Not just energy - Searching for edible crab and cod in an offshore wind park

In order to investigate these questions, various examinations and experiments were carried out in the wind farm, in the 500 m safety zone and in the surrounding waters of the wind farm:

• Assessing the population of edible crab (Cancer pagurus) and a possible gradient in their abundance applying crab pots in the 500 m safety zone and surrounding waters.
• A systematic analysis of the plankton for the detection of edible crab larvae and for the assessment of the wind farm as a possible reproduction area for edible crabs in the southern North Sea.
• Sampling of cod (Gadus morhua) with hand rods to analyse the importance of wind farms for cod nutrition, habitat use and trophic relationships.

An investigation of fish eggs and fish larvae (Ichthyoplankton) had already taken place in winter to find out whether cod uses wind farm areas for reproduction. Since it is only possible to get access to a wind farm under high safety conditions, the scientists had the opportunity to use the WindMW Crew Transfer Vessel (CTV) "Gesa" (operated by North Frisian Offshore GmbH) in order to carry out their investigations.

The first day of the experiments started already with a surprise: After the first crab pots had been set out, more than 30 cod with up to 4 kg in individual weight were caught during a 3-hours of fishing. Although high cod abundance at the facilities above the scour protection was known from anecdotal reports, the Thünen team did not expect such a high presence. During the whole sampling period, a total of 120 cod were caught and sampled. After the catch, length and weight of each fish were determined and gonad and liver weight as well as otoliths were taken to determine the age and condition of the fish. In addition, fish stomachs were sampled to investigate feeding ecology of the individuals in order to assess whether wind farms can provide a habitat of high quality. Already during sampling, swimming crabs, benthic crabs and benthic fish species such as butterfish (Pholis gunnellus) and gobies were identified in the fish stomachs. In addition to cod, grey gurnard, ling, wrasse and mackerel were caught as "by-catch", suggesting that wind farms may be a valuable habitat for a variety of fish species.

In the following days, pots filled with edible crab were hauled up on board, equipped with new bait and released again. It is known that edible crabs prefer rocky ground and the Thünen team expected them to live in close proximity to the facilities. This assumption was confirmed on the second working day. On average, not less than 6-7 crabs were caught per pot. At the eastern edge of the wind farm, the pots were set with increasing distance to the wind farm to determine whether the edible crab is migrating from the wind farm to other areas, which in research jargon is referred to as the "spill-over effect". Here, interesting results are emerging that need to be evaluated further. Over the entire period, about 330 kg of edible crab were caught with comparatively low fishing effort. A total of 105 crab pots, which were placed at a distance of 300 to approx. 1000 m from the facilities, were hauled on board.

After a successful week in the wind farm, the Thünen team led by project manager Vanessa Stelzenmüller and coordinated by Antje Gimpel left the Island of Helgoland on June the 24th 2019. We would like to thank WindWM and the crew of the "Gesa" for their great support and excellent cooperation. We are looking forward to come back!

If any questions remain, please contact Dr. Vanessa Stelzenmüller or Dr. Antje Gimpel.