Cruise report Hywind Tampen December 2025 and Mach 2026

Hywind Tampen is the first floating windfarm in Norwegian waters. With its 11 turbines it is among the biggest, floating wind farms in the world. It is built and operated by Equinor and supports oil and gas installations

Gullfaks and Snorre with renewable energy. It was constructed during 2022-2023 and holds a

total capacity of 88 MW. For more details on Hywind Tampen, structure and design look at (rapport Equinor,

2019).

1.1. The Hywind Tampen turbine structure

(text and figures taken from Equinor report: Hywind Tampen PUD del II – Konsekvensutredning).

The wind turbines are standard offshore wind turbines placed on a floating concrete base (Fig. 1). The blades are produced in fiberglass/epoxy. The turbines have a total height of approximately 190 meters from the bottom of the chassis to the centre of the nacelle. The distance between the sea surface and the blade tip is approx. 23 meters. The rotor has a diameter of 167 metres. Each wind turbine is anchored to the seabed with three anchor lines consisting of steel wire and chain (138 mm). Each of the anchor lines extend approximately 800 meters from the installations and is moored to suction anchors on the seabed. A total of 19 suction anchors of which 9 act as common anchors for 2 or 3 turbines.

The wind turbines are linked together with power cables that run between the turbines (internal cables). The power cables hit the seabed 400 - 500 meters from the turbine. Where the power cables hit the seabed, a small anchor will be installed to hold the cables in place. The internal cables that link two wind turbines together will be around 2 km long. The export cables are static cables that are buried.

1.2. Aim of cruises

These two cruises are following ups of previous yearly cruises in 2023 (Utne-Palm et al. 2023) and in 2024 (Utne-Palm et al. 2024). There is little knowledge related to how floating wind farms effect the marine environment, as this technology is still relatively new, and HT being the largest floating OW in the world. Thus, data gathered on these cruises is novel. Further, sixteen of the twenty selected OW sites in Norway are in deep water area where floating turbines are needed. Thus, any knowledge we can get from HT regarding environmental effects is of great value to the development of offshore wind in Norway. Offshore wind impacts the maritime environment, primarily through underwater noise from constructions and turbine operation, changes to wind-driven ocean currents (wake-effect), electromagnetic fields from subsea cables, habitat alterations (reef-effect), and chemical and solid discharges, from corrosion (microplastic from turbine blades) and the continuous leaching of heavy metals from anti-fouling and galvanic protection systems. We have aimed at covering as many as possible of these factors on our cruises.

Wake-effect: The windfarm removes wind, causing a wake, that may affect the water stratification and current system in the area around HT. That in turn might affect plankton distribution. To study this effect, we conducted several CTD transects across the windfarm and shelf area. On selected stations (15 in total), we also took water-bottle samples for nutrients and chlorophyll a as well as plankton samples (WP2 plankton net), on the March cruise eDNA was also taken at these 10 stations. Throughout the cruise, we collected current measurements using the vessel-mounted ADCP. We further deployed and retrieved 3 ADCP moorings; one within HT, one south and one north of HT.

Underwater noise: We deploy two hydrophone moorings one at 600 m and on at 2 km distance to turbine 10 (Hy10). The hydrophone data from these two moorings will be used to evaluate sound propagation models for HT (part of the NFR project WindSys). All moorings were deployed on the December cruise and was to be picked up on the March cruise.

Fish distribution: Thus, to look at possible attraction or avoidance effects on pelagic fish distribution, we conducted several acoustics transects through and around the wind farm with RV G.O. Sars aiming at completing several transect from ~5 nm outside and through HT following the same flower-like transect that we used in previous years.

For ground truthing of acoustic findings we use a trawl with an open cod-end equipped with a camera (DeepVision), and we sample eDNA from different depths.

Reef-effect: Our aim was to gain more knowledge about the effects of floating offshore wind farms on the bottom environment. The wind turbines at Hywind Tampen are secured to the soft bottom by suction anchors and chains. All subsea hard structure: the anchors, chains and the submerged part (90 m) of the turbine pilar, introduce a new type of habitat in this softbottom area. A habitat that we expect will be colonized by hard bottom species, resulting in additional food sources and shelter for higher trophic levels (Lindboom et al. 2011). Studies have shown that ecosystem processes and properties can be sensitive to changes in food web generated by OWF installations (Burkhard et al., 2011).

