Abstract
To obtain information on the occurrence of mussel (Mytilus spp.) larvae in the German Bight (North Sea), plankton sampling was carried out at 12 test locations in offshore areas 10–40 nautical miles offshore and along a transect extending from a nearshore area of the Weser estuary near Bremerhaven to an offshore site near Helgoland in the years 2003, 2006, 2008, 2011–2013, and 2014–2015. Furthermore, the settlement of Mytilus spat was estimated on test moorings equipped with suspended artificial collectors. Larvae were distributed heterogeneously in time and space, with the highest numbers recorded in May of each sampling year (80–25,000 larvae m−3). Lower larval numbers were observed in the offshore areas (>20 nautical miles off the coast) compared with those in inshore locations (<20 mi). The numbers of settled Mytilus spat (ranging from 0 to 1,100 mussels m−1) were low offshore compared with the inshore data of several tens of thousands of settled spat·per meter collector. Water samples were also obtained to assess the nutrient concentrations, chlorophyll contents, and the particulate organic carbon (POC)/total particulate nitrogen (TPN) ratios as potential indicators of the availability and quality of food for blue mussels. The measured environmental parameters indicated favorable food conditions for mussel growth in the offshore areas. However, further detailed studies on larval distribution and settlement dynamics are needed to fully assess the potential of offshore areas for seed mussel resources and grow-out cultivation.
Introduction
The majority of European coastal countries with direct access to the sea are engaged in bivalve cultivation, including Norway, Sweden, Denmark, the Netherlands, France, and Germany (FAO, 2024). Mussel farming along the German North Sea coast has a long tradition (Kleinsteuber and Will, 1988), and although the exploitation of natural beds of the blue mussel (Mytilus edulis) in the German Wadden Sea has been taking place for centuries, an extensive, combined fishery–culture system has only been developed since the 1950s (Korringa, 1976). Here, farmers use a technique described by Seaman and Ruth (1997), commonly called “on-bottom cultivation” (Johansson et al., 2026; Hickman, 1992). The seed is collected from dense wild mussel beds in subtidal or intertidal parts of the Wadden Sea area and then re-laid at densities of 30–40 mt ha−1 on specific, permanently water-covered culture plots (Hickman, 1992), allowing an annual harvest of up to 100 mt ha−1 (Seaman and Ruth, 1997). A disadvantage of on-bottom culture is the increased predation pressure from eider ducks, starfish, and/or crabs (). Furthermore, this cultivation method is dependent on the availability of wild seed mussels (Walter et al., 2003). In recent years, however, recruitment and settlement success have declined in Lower Saxony (Melle, 2022), resulting in a reduced seed availability. Consequently, mussel farming activities increasingly shifted from Lower Saxony to Schleswig-Holstein, where the settlement of Mytilus larvae remains comparatively high (Torben Wagner, Schleswig-Holstein Mussel Growers Association, personal communication).
Due to poor recruitment over several years and the resulting low settlement rate, research was carried out to investigate whether suspended “off-bottom” culture techniques (Sreedevi et al., 2014; McKindsey et al., 2011) could be used to obtain seed mussels in an inshore area by using collector ropes hanging down from a transshipment facility into the water body of the Jade estuary (Walter and Liebezeit, 2001, 2003; ) and at an open ocean test facility at the offshore lighthouse “Roter Sand” (; ). The infrastructure employed was a typical longline system floating at the surface with buoyancy deployed, anchored at both ends to the seafloor, holding dropper lines suspended perpendicular into the water column (). These dropper lines, also called collector ropes, serve typically as settling substrates for mussel spat (Walter and Liebezeit, 2003; ; ). The number of Mytilid larvae in the water column is therefore an intrinsic factor influencing aquaculture production performance through the colonization of these substrates.
Due to extensive nature conservation areas along the German North Sea coast and the limited coastline available for aquaculture development, the expansion of production volume is only possible through the utilization of more exposed offshore farming sites (). With this in mind, a multi-use concept (Schupp et al., 2019) was suggested to maximize the benefit of a certain site. This approach may provide an alternative to heavily utilized coastal waters where increasing competition among stakeholders restricts further development opportunities (Maar et al., 2023). Here, synergy can be realized by co-use areas, where offshore wind farms (OWFs) are planned or are already in operation (Buck and Krause, 2012; ). This encompasses various aquaculture species including macroalgae (e.g., Saccharina latissima) (), blue mussels (; Rumes et al., 2021), oysters (e.g., Ostrea edulis and Crassostrea gigas) (Pogoda et al., 2011, 2013), crustaceans (e.g., Homarus gammarus) (Krone, 2012; ), and finfish (e.g., Scophthalmus maximus and Gadus morhua) ().
This study investigated the occurrence and the distribution of mussel larvae in the coastal area 10–40 nautical miles off the German North Sea coast in close vicinity to OWF sites to support the multi-use concept for future farm siting. While inshore mussel distribution and settlement is known, there are limited data available for offshore areas of the German Bight ().
