej 100, DK-8520 Lystrup, Denmark

4Danish Shellfish Centre, Øroddevej 807900 Nykøbing Mors

AcknowledgementsThanks to P.E. Amby, site coordinator Horns Rev 1, Vattenfall for detailed information on drift and logistic in a WF and wave data, K. S. Hansen, seniorresearch scientist, DTU Mechanical Engineering, L. E. Jensen, Manager, Wind Power Analysis, DONG Energy for providing wind data. Dr. A. L. Agnalt, Instof Marine Research, Norway , and Per Thomsen, manager, Norwegian Lobster Farm for providing useful information on lobster biology. This study wasfunded by The Danish Food Industry Agency, Ministry of Food, Agriculture and Fisheries.

ReferencesMaar, M., Bolding, K., Petersen, J.K., Hansen, J.L.S., Timmermann, K. (2009) Local effect of blue mussels around turbine foundations in an ecosystem model of Nysted off‐shore wind farm, Denmark. Journal of Sea Research 62: 159‐174.Møhlenberg, F., Holtegård, L.E., Hansen, F.T. (2008) Miljøneutral udvidelse af havbrugsproduktion. Dansk Akvakultur Rapport. DSC and DHI.

Figure 2. Wind speeds from the three different sites.