G.O. Sars is not allowed closer then 500m to the turbines, thus, in 2023 we used an acoustic kayak drone within the 500 m range to the wind turbines. With the kayak drone we managed to get as close as ca 20 m to the turbines, however, this was not close enough for the acoustic to cover the fundament and the area close by where we expect to find fish aggregations (Reef-effect). In 2023 we also conducted ROV transects along six anchor chains and 3 suction-anchors including control transect (no structure), to look for possible reef effect around the bottom mounted structure. During the 2025 and 2026 cruise we aimed at repeating these transects to look for possible changes in fauna around the structure after three years of operation.  

As part of the WindSys project we have deployed what we call an ObsFAD at HT, an observation rigg holding an echosounder, CTD, hydrophone, telemetry and a Fish Aggregation Device (set of underwater buoys at ~25 m depth). Hopefully this device will aggregate pelagic fish, and allow behavioural studies related to the temporal variations in windfarm noise. For the WindSys project, where Equinor is a collaborator, we have also deployed a Lander at HT. This lander holds, a bottom mounted multibeam echosounder, ADCP, hydrophones, CTD, oxygen – and turbidity sensors. Thus, the Lander will give us the possibility to observe fish behaviour related to sound and other environmental factors. Data is downloaded once a year from these to platforms, next time is November 2026.

2.1. Hywind Tampen (7 to 17th of December)

A detailed logbook of all activities during the cruise is available in Appendix (6. All stations).

Given bad weather predictions, with only a narrow window of less strong wind, over the next 1-2 days, we steamed directly to HT to deploy ADCP’s and hydrophones, hoping for the sea to calm down for ROV deployment. Or plan 1 was to conduct the follow up ROV transect along the chain and suction anchors at HT, and to retrieve a Lander frame that we had left on the bottom inside HT on a previous cruise. To be able to use ROV Freyja we need calm weather (< 2m waves). The Tampen area is known for its rough seas and wind, so we had to use any calmer window efficiently.

2.1.1 Deployment of Hydrophones and ADCP moorings

Before entering Hywind Tampen we contacted Equinor to open our working licences at HT. We started with deploying the two hydrophone moorings. Hydrophone mooring 1 was sat at 600 m from Hywind Turbine 10 (Hy10). Hydrophone mooring 2 was placed at 1400 m from Hydrophone 1 and 2000 m from Hy10at (positions are given in Table 1). This was followed by the deployment of the three ADCP moorings. ADCP1 was deployed within HT between Hy02 and Hy03, close to suction anchor no. 7 (SA07). ADCP2 was deployed ~5km south of turbine no. 6 (Hy06), and ADCP3 was deployed ~5km north-east of turbine no. 1 (Hy01). Positions of the ADCP moorings corresponded to our earlier deployments in March 2023.

After deployment all 5 moorings were added to Barentswatch, and information on position and type of equipment was sent to Equinor, and fishers organisations.

Hydrophone mooring design

The hydrophone mooring consisted of two hydrophones and CTD mounted in a float and attached by rope to an acoustic release which is attached by chain to a bottom anchor in form of two train wheels (Appendix fig. 4 and 5). Hydrophone mooring 1 holds two Aural M3 hydrophones, while Hydrophone mooring 2 holds two SoundTrap hydrophones.

ADCP mooring design

The moorings consisted of a Signature250 ADCP mounted in a float and attached by rope to an acoustic release which is attached by chain to a bottom anchor in form of train wheels (see Appendix 1, 2 and 3).  ADCP’s have a CTD on their mooring, clamped onto the rope below the ADCP. The mooring is equipped with an acoustic release. The acoustic triggers act as transponders. When desired, we can send a signal from G.O. Sars and measure distances.

2.1.2. Retrieve Lander frame with Dyneema rope 

A Lander’s bottom plate and Dyneema rope (300m long) were lost when retrieving the Lander in summer 2025 (Fig. 2). The Lander frame was within HT between Hy08 and Hy07. Followed by asking Equinor Kokstad to open or working licences we deployed ROV Freyja.

Method: We used a detailed map of underwater structure (provided by Equinor) to keep >250m away from HT cables and chains in our search. Before deploying ROV we also considered current direction, given that there was a 300 m long rope attached to the lander bottom plate. We used the ROV to connect a rope from the aft winch to the lander bottom plate, while paying attention to the current we could lift in the ROV while also lifting lander bottom plate.

Lost the ROV: The tether tired off when latching the ROV on board, due to one big swell. We asked Equinor for permission to use the mob boat to pick up the ROV as it was drifting towards turbine Hy08. Using the mob boat to tow it back out of the 500 m restriction zone, where GO Sars could pick it up.