The present study was conducted to provide data on 1) mussel larval occurrence at OWF sites, located directly on the border of the Economic Exclusive Zone (EEZ) with the coastal sea realm due to commercial feasibility, and 2) spat settlement on suspended dropper lines, while also offering 3) insights into the timescale and density of spat settlement. These three objectives were complemented by measurements of the chlorophyll concentrations and C/N ratios to evaluate whether sufficient food resources for blue mussels are available at the selected offshore sites, thereby assessing their suitability for seed collection and grow-out cultivation.
Materials and methods
Test sites at OWFs and transects
To identify offshore locations with suitable environmental conditions for multi-use of OWF and aquaculture facilities, a number of OWF sites within the German Bight were identified in 2002. Site selection was based on typical factors described for offshore aquaculture in and in . The term “offshore” was defined by referring to operations beyond the three-nautical-mile limit, as defined in . The criteria for this survey were as follows:
Proximity to an OWF to allow and benefit from the multi-use concept and a synergetic cooperation between the two user groups (Schupp et al., 2019; Michler-Cieluch et al., 2009; Wever et al., 2015);
Distance from the next harbor to reduce costs and allow commercial potential;
Minimum water depth, as areas in the German Bight can generate high waves. In shallow water, wave motion changes from orbital to elliptical, thereby generating higher energy, which necessitates the use of more robust and expensive technology for mussel aquaculture. For the German Bight, the water should be at least 15 m deep (Heasman et al., 2024; Lojek et al., 2024).
Water quality parameters within the ranges known to support mussel filtration performance, growth, and survival, thereby providing suitable conditions for mussel cultivation (see Table 1);
Presence of mussel larvae in sufficient quantities in the water column to support successful settlement on the substrates (see Table 1); and
Settlement on collectors to ensure successful mussel cultivation.
Table 1
| Parameter | Optimum | Minimum | Reference |
|---|---|---|---|
| Chlorophyll (µg L−1) | 4–8 | >1–2 | ; Larsen et al., 2018; Gennari et al., 2026 |
| Larval concentration (larvae m−3) | 50,000–100,000 | >5,000 | Riisgård et al., 2015; Padin et al., 2024; |
| Settlement (individuals m−2) | 1,000–5,000 | >500 | van den Bogaart et al., 2023; van Broekhoven et al., 2024 |
Optimum and minimum data as prerequisites for offshore blue mussel (Mytilus edulis) aquaculture: chlorophyll, larval concentration, and settlement success.
In the following years, further sampling sites were examined as a number of OWF operators showed interest in collaborating on the projects at a later stage and additional research conducted (e.g., Gimpel et al., 2015). Sites that had previously yielded promising results were examined in subsequent years. In addition, one transect between the City of Bremerhaven and the offshore Island of Helgoland with seven transect stations (TS A–G) was set to:
Intensify the examination of the larval drift around test station 3 (Nordergründe) with adjacent stations TS B–TS E (Figure 1, Table 2) and
Improve understanding of the larval distribution patterns in adjacent areas that may be suitable for mussel cultivation.
Figure 1
Table 2
| Test site, no. | Cluster | Location | Justification | Year | Position | Sampling |
|---|---|---|---|---|---|---|
| 1 CS | C | L: Vortrapptief OWF: Amrumbank West, TD for cluster D | Vicinity to an OWF with potential to conduct mussel farming | 2003 | 54°34.80′ N–8°12.95′ E | B, ML, MS |
| 2 CS | C | L: Hever TD for OWFs: Amrumbank West and Butendiek, TD for cluster D | Transect for wind farms; location known with high density of mussel larvae () | 2003 | 54°20.49′ N–8°18.77′ E | B, ML, MS |
| 3 CS | A | L: Nordergründe OWF: Nordergründe | Vicinity to a nearshore OWF with potential to conduct mussel farming; location known with high density of mussel larvae (; ) (this location was key to the selection of the transect, see no. 11) | 2003, 2006, 2008, 2011–2013 | 53°50.14′ N–8°06.39′ E | B, ML, MS |
| 4 CS | B | L: North Juist (I) TD for OWF: Nordergründe and cluster A | Transect for wind farms and data on larval drift from NL to DE | 2003 | 53°48.28′ N–6°57.73′ E | B, ML, MS |
| 5 CS | B | L: North of Juist (I) TD for OWF: Nordergründe and cluster A | Transect for wind farms and data on larval drift from NL to DE | 2003 | 53°45.82′ N–6°39.96′ E | B, ML, MS |
| 6 EEZ | B | L: North of Borkum (I) OWFs: Borkum Riffgrund I/II, DolWIN alpha/gamma/epsilon, Nordsee I | Vicinity to an OWF with potential to conduct mussel farming | 2003 | 53°54.12′ N–6°39.84′ E | B, ML, MS |
| 7 EEZ | B | L: Borkum Riffgrund OWFs: Borkum Riffgrund I/II, DolWIN alpha/gamma/epsilon | Vicinity to an OWF with potential to conduct mussel farming | 2003 | 53°51.57′ N–6°21.89′ E | B, ML, MS |
| 8 EEZ | B | L: OWW German Bight Western Approach OWFs: Nordsee II/II, Delta Nordsee I/II, Godewind I/II/III | Vicinity to an OWF; data collection for the larval dilution effect at areas in larger distances | 2003 | 54°07.83′ N–6°53.82′ E | B, ML, MS |