2.1.3. CTD transects, with water sampling and plankton net

After asking Equinor Kokstad to open licences we conducted a CTD station transect across HT starting at the Tampen plateau and down the slope (from South-West to North-East). We also did two control transects over the same depth range across the shelf north and south of Hywind Tampen (Fig. 3, left). At every CTD station on the cross shelf transects (Fig. 3 left) we also took water samples for nutrients and chlorophyll a as well as plankton net (WP2) hauls.

When we were within HT we always stopped the CTD or WP2 net at 20 m above seabed due to underwater structure (e.g. floated cables, anchor, chains).

A CTD transects with nutrients only were also conducted running through HT from South-West to North-East and from North-West to South-East (see Fig. 3 mid). Furthermore, we did an extended cross shelf CTD transect (Fig. 3 right), that was an extended version of the central Mid cross shelf transect (Fig. 3 left).

Nutrients

We used IMR’s standard sampling depths: 5, 10, 20, 30, 50, 75, 100, 125, 150, 200, 250 and 300 m.

Chlorophyl

We sampled chlorophyl a from surface to 100 m depth in the same standard intervals as for the nutrients (see above).

Zooplankton sampling

Plankton samples were always taken by two net hauls. First from 100 m to surface and then from bottom to surface using WP2 net. Our plan was to take from the pycnocline to surface, but as the water column was so well mixed by all the bad weather conditions, we could not see a clear pycnocline and decided to sample upper 100 m as standard. Plankton samples were only taken at the 15 stations in the cross shelf transect (Fig. 3 left).

2.1.4. Studying Reef effect along subsea installation – by ROV

On a cruise to HT in March 2023, ca one year after the building of HT started, we conducted 12 ROV transects, around suction-anchor 6, 9 and 12 (SA06, SA09, SA12). In 2023 we used REEV Oceans ROV “Aurora”. On these cruises we aimed at repeating the 12 video-transects, see Fig. 4 for position related to HT structure, and Table 2 for more detailed transect position.

Followed by asking Equinor Kokstad to open licence we revisited two of the suction anchors (no. 6 and 9, SA06 and SA09).

In 2025 we conducted three ROV (Freyja) transect starting at the control transect going from North-West towards SA06. In the same dive we also did the transect along the chain from SA06 towards turbine Hy10, and the control transect from South-West towards SA06, before we had to surface due to bad weather coming in.

Method: During the ROV transect G.O. Sars positioned outside the safety zone (> 500 m from any turbine) close to the suction anchor. The ROV enters at the suction anchor and either follows the anchor chain 350-500 m towards the turbine Hy10 or 350-500 m along the control line (Fig. 4). This is the same method and procedures that we used in 2023. We used detailed map of underwater structure to keep >500 m away from cables and chains.

Due to bad weather (swells > 2m) we only managed to conduct transect line 2, 3 and 4 (Table 2). That included three control lines and one chain line around suction anchor 6 (SA06). 

2.1.5 Core samples for bacteria and pollutants

Running ROV transects gave us the possibility to take sediment samples in accurate positions in a gradient design from the wind turbines, to look at bacteria and pollutants. Our plan was to take a push core at the beginning and end of each ROV transect (12 transect, 24 core samples).

Method: 8. We used sterile equipment, cleaned knife and core tube with alcohol between samples. We slice off top 5 cm of core sample, placed it in a sterile plastic bag and stored it in -80°C freezer.

2.1.5. Acoustic study – and trawling with DeepVision

Hywind Tampen might attract fish to the added structures, or it might scare fish away by noise and activity. We therefore want to look at any possible attraction or avoidance effect by conducting an acoustic transect within Hywind Tampen to 5 nm from the farm using vessel mounted broadband echosounders (Kongsberg Discovery EK80; 18, 38, 70, 120, 200, 333 kHz). We did acoustic transects through HT (yellow line) from North to South and the opposite way (direction based on current and wind), and in a flower shape (blue lines) around and across HT as well as across (red lines) and along (green arrows) (Fig. 5).

DeepVision

To identify organisms in the major scattering layers observed by the acoustics we used an open-ended pelagic trawl with the DeepVision camera system in several trawl hauls along the park, taking pictures of the catch as it passes through the trawl.

2.1.6 Continuous underway measurements

Throughout the cruises, currents throughout the water column were monitored using the research vessel-mounted ADCPs (Teledyne RDI Ocean Surveyor at 75 kHz and a Kongsberg EC150 (150kHz)), this was synchronised with the scientific echosounder and controlled using UHDAS.

Likewise, was G.O.Sars bottom mounted broadband echosounders (Kongsberg Discovery EK80; 18, 38, 70, 120, 200, 333 kHz) on throughout the cruise.