| 9 EEZ | B | L: OWW East of German Bight Western Approach OWFs: Borkum Riffgrund III | Vicinity to an OWF; data collection for the larval dilution effect at areas in larger distances | 2003 | 54°07.40′ N–6°20,92′ E | B, ML, MS |
| 10 EEZ | D | L: OTS ODAS OWF: Butendiek | Vicinity to an OWF with potential to conduct mussel farming; location known with high density of mussel larvae () | 2006, 2008 | 54°59.46′ N–7°54,37′ E | B, ML, MS |
| 11 CS | C | L: North of Helgoland (I) OWF: Meerwind Süd/Ost | Vicinity to an OWF with potential to conduct mussel farming; official test site for OOA research of extractive species | 2014–2015 | 54°23.76′ N–7°45.23′ E | B, ML |
| TS CS | A | TR: Bhv. to Helgoland (I) TD: for OWF Nordergründe | Vicinity to an OWF with potential to conduct mussel farming; official test site for OOA research of extractive species; location known with high density of mussel larvae (; ) | 2011–2013 | Transect with seven TS from Bhv. (TS A) to Helgoland (TS G) TS A: 53°37.37′ N–8°28.48′ E TS B: 53°41.26′ N–8°21.33′ E TS C: 53°44.56′ N–8°13.80′ E TS D: 53°50.17′ N–8°06.09′ E TS E: 53°55.72′ N–7°57.54′ E TS F: 54°03.63′ N–7°55.82′ E TS G: 54°08.94′ N–7°55.45′ E | B, ML |
List of test sites to evaluate the potential for offshore aquaculture of mussels (Mytilus spp.) and their survey justification, positions, and sampling procedure.
B = basic data (DIN, RDP, SI[OH], Chl.); Bhv., City of Bremerhaven; CS, Coastal Sea; DE, Germany (Deutschland); EEZ, Economic Exclusive Zone; I, island; L, location; ML, mussel larvae; MS, mussel settlement; NL, the Netherlands; OOA, open ocean aquaculture; OTS, offshore test station; OWF, offshore wind farm; OWW, offshore water way; TD, transect data; TS, transect station; TR, transect route; TS, transect with seven stations from Bremerhaven (TS A) to Helgoland (TS G).
This transect was additionally identified as important due to the adjacent areas providing potential and benefits for mussel cultivation as these sites are closer to official shipping routes (easy access), various harbors (short ways to load/unload, processing, and equipment transfer), and natural mussel beds (which provide large numbers of larvae).
Subsequently, 11 locations (1–11) at 10–40 nautical miles off the German North Sea coast, as well as a transect comprising seven closely spaced stations (TS A–TS G), were selected for further investigation (Figure 1, Table 2).
In January 2003, the first nine selected offshore locations were equipped with test moorings on a seagoing expedition using the research vessels RV Heincke and RV Uthörn [both operated by the Alfred Wegener Institute (AWI) Helmholtz Centre for Polar and Marine Research]. The test moorings were deployed adjacent to the proposed OWF sites (test moorings 3–11) and in areas identified previously with promising positive site selection criteria (test moorings 1 and 2). For safety reasons, moorings were placed in close vicinity to already existing larger waterway marker buoys (“Ortstonne”) and at least 0.5 nautical miles away from the respective wind farm area.
The mooring’s marker buoy provided a buoyancy of 300 kg and was connected to a 2-mt concrete block with a 22-mm steel wire and a heavy buoy chain (Figure 2A). In the “Mytilus community zone” (Krone et al., 2013) at 3 m below the surface, a 1-m × 1-m metal frame was fixed to the wire, providing a holding device for various test units. The deployment depth was selected based on long-term observation indicating maximum settlement at 3 m below the surface (Dobretsov and Miron, 2001) and low settlement at depths > 6m (Joschko et al., 2008; Walter and Liebezeit, 2001). In addition, the depth was chosen so that the system could be installed several meters below the water’s surface in order to avoid the strong forces due generated by wave action (Lojek et al., 2024) and to minimize dislodgement of newly settled mussels.
Figure 2
The test units used were spat collectors clamped into the frame, where mussel larvae settle after metamorphosis. Each collector consisted of a polypropylene carrier rope (diameter = 10 mm) with four inserted transverse elements to enlarge the surface area. These elements were made of 15-cm-long pieces of the same polypropylene rope, which were frayed manually in approximately 1,100 single fibers to produce a bow-tie-like bundle, allowing a large surface availability for spat attachment (Figure 2B). This type of collector is equivalent to the type used by Tortell (1976) and , as well as the reference collector in , which has previously been shown to support high settlement densities of mussel spat (; Walter and Liebezeit, 2003).
After the test moorings were deployed in January 2003, samples were collected once a month (in February, March, April, May, July, and September) during a 5-day trip with the research vessel RV Uthörn in 2003 and in 2004. In the years that followed, prior to recruitment, which sets the number of mussel larvae in the water column, and the subsequent metamorphosis, which leads to settlement on substrates, new collectors were installed in the frames of the test stations. The removal of the collectors and the installation of new ones proceeded as follows: each month, four bundles were removed and were replaced with four new bundles. The following month, another four bundles were removed and similarly replaced with new bundles. The remaining bundles served as spares in the event of any being lost and for long-period observation.