2.2. Activity Cruise 2 to 12 March 2026

A detailed logbook of all activities during the cruise is available in Appendix (6). This cruise started off with deployment of Landers in two areas in the North Sea where Norway is planning offshore wind in the near future (Sørvest F and Vestavind F). After deploying the Landers we continued to Hywind Tampen.

2.2.1. Deploy Lander’s at offshore wind sites

Five Landers were deployed: three in Southern North Sea (Sørvest F), and two in the Utsira area (Vestavind F). Two Landers were deployed inside the OW area “Sørvest F” and on in a control area south-west of Sørvest F. Further were on lander deployed inside the OW area “Vestavind F” and one in a control area south-west of Vestavind F.

Lander details

The lander is built by Develogic. It holds three hydrophones type HTI-99 UHF, ADCP (Acoustic Doppler Current Profiler) Nortek Signature100, CTD (Conductivity, Temperature and Depth) Seabird SBE37-SMP MicroCAT, oxygen sensor (Aanderaa Oxygen Optode 4330) and transmissometer, WBAT (Wide Band Autonomous Transceiver) Simrad, with two T218 transducers, Simrad ES70-7CD split-beam transducer and Simrad ES120-7CD split-beam transducer, data logging system and batteries (Fig. 6 and Table 4).

Method: Deployment was done from the ship hangar traverse beam block winch. Position was monitored by the HIPAP system. When retrieving the Lander: The Lander housing (in titanium) is connected to a steel platform that functions as a weight. The Lander holds a built-in acoustic release system, that releases the Lander from its platform. A connection wire between the Lander and the platform makes it possible to pick up the entire device without using an ROV. Unfortunately, these five landers delivered without such a connecting wire, therefore this must be added on before next deployment.

Table 3. Position coordinates of the 5 Landers deployed in Sørvest F and Vestavind F. ID corresponds to table in Figure 7.

Table 4. Offshore wind landers sensors.

2.2.2. Activity at Hywind Tampen 2026.

The planed activity at HT, was to repeat the hydrographic, plankton and acoustic studies conducted on our December 2025 cruise (see section 2.1). Conduct the rest of the ROV transects (see section 2.1.4). Retrieve the three ADCP rigs, and the two hydrophone rigs deployed on the last cruise (see section 2.1.1). Further we took eDNA, to ground through what we saw on the acoustics. On this cruise we had asked for ROV Ægir, as it is able to work in rougher sea than ROV Freyja. With Ægir on board we could not trawl, and we did therefor not get any DeepVision data. However, Ægir was chosen as we wanted to prioritise finishing the ROV transect along the chain and suction anchors and to retrieve our ADCP and hydrophone rigs.   

2.2.2.1. CTD transects, with water sampling eDNA and plankton net

CTD transects

Due to bad weather conditions ahead we prioritize conducting the cross-shelf transects (Fig. 3 left), as this was the transect where also plankton and eDNA were going to be collected.  Furter, we sampled station 3 to 6 on this transect passing through the park going from South-West to North-East (Fig. 3 central). 

Zooplankton

Zooplankton community sampling was extended during the 2026 cruise, to assess samples for DNA studies. At each station, four WP2 net hauls were conducted: two hauls from 100 m to the surface and two hauls from near bottom to the surface. Due to vessel drift and the time required for each haul, sampling was performed in two stages: one full-depth haul and one 100 m haul were completed first, after which the vessel was repositioned before conducting the remaining two hauls.

Initial sampling design targeted the pycnocline-to-surface layer; however, as the water column was well mixed and no distinct pycnocline was observed during the survey, the upper 100 m was adopted as a standardized sampling depth for the shallow hauls. Zooplankton samples were collected at 15 stations along the cross-shelf transects (Fig. 3 left).

The first two hauls at each station were processed onboard according to standard IMR protocols (Appendix 8). Material from the remaining two hauls was preserved directly in approximately 80% ethanol for subsequent genetic analyses, including DNA metabarcoding of genomic DNA. Genetic analysis and taxonomic identification of specimens to the highest possible resolution is ongoing. The integration and comparison of genomic DNA data, eDNA metabarcoding results, and taxonomic identifications will enable a more comprehensive characterization of plankton community composition.

eDNA sampling

Environmental DNA (eDNA) analysis provides a non-invasive approach for assessing species presence in marine environments through the collection and analysis of water samples. Water samples for eDNA analysis were collected along three cross-shelf CTD transects, in addition to a supplementary station located within the wind farm (Fig. 3 left and mid). Furthermore, an additional sample was obtained from the onboard water system during transit between Sørlige Nordsjø II and Hywind Tampen.

At each station, triplicate water samples (5 L each) were collected from three discrete depths: near-surface, mid-water column, and near-bottom. Surface samples were obtained at approximately 10 m depth, while bottom samples were collected approximately 20 m above the sea floor. Sampling closer to the seabed was restricted due to the presence of wind farm infrastructure. Prior to water sampling, a CTD cast was conducted to assess vertical stratification, including the presence of a pycnocline. As the water column was generally well mixed during the sampling period, the mid-water depth was selected to represent intermediate conditions.

A total of 82 samples were collected across 10 stations (3 stations per transect + one additional inside HT), corresponding to 8–9 samples per station. At the two stations located within Hywind Tampen, all nine samples were obtained. At stations outside the wind farm, one of the three near-bottom replicates was omitted due to a limited number of available filters. In addition to environmental samples, multiple controls were included: one air control, freshwater controls collected before and after cleaning of sampling equipment (four in total), and three additional near-surface samples collected from a hose during transit to Hywind Tampen.

Filtration was conducted onboard using three 500 mL filter holders (Nalge Nunc International Corporation, Rochester, USA) connected to an EZ-Stream peristaltic pump (Merck KGaA, Darmstadt, Germany). Water samples were filtered onto 2.0 µm glass fiber filters (Merck KGaA). Immediately following filtration, filters were preserved in ATL buffer (Qiagen GmbH, Hilden, Germany) and stored until subsequent DNA extraction. Between samples, all filtration equipment was decontaminated using a bleach solution followed by rinsing with freshwater to minimize the risk of cross-contamination.

2.2.2.2. Studying Reef effect along subsea installation – by ROV “Ægir”

In 2026 we conducted four ROV transects starting on a control transect from North-West towards SA09, continuing with transect from SA09 along the chain towards Hy09 (Fig. 4). Followed by a control transect from South-West towards SA09, continuing with a chain transect from SA09 towards Hy10 (line 5, 6, 7 and 8, Table 2, start stop positions on this cruise is given in Appendix 7). With a core sample next to the suction anchor (SA09). We also tried to get a core sample at the end of the chain transect (closest to the turbine), but something was wrong with the corer, so we only got one core sample. 

Technical problems with the ROV stopped us from continuing and finalising our transect study. 

2.2.2.3. Acoustic study

Like in December 2025 we did acoustic transects through HT from North to South and the opposite way (direction based on current and wind), and in a flower shape around and across HT as well as along HT (Fig. 5). Having ROV Ægir onboard we could not trawl, as part of its equipment covers the trawl deck. Thus, on this cruise we must use the eDNA samples, to ground through what we see on the acoustics.

3.1. Hydrography

3.1.1. Hydrographic data from 2025 cruise

Preliminary results from the CTD transects across the shelf and from north to south through the wind farm (Fig. 9 and 10) indicate a well-mixed water column within and around HT. Signs of stratification are related to the circulation in Norskerenna at the deep ends of the transects. They include colder and slightly more saline bottom waters and surface elevated salinities. This is confirmed in the E-W transect that extends the mid transect into the deepest part of the trough (Fig. 11).

Vessel-mounted ADCP data are currently being processed.

3.1.2. Hydrographic data from 2026 cruise

Preliminary CTD transects across the shelf and from north to south of Hywind Tampen (Fig. 12, 13 og 14) show a largely well-mixed water column within and around the wind farm when expressed in terms of practical salinity (PSU) and temperature (℃).

Salinity profiles show minimal vertical variability across all transects, with changes confined to small decimal-scale variations. Temperature exhibits a weak decrease with depth, particularly in the deeper portions of the transects. This vertical temperature structure is most pronounced along the mid-shelf transect and likely reflects larger-scale circulation in Norskerenna.

3.2. Plankton biomass 2025 and 2026

Plankton samples were taken at each station on the cross shelf transect (Fig. 3. Left). The biomass of different size groups of plankton caught in the WP2 from bottom – 0 m or from 100 – 0 m, during the cruise in December 2025 and March 2026 is shown in Figure 15.  There was a higher biomass in the upper 100 m as expected, and the biomass was, however, significantly larger in December compared to March (Fig.16).  Otherwise, there were now clear trends in the biomass distribution related to HT Species composition has not yet been looked at.

3.3. ROV study of possible Reef and structure effect 2025 and 2026

The Station overview of the ROV transects follows the transect names used in 2023 cruise report (Table 2).

The benthos biologists made annotations of animals as they appaired on the video during transects. Number of organisms within some organisms’ groups were counted during the control transects and the chain transect as well as around the suction anchor (SA06), and this is given in Table 5. In 2026 they aditionaly made a descriptive annotation of biotope and the organism most frequently observed (see below).

Table 5. Noted organisms’ groups along the ROV transects in 2025 (HT01-HT04) and 2026 (HT05-HT08). Transect HT01: was a transect from suction anchor 06 (SA06) along the chain towards turbine HY10; Transect HT02: control transect from SA06 towards north-west; Transect HT03: control transect from SA06 towards south-west; HT04: video observations around suction anchor SA06; Transect HT05: control transect from SA09 to north-west; Transect HT06: from SA09 south-east along chain towards Hy09 (Hywind Turbine 9);  Transect HT07: control transect from SA09 towards north-west; Transect HT08: chain transect from SA09 going north-east to Hy10 (Hywind turbine 10). Taxon are given when possible to determin, also common name in English and Norwegian are given. 

Annotations from ROV transect 2026

HT05 (06.03.26 at 15:42 UTC)

Control transect starting in North-West moving South-East 500m off the chain towards suction anchor SA09.

Muddy sand with some burrows. Tracks from burrowing sea urchins all the way.

The sea urchins Gracilechinus and Spatangus dominated the transect in addition to Ditrupa tubes. The sea star Pontaster tenuispinus and Sabellidae tubes were also frequently observed.

Some saithe (Pollachius virens) were following the ROV along the video line, and several flatfish were observed, including Glyptocephalus cynoglossus and Lepidorhombus boscii. Ling (Molva molva), and greater forkbeard (Physcis blennoides) were also seen.

Other taxa: Astarte sp., Parastichopus tremulus, Aporrhais sp., Crangonidae, Pandalidae, Colus, Filograna.

A push core was taken at the anchor site.

HT06 (06.03.26 at 17:05, UTC)

Transect starting from suction anchor SA09, continuing South-East along the anchor chain.

Filming the suction anchor before the video line. Several ling (Molva molva) at the anchor. Also tusk (Brosme brosme) and saithe (Pollachius virens) swimming around. Some sea stars, hydroids and barnacles.

Muddy sand with some burrows along the video line. Tracks from burrowing sea urchins all the way.

The sea urchins Gracilechinus and Spatangus were dominating. Hermit crabs with Hormathiidae anemones were frequently observed. Ditrupa tubes and the sea stars Pontaster tenuispinus and Asterias rubens were common. Also, several Molva, Colus, Gadiculus, Parastichopus and Munida were seen in the video line.

The transect stopped 480 m in the line when the chain rose above the sea floor.

Attempt to take push core at the end of the line failed, due to problems with the core.

HT07 (07.03.26 at 08:05 UTC)

Control transect in North East direction 500 m off chain towards suction anchor SA09.

A push core was taken at the start of the video line approximately 500 m from SA 09.

Muddy sand with some burrows. Tracks from burrowing sea urchins all the way.

The sea urchins Gracilechinus and Spatangus were very abundant. High densities of Ditrupa tubes were also observed.

Common species along the transect were the sea cucumber, Parastichopus tremulus and the sea star Pontaster tenuispinus. Sabellidae tubes, shrimps (Pandalidae), Hormathiidae and Colus snails were also common, in addition to saithe (Pollachius virens) and silvery pout (Gadiculus argenteus). A few Astarte shells, ling, squat lobsters and hermit crabs were also seen.

HT08 (07.03.26 at 09:08 UTC)

Transect from suction anchor (SA09) on chain North East direction towards Hy10.

Started at the suction anchor, but more detailed filming of this area was done in the previous dive.

Muddy sand with some burrows. Tracks from burrowing sea urchins all the way along the video line.

The sea urchins Gracilechinus and Spatangus dominated, and Squat lobsters (Munida), were also abundant along the chain. Hermit crabs with Hormathiidae anemones were frequently observed, in addisjon to ling (Molva molva), sea stars (Asterias rubens, Pontaster tenuispinus and shrimps (Pandalidae).

Some sea cucumbers (Parastichopus tremulus), brittle stars, sponges, seaweed fragments, tusk (Brosme brosme) and silvery pout (Gadiculus argenteus).

Poor visibility towards the end of the line due to movement from the chains and the videoline was stopped after approx. 350m

Attempt to take a pushcore at the end of the line failed, due to problems with the core.

Litter observed: A part of a loading strap on the chain.

Preliminary results indicate a lower number of irregular sea urchins and bristle worms along the chain, compared to the control (Fig 17a), while most of the other species’ groups where higher in number along the chain. In particular squat lobster, hermit crabs, sea anemone, shrimp, sea star, brittle star and Hydrozoa where much more abundant along the chai (Fig 17b), but also sea cucumber, whelk and fish where more common along the chain transect.

a)

b)

3.4. Push cores’

We aimed at taking a core at each end of the ROV transect lines (12 transect lines 24 core), however the cores were not closing properly, so the sediment seeped out. Position of the two core samples taken are given in table 6.

Table 6. Information on core position.

3.4. Acoustic study – and trawling with DeepVision transects

A total of 19 acoustic transects were performed, one through the park and 18 outside. Acoustic data was continuously collected along the transects using Kongsberg Discovery EK80 scientific echosounders operating in broadband mode (Table 7). All echosounders transmitted simultaneously.

The 18 transects outside Hywind Tampen, “flower transects”, started at 500 m distance to the turbines and conduct transects outwards to 5.4 nm from Hywind Tampen (or in opposite direction). The transect design is a compromise between design for quantity calculation (more or less homogeneous coverage) and gradient study from the windfarm. Total length of transect is ca 105 nm. The transects are shown in (Fig. 18).

This flower-transect has been used on all previous cruises (Figure 15., Utne Palm et al., 2023). In addition, several transects were performed across the park to capture potential edge effects, such as potential accumulation of biomass near the edges of the park (see Fig. 5). Below are examples of data from acoustic transects outside, through and across the Hywind Tampen (38 kHz) (Fig. 19, 20 amd 21). Vertical lines indicate the start and end of the polygon covering the wind park. In this initial analysis, this polygon is used to describe the boundaries of the park which is an artificial boundary.

Acoustic backscatter data (Sv) from EK80 transects crossing the Hywind area were preprocessed by removing low-quality samples, block-averaging in range and time, and computing cumulative along-track distance. From the echograms we derived a set of integrative indices (center of mass, abundance, density, spread, etc.) and defined “inside” the wind farm using a time window mapped to along-track distance. For each transect, we compared conditions inside this window to nearby “outside” conditions restricted to a local corridor (±5 km from the window) to avoid bias from distant parts of the transect. Differences in density index D and depth averaged Sv (10–300 m) between inside and outside were assessed using permutation tests on the mean and median, which do not rely on distributional assumptions. A complete and more detailed analysis will be carried out when the full dataset is available.

The example echograms show the effect of time on the distribution of organisms within the wind park, which is most clearly visible when comparing T3 and T4. T3 was performed as the mesopelagic layer, a dominating feature within the park in addition to demersal fish, ascended to the surface and backscattering became evenly distributed throughout the water column.

Very preliminary analysis of the limited dataset showed that across the two independent transects crossing the wind farm (T2 and T5), we found consistent but transect specific patterns. In T2, the density index D did not differ significantly between inside and local outside, although a pronounced peak in D was observed in the edge zone immediately adjacent to the boundary. In contrast, T5 showed a clear and statistically significant increase in D inside the wind farm relative to the local outside both in time and along track distance. Depth integrated Sv (10–300 m) tended to be slightly lower inside than outside in both transects, but this reduction was small (order 0.3–1 dB) and more evident in the mean than in the median, indicating subtle changes in the distribution of scatterers rather than a uniform shift. Together, these preliminary results suggest that the across farm structure of scatterers can vary between crossings, likely influenced by time of day and background conditions, with T5 showing a more pronounced accumulation inside the wind farm than T2.

Along-transect (through HT) preliminary analyses for T3 and T4 show a consistent pattern of reduced density and backscatter inside the wind farm compared to the local outside corridor. For both transects, the density index D is significantly lower inside than outside in both time and distance domains. Depth integrated Sv over 10–300 m is also significantly lower inside than outside in both T3 and T4, with a larger reduction in T3 (mean and median differences of about 1–1.7 dB) and a somewhat weaker but still significant reduction in T4 (mean difference ≈ 0.3 dB, median ≈ 2.4 dB). Edge regions in T3 do not differ from the local outside, whereas in T4 the edges already show a modest but significant decrease in D relative to outside, suggesting a broader transition zone. The initial analysis of the 2025 acoustics showed a tendency of a systematically lower backscatter within the wind farm than in the adjacent local surroundings.

However, this pattern was not seen in the 2026 acoustics data. Example transects from the 2026 data are shown in the figure 22 (38 kHz data). Figure 22 shows transect T19 through the park. G. O. Sars entered the park from the south at approximately 16:01, just before the onset of diel vertical migration (as seen from approximately 8 km (along-track distance).

No conclusions can be drawn from this initial analysis, a full analysis of all crossings will be performed and compiled with data from previous cruises.

3.5. Pelagic trawling with DeepVision

In total we conducted seven trawl hauls (Table 7). The preliminary results from the pelagic trawling with DeepVision, show that mesopelagic fish as well as gelatinous plankton is dominating in the area (Fig. 23), this corresponds with our findings from previous years (Utne Palm et al., 2023 and 2024).

Table 7. Time and position of start and stop for the seven pelagic trawl stations with DeepVision, conducted in 2025.

3.6. eDNA

The eDNA data from the March cruise has not yet been analyzed.

Burkhard B, Opitz S, Lenhart H, Ahrendt K, Garthe S, Mendel B, Windhorst W. 2011. Ecosystem based modeling and indication of ecological integrity in the German North Sea—Case study offshore wind parks. Ecological Indicators 11(1):168-174. https://doi.org/10.1016/j.ecolind.2009.07.004

Lindeboom HJ, Kouwenhoven HJ, Bergman MJN, Bouma S, Brasseur SMJM, Daan R, Dirken S, van Hal R, Hille Ris Lambers R, ter Hofstede R, Leopold MF, Scheidat M. 2011. Short-term ecological effects of an offshore wind farm in the Dutch coastal zone; a compilation. Environmental Research Letters 6. https://iopscience.iop.org/article/10.1088/1748-9326/6/3/035101

Report from Equinor. Hywind Tampen PL050 - PL057 - PL089 PUD del II - Konsekvensutredning Mars 2019.https://cdn.equinor.com/files/h61q9gi9/global/59db109a1ab7991e6b7546ef9b161dcfa74ec514.pdf?hywindtampenpud-del-II-konsekvensutredning-mars-2019-equinor.pdf  

Utne-Palm AC, de Jong K, Pedersen G, Saltskår J, Hovland T, Renner A, Mihaljevic M, Wilhelmsen JF, Drivenes L, Bergamasco A, Falcieri F, Majaneva M, Critchley MJ, Kühn S, Viotti F, Akkaş K. Cruise report Hywind Tampen 19 to 26 October 2024. Toktrapport No. 7 2025. https://www.hi.no/hi/nettrapporter/toktrapport-en-2025-7

Utne-Palm AC, Søiland H, Sveistrup AK, Renner A, Ross R, Moy F, Bakhoday Paskyabi M, Totland A, Hannaas S, de Jong K, Gonzalez-Mirelis G, Hovland T, Pedersen P, Wilhelmsen JF, Majaneva MA, Heum SW, Skjold W, Vågenes S, Skaret G, Corus F, Voronkov A, Vågenes P, Kielland L. Cruise report Hywind tampen 13 to 28 March 2023. Toktrapport No. 10, 2023. https://www.hi.no/hi/nettrapporter/toktrapport-en-2023-10

ADCP: is an Acoustic Doppler Current Profiler. It is a hydroacoustic current meter similar to a sonar, used to measure water current velocities over a depth range using the Doppler effect of sound waves scattered back from particles within the water column.

Acoustic releaser: is for the deployment and subsequent recovery of instrumentation from the sea floor, in which the recovery is triggered remotely by an acoustic command signal.

CTD: is an oceanography instrument used to measure the electrical conductivity, temperature, and pressure of seawater. Conductivity is used to determine salinity and depth is calculated from pressure.

Hydrophone: is a microphone designed to be used underwater for recording or listening to underwater sound. Most hydrophones are based on a piezoelectric transducer that generates an electric potential when subjected to a pressure change, such as a sound wave.

Multi-frequency echosounder: Recent research has shown that simultaneous use of several discrete echo sounder frequencies (multifrequency) not only improves fish stock estimates but can also be used to identify species. This is because each specie has a unique acoustic frequency response. This new and growing understanding greatly improves the value of hydroacoustics to obtain information about marine resources. For more information see: https://www.kongsberg.com/maritime/support/themes/simrad-content-s/scientificapplications/multifrequency-echo-sounder-operation/  

ROV: stands for remote operating vehicle. ROVs are unmanned, highly manoeuvrable underwater machines that can be used to explore ocean depths while being operated by someone at the water surface. For more details see: https://www.revocean.org/vessel/rov/ 

DeepVision: is a camera system that is mounted at the cod-end entrance of the trawl. Here it takes pictures of the catch as it passes through the trawl. It is used to sample fish without having to land or injure the fish, as the fish passes through the open trawl cod-end. For more details see: https://www.deepvision.no/deep-vision-formarine-research 

1. ADCP mooring 1

2. ADCP mooring 2

3. ADCP mooring 3

4. Hydrophone mooring 1

5. Hydrophone mooring 2

6. All stations