The collected data were evaluated and the study areas were re-evaluated accordingly. In the following years (in 2006, 2008, 2011–2013, and 2014–2015) (Figure 3), the test moorings that showed the best results from a mussel farming perspective (see Table 1) were re-examined between May and August in order to further improve the data for evaluating the site selection. This resulted in four data collection periods on a timescale from 2002 to 2015, which are shown in Figure 3. Table 2 lists the study sites and the samples taken.
Figure 3
Data acquisition of larval numbers and spat settlement of post-larvae
To determine the number of mussel larvae in the water column, 50-L samples of seawater (three replicates) were taken from a depth of approximately 3 m using a submersible pump (; Roegner, 2000). Depending on the sea status and current velocity at the time of sampling, the pump inlet was weighted down to guarantee a depth of 3 m. The seawater samples were filtered through a plankton net (mesh size = 125 µm), fixed in buffered formaldehyde (4%) afterward, and stored at 4 °C for later analysis. In the laboratory, mussel larvae were quantified on a counting grid using a binocular microscope (magnification, ×20–×50). Due to limitations in morphological identification, particularly during the early larval stages, mussel species were not routinely identified ().
Tightened bundles fixed in the metal frame devices were sampled monthly during the first and second data acquisition periods to follow the settlement of post-larvae during the entire study period. In 2003, some remaining ropes were not collected until the end of September to provide information on long-term settlement and growth. Collectors attached to the same mooring were treated as subsamples rather than fully independent experimental units, and the results should therefore be interpreted as indicative of settlement patterns at each site. Due to the fact that only a single mooring system was deployed at each site, the mooring itself represented the effective experimental unit. Consequently, site-specific estimates of spat settlement may also reflect local technical or operational effects associated with an individual mooring system. The present study should therefore be regarded primarily as an exploratory assessment of settlement patterns and site suitability rather than a study with a fully replicated experimental design. Removed bundles were preserved in buffered formaldehyde (4%). All bundles were later rinsed with water and the settled mussel spat removed. When smaller mussels remained on the filaments, the whole bundle was bathed in sodium hypochlorite solution to facilitate detachment of the byssus threads. Mussel spat was separated from other fouling organisms and counted. The length of small mussels (<4 mm) was measured using a binocular microscope and a micrometer. Individuals with lengths >4 mm were measured using vernier calipers.
Water sampling
Water samples for the determination of chlorophyll, particulate organic carbon (POC), and total particulate nitrogen (TPN) were collected at a depth of 3 m with the same electric pump used for larval sampling. Following Walter and Liebezeit (2001), all water samples were filtered through 300-µm plankton gauze to remove larger particles (Evans and O'Reilly, 1980). Chlorophyll samples were taken during all data acquisition times, while POC and TPN were only taken during the first data acquisition.
To measure the chlorophyll (Chl. a, b, c1, and c2) and pheophytin contents, 800 ml of seawater was filtered through Whatman GF/C® filters (n = 3). The filters were cut into small pieces and stored at −20°C in 1.5 ml Eppendorf cups. These were transferred into test tubes with 10 ml aqueous acetone (90%) and treated ultrasonically (Ultra Sonifier W 250, Branson Ultrasonics Corporation, Danbury, CT, USA) for 20 s while cooled on crushed ice. Samples were then incubated at 4°C in the dark for 2 h, followed by centrifugation for 10 min at 4,000 rpm. The absorption of the supernatant was measured (optical path length = 5 cm) at 750, 630, 647, 663, 664, and 665 nm (Parsons, 1963; Parsons and Strickland, 1963) using a spectrophotometer (Milton Roy Genesys, Libra S60 Biochrome, Thermo Fisher Scientific, Waltham, MA, USA). The reference was aqueous acetone (90%). Thereafter, 0.1 ml HCl was added and the measurement at 665 nm was repeated. After correction of the extinction for all wavelengths by the extinction at 750 nm, the chlorophyll contents were calculated employing the equations given by Jeffrey and Humphrey (1975). The total chlorophyll (TCHL) is therefore the sum of a, b, c1, c2-chlorophyll, and pheophytin.
Samples for the determination of POC and TPN were filtered over pre-combusted (450°C, overnight) glass fiber filters (Whatman GF/C®), washed with 2 ml doubly distilled water, and dried at 65°C. After weight determination, the samples were acidified with 1 M HCl to remove inorganic carbonates and re-dried. POC/TPN was then determined by high-temperature combustion (Nitrogen Analyser 1500, QuAAtro30 Continuous Segmented Flow Analyzer + Autosampler, Carlo Erba Strumentazione, Milan, Italy). Acetanilide was used as the calibration standard.
To determine the seasonal availability of inorganic nutrients as an indicator of phytoplankton growth conditions, ammonium, nitrate, phosphate, and silicate were analyzed in the seawater according to Grasshoff et al. (1999) on replicate samples. The reproducibility values were ±3.0% (reactive dissolved phosphate, n = 94), ± 5.2% (nitrite, n = 95), ± 3.3% (nitrate, n = 93), ± 4.5% (ammonium, n = 95), and ±2.5% (silicate, n = 94).
Statistical analysis of data
Statistical analysis was performed using the Statistical Package for Social Sciences (SPSS 12.0). Normality was assessed using the Kolmogorov–Smirnov test. To assess differences in the larval abundance and spat settlement density in relation to distance from the shore (groups 1 and 2 respectively <20 and >20 nautical miles offshore), the non-parametric Mann–Whitney test was applied. Significance was accepted at p < 0.05 (Sachs, 1972).
Results
Sample acquisition
Due to stormy weather conditions on two cruises in 2003, some of the test moorings could not be retrieved to collect data on mussel spat. During periods of favorable weather conditions, water, mussel larva, and mussel spat samples were obtained and the metal frame was supplied with new collector bundles. Due to harsh environmental conditions, two of the moorings were lost. The buoys at locations 1 and 2 disappeared after the sampling in May. One of the mooring’s marker buoys was retrieved 1 year later by the Swedish Water and Shipping Agency in the open ocean off Stömstad, while the other entire mooring was loaded on board a shrimp cutter, who ignored the safety distance and maneuvered too close to the test area, where the fishery equipment got entangled with the mooring device.
First data acquisition: larval numbers, settlement of post-larvae, chlorophyll, POC, and TPN
During the first data acquisition in 2003, the abundance of planktonic mussel larvae showed a heterogeneous spatial distribution among the various test locations in the offshore area of the German Bight (Figure 4A). Low numbers of 25 larvae m−3 ( ± 13.65 SD) at test site 3 and up to 1,030 larvae m−3 ( ± 126.11 SD) at test site 5 were recorded in April, with numbers <1,000 at the remaining nearshore and further offshore stations (test sites 1, 2, 4, and 6–9). Both test locations off the Schleswig-Holstein coast (test sites 1 and 2) showed the highest larval contents, i.e., 2,958 larvae m−3 ( ± 400.26 SD) at test location 1 and 24,900 larvae m−3 ( ± 1,500 SD) at test site 2 in May, whereas larval abundance at locations off the Lower Saxonian coast was considerably lower (≈80–1,500 larvae m−3). In July, the numbers of larvae decreased at all test sites, again with higher larval densities at locations 1 and 2 and lower numbers off the Lower Saxonian coast (≈50–1,300 larvae m−3). Overall, larval abundance was significantly lower in the offshore areas (>20 nautical miles off the coast) compared with that in inshore locations (<20 miles) (Figure 4A).
Figure 4
On the suspended bundles, first settlement of post-larvae was recorded in April 2003 at test locations 1 and 4–6, with mean values of 4–29 individuals m−1 (Figures 4B, 5A). Due to harsh weather conditions, the settlement record in May was incomplete. The numbers were low (4–56 individuals m−1), with the exception of the Weser estuary (location 3) where higher numbers were observed (800 ± 400 individuals m−1). After the larval peak from May–July, a maximum of 300 individuals m−1 was recorded. Long-time records from bundles deployed for the full experimental period (from January to August) showed a further increase in the number of mussels settling at the majority of the locations, ranging from 11 to 1,100 individuals m−1. No significant relationship was detected between distance from shore and spat settlement density.
Figure 5
The inorganic nutrient concentrations were high in the winter and in several coastal areas (≈40–70 µmol L−1 depending on the distance from the coast) and decreased toward March (≈30–60 µmol L−1), remaining at a relatively low level until September (≈8–20 µmol L−1). Detailed information is given for test site 3 because this station was selected for continued monitoring in the following years (Figure 6A). Reactive phosphate showed a decrease from March (≈0.5 µmol L−1) to July (≈0.3 µmol L−1) (Figure 6A). The chlorophyll contents showed a peak of approximately 16 µg L−1 in April and summer values of approximately 7 µg L−1 (Figure 6B). The POC/TPN ratios, on the other hand, showed little temporal variation and leveled at ≈6.5–7 (Figure 6B). A slight decrease was, however, noticeable for the April samples (≈5.5). Marked changes occurred in the POC/TCHL ratio, with a decrease from 380 in January to 30 in April and a slight increase to 50 in May and July.
Figure 6
Based on the results of the first monitoring period (mussel larvae and spat settlement), test sites 3 and 10 were selected for continued investigations during the second data acquisition period. By evaluating the given data, test site 3 was characterized and evaluated as being suitable for offshore mussel farming, which is supported by the fact that it is in the vicinity of the wind farm Nordergründe and has the shortest distance from shore. In addition, due to the interest of another wind farm operator, test site 10 was taken as a test site as well.
Second data acquisition: larval numbers, settlement of post-larvae, chlorophyll, phosphate, and nitrate/nitrite
During the second data acquisition, larval abundance at test site 3 decreased in 2006 from 4,678 larvae m−3 ( ± 206.18 SD) in May to 640 larvae m−3 ( ± 69.58 SD) in July and in 2008 from 10,155 larvae m−3 ( ± 625.73 SD) in May to 2,762 larvae m−3 (± 407.43 SD) in July (Figure 7A). At test site 10, larval abundance decreased in 2006 from 2,758 larvae m−3 ( ± 243.82 SD) in May to 206 larvae m−3 ( ± 23.44 SD) in July and in 2008 from 2,762 larvae m−3 ( ± 407.43 SD) in May to 627 larvae m−3 ( ± 83.50 SD) in July (Figure 7A).
Figure 7
In 2006, settlement of post-larvae increased in July at test site 3 from 1,375.00 individuals m−1 ( ± 117.84 SD) to 1,432.67 individuals m−1 ( ± 54.49 SD) in August and at test site 10 from 373.67 individuals m−1 ( ± 75.18 SD) in July to 535.33 individuals m−1 ( ± 140.01 SD) in August. A similar trend was observed in 2008, although the settlement densities were slightly lower (Figure 7B).
The chlorophyll and nutrient data were obtained from various databases from the Federal Maritime and Hydrographic Agency of Germany (BSH), such as the MURSYS reporting system () and the RGE BLMP—Working Group for the North Sea and Baltic Sea Monitoring Programme (). The phosphate concentrations at test site 3 were approximately 1.3 µmol L−1 and at test site 10 were 1.1 µmol L−1, while the dissolved inorganic nitrogen (DIN) concentrations leveled around 11.4 µmol L−1 at test site 3 and 28.5 µmol L−1 at test site 10. The chlorophyll concentrations were approximately 4–5 and 5–6 µg L−1, respectively.
The results of the second data acquisition period indicated that test site 3 exhibited comparatively favorable conditions for mussel cultivation. However, due to the fluctuation in the larval concentration, it was decided to examine test site 3 once more.
Third data acquisition: larval numbers, chlorophyll, phosphate, and nitrate/nitrite
During the third data acquisition period, a transect was set with seven additional stations (TS) to obtain more detailed information around test station 3 and along the shipping route off the coast of Bremerhaven to the Island of Helgoland. The transect starts with TS A at Bremerhaven and goes along via test station 3 to Helgoland being TS = G (Table 2) and was serviced in the years 2011–2013. Test site 10 was not further examined as the evaluation resulted in a too low settlement of mussel larvae and the fact that this site was too far off the coast.
The larval concentration during the third data acquisition period showed a heterogeneous distribution between sites and timescales. Larval abundance generally increased with distance from the coast, reaching its highest concentration in the vicinity of TS C and TS D, as well as at test site 3, and then decreased again toward Helgoland. Figure 8 shows this development in the years 2012 and 2013; however, in 2011, a contrary trend was recorded. Here, the larval concentration was very high in nearshore environments (30,997 larvae m−3 ± 17,341.63 SD at TS A and 32,452 larvae m−3 ± 13,828.03 SD at TS G). Other TS leveled between 14,793 and 24,566 33 larvae m−3, except those halfway to Helgoland and test site 3, with 6,936 larvae m−3 ± 357.50 SD and 4,642 larvae m−3 ± 408.97 SD, respectively.
Figure 8
According to data obtained from the MURSYS database () and the BLMP monitoring program (), the phosphate and DIN concentrations along the transect, as well as at test site 3, were sufficient to support phytoplankton production. Following these databases, the nutrient concentrations did not vary much since the last 50 years. Only natural events, such as the River Elbe flooding, could cause higher concentrations due to the runoff of nutrients from agriculture fields.
The recorded data at test site 3 and along the transect from Bremerhaven to Helgoland provided further evidence that test site 3 may represent a suitable location for mussel cultivation. Due to the fact that north of Helgoland another newly established OFW started its operation, the company running the OWF was interested in mussel farming as well. Consequently, a fourth monitoring period was conducted at this test site.
Fourth data acquisition: larval numbers, chlorophyll, phosphate, and nitrate/nitrite
Although the nutrient and chlorophyll concentrations remained within the ranges considered favorable for mussel growth (; ), larval abundance was low in 2014 with 1,882 larvae m−3 ± 364.67 SD and in 2015 with 1,000 larvae m−3 ± 184.28 SD. These numbers were considered insufficient to justify further investigations at this site.
Discussion
A fundamental question is whether the search for expansion areas for coastal aquaculture of extractive organisms must necessarily extend into the open ocean at greater distances from shore and/or into exposed areas (Jouffray et al., 2020; Sclodnick et al., 2024; FAO, 2024). It is well known that there is severe overuse in the nearshore waters of the coastal states of Europe (). Therefore, either new or intensified uses should not further burden the coastal seas (Lester et al., 2018; Horn et al., 2025). However, to ensure Europe’s food security and reduce dependence on imports from countries outside Europe, future increases in aquatic food production may require the development of both land-based and offshore production systems. The concept of employing multi-use strategies (; Schupp et al., 2019) may represent an economically viable alternative while also conserving other areas that remain unused due to the expansion of aquaculture at OWF sites.
Despite the low larval density and the poor settlement success at test site 3, it was decided to continue testing the site as there was a wind farm being built there.
Food concentration and quality
As part of the site selection criteria, the water samples were analyzed for phytoplankton content in terms of chlorophyll and the C/N ratios and nutrient concentrations. This provided important information about food availability, which is a key component in the successful settlement and growth of mussel spat on artificial substrates in offshore areas. This assessment was undertaken to determine whether sufficient food resources are available offshore for newly settled mussels. Painter et al. (2018) and Heasman et al. (2024) argued that the concentrations of organic matter, as well as nutrients, will decrease with distance from the coast. The data provide a preliminary basis for relating phytoplankton abundance and composition to mussel growth. There are certainly a number of other parameters that provide information about food quality, such as essential fatty acid and protein composition (), lipids (Laudicella et al., 2020), and amino acids and vitamins (), among many others. These parameters could be incorporated into future site selection frameworks () when first attempts for the commercial implementation of offshore mussel aquaculture are put forward.
The chlorophyll concentrations remained within the ranges considered suitable for mussel growth throughout the year, with only a brief decline toward the lower end of this range during spring (Table 1). The inorganic nutrient concentrations did not indicate nutrient limitation, suggesting that primary production was not substantially constrained during the main productive period. The seasonal nutrient dynamics were consistent with the expected pattern for the North Sea, where the spring bloom is typically dominated by diatoms. Consequently, food suitable quality and quantity were likely available for mussel larval settlement and early post-settlement growth from spring through summer and potentially until the autumn phytoplankton bloom, although the full extent of this resource requires further investigation. The consequently low POC/TPN ratios suggest a particulate pool enriched in nitrogen-containing organic matter, potentially reflecting substantial contributions from phytoplankton and/or bacterial biomass rather than terrestrial detritus. Sarà et al. (1998) observed low POC/TPN ratios in connection with high phytoplankton abundance and noted that higher ratios indicate a high detrital contribution.
If contributions from a bacterial biomass are indeed present, then the available food for filter feeders will be of a reasonable quality, even under non-bloom conditions. Similarly, the POC/TCHL ratios in the spring and summer are indications of a high contribution of non-detrital carbon to the particulate pool, while the high January values clearly suggest a pronounced detrital component (e.g., Steele and Baird, 1965). These observations suggest that food quantity and quality were unlikely to limit the larval and early post-settlement growth between the late spring and autumn.
Larval numbers and settlement of post-larvae
Favored by a preceding spring phytoplankton peak (Niesel and Günther, 1998), spawning of Mytilids along the North Sea coast of Germany generally occurs from May to June (Pulfrich, 1995, 1997; Walter and Liebezeit, 2001). Although larvae were also present later in the season, their abundance in the water column was considerably lower than that during the main peak (Figure 4). The fate of Mytilus larvae during their planktonic development remains incompletely understood. Herlyn and Millat (2000) showed that mussel fishing substantially reduced the mussel bed area and biomass, which was associated with lower recruitment of juvenile mussels, suggesting that habitat availability can be an important factor influencing recruitment success.
Mussel beds are predominantly located in the tidal backwaters of the Frisian Islands (Wadden Sea), in front of the East Frisian Islands toward the open sea and close to the estuaries of the rivers Weser and Jade. Here, hard substrates suitable for settlement are more or less absent due to the geological formation of the Wadden Sea seafloor; however, the mussel beds themselves act as the only natural hard substrate. Successful settlement of larvae, which derive from these areas, is dependent on the timing of larval release with wind forces and currents (Young et al., 1998; Metaxas, 2001). If suitable settlement substrate is unavailable near the spawning area, M. edulis larvae potentially delay metamorphosis for a number of weeks (James et al., 2019; Nanninga and Berumen, 2014; ). This may facilitate settlement on substrates located in considerable distance from the coast (McQuaid and Phillips, 2000). However, substantial mortality may occur when larvae settle in unsuitable offshore habitats (Leis, 2021; Roegner, 2000; ). During the extended dispersal time in the water column, Mytilus larvae are able to survive periods of exposure to physical stresses, such as water turbulence and unfavorable currents (; Morgan, 1995; Richards et al., 1995), and to biological constraints such as predation (Young and Chai, 1987; Rumrill, 1990) and starvation. Predation by fish and invertebrates, together with starvation, represents the major source of larval mortality (Thorson, 1966; Mileikovsky, 1971; Purchon, 1977; Jørgensen, 1981). Furthermore, due to the dilution effect of long drift times to offshore settlement, sites result in declining densities of mussel larvae in the water column (Young et al., 1998; Metaxas, 2001; ).
Consequently, larval abundance would be expected to decrease with increasing distance from the coast. For a coast-dependent population, an excessive export to the open North Sea would represent a loss from the parental stock. The gradient in larval numbers from the inshore to the offshore areas in the German Bight confirms this assumption. Inshore larval densities typically range from 9,000 to 190,000 larvae m−3 (Heiber, 1988; Pulfrich, 1995; ; ; Nehls et al., 2001; Walter and Liebezeit, 2001). The concentration of larvae was very low off the Lower Saxonian coast, even at the location closest to the inshore spawning areas in the Wadden Sea, despite sufficient nutritional quality of the phytoplankton. Regional differences in larval abundance and recruitment success may result from multiple interacting factors. These include climate change effects (e.g., increased sea surface temperature, precipitation, and extreme weather events) (), hydrodynamic processes (Folmer et al., 2014), habitat exposure (Folmer et al., 2017), changes in predation pressure and larval transport pathways (), and post-settlement survival, all of which may be influenced directly or indirectly by climate change and can differ regionally (Troost et al., 2022). In addition, the anti-clockwise current regime in the German Bight (Figure 1) explains the resulting shift in the drift of mussel larvae. However, in contrast to the findings off the coast of Lower Saxony, a comparatively high number of larvae was recorded approximately 10 nautical miles off the coast of Schleswig-Holstein.
The distribution of larvae in the water column is dependent on various biological and physical factors, such as the reproductive success of a closed population and the distance from different population groups, the time between spawning and metamorphosis and settlement, the growth rate, mortality, geographical origin, and oceanographic events such as current velocity and direction (Robins et al., 2013, 2015; Leis, 2021; ). Currents in the coastal waters of the German Bight are predominantly tidal, corresponding to a tidal dynamic, but with a net current that moves in an anticlockwise direction from cluster B in the west via cluster A to the east, and then via cluster C into a northerly direction to cluster D (Figure 1), which can take weeks to months and will therefore have an influence on settlement (James et al., 2019; Nanninga and Berumen, 2014; ; ). Concentrations of approximately 3,000 larvae m−3 were recorded by Rees (1954) in the center of the North Sea, 200–300 nautical miles away from their origin at the north British coast. The results indicate that, in the Schleswig-Holstein region, the mussels were of local origin, whereas off the coast of Lower Saxony, mussel larvae drifting with the counterclockwise-directed coastal current () may have originated from a more westerly origin (Belgium, the Netherlands). Thus, the coupling of these events (phytoplankton density and larval drift density) is decisive for offshore settlement. Since no universally accepted threshold concentration of M. edulis larvae exists for successful offshore seed mussel collection, productive mussel farming areas have been associated with seasonal larval densities ranging from approximately 10,000 to >100,000 larvae m−³. Successful recruitment however, depends not only on larval supply but also on hydrodynamic conditions, the availability of suitably settlement substrate, and post-settlement survival (Riisgård et al., 2015; Padin et al., 2024).
The ongoing expansion of OWFs in the German Bight is expected to substantially increase the amount of artificial hard substrate available for colonization by M. edulis (Adams et al., 2014; Maar et al., 2023). Previous studies have shown that offshore wind turbine foundations (jacket type) can support large mussel populations (Joschko et al., 2008), which may act as additional sources of larvae. Given the projected increase in the number of turbine foundations in the German EEZ, the larval production and concentrations in the water column are likely to increase, potentially enhancing settlement success and altering the present-day dispersal and connectivity patterns. Consequently, the larval and spat distribution patterns documented in this study represent an important baseline of pre-expansion conditions against which future changes in mussel recruitment, population connectivity, and ecosystem functioning associated with OWF development can be evaluated. Although the jacket foundation has an approximately two to three times larger colonization surface compared with the monopiles normally used in the German North Sea (DWG, 2024), it is clear what quantities of mussels are present, which reproduce in addition to the mussels in the back mudflats. It can be assumed that approximately 2 tonnes of mussels are settling per monopile foundation and that there will be approximately 18 foundations in cluster A (→ 36 tonnes/year), 641 foundations in cluster B (→ 1.282 tonnes/year), 246 foundations in cluster C (→ 492 tonnes/year), and a further 80 foundations in cluster D (→ 160 tonnes/year) (DWG, 2024) (Figure 1). The larval concentration in the water column will be significantly increased in the future, which in turn will also lead to increased settlement success.
Mytilus pediveliger larvae preferably settle on threadlike structures, e.g., hydroids, algae, or the byssus of adult mussels (; Eyster and Pechenik, 1987; Satuito et al., 1995). This behavior could be used in the collection of young mussel spat on artificial collectors for the suspended culture of mussels and for monitoring the number of plantigrades in the water column (King et al., 1990). For this study, a robust collector type tested by , which could easily be replaced, was used. Previous studies have shown that a tufted polypropylene collector was successful in attracting spat in densities numbering tens of thousands of individuals per meter (Tortell, 1976; ; Walter and Liebezeit, 2003).
The settlement densities recorded in the German Bight were between one and several orders of magnitude lower than those reported from inshore locations in the North Sea. In the Wash (UK), 100,000–400,000 individuals m−1 of Mytilus spat settled on this collector type (). At the Lower Saxonian Wadden Sea coast (Germany), mean numbers of monthly settled blue mussel spat of 33,000–72,000 individuals m−1 were recorded (Walter and Fehrs, 2004). Even long-term collectors exposed to repeated settlements events, although they also experienced losses caused by predation or wave action, showed higher densities in inshore compared with offshore areas by at least one order of magnitude (28,000 individuals m−1 from March to August 2000) (Walter and Liebezeit, 2001).
The low larval concentrations observed at several sites likely contributed to the comparatively low settlement densities. Future increases in offshore hard substrate associated with the ongoing “mytilisation” of the North Sea (Kerckhof et al., 2021) may alter this pattern. Many studies investigating the appearance and settlement of Mytilus spp. in offshore environments focused on their attachment to the rigs of oil and gas platforms and the foundations of wind turbines and transformer stations. These investigations primarily include fouling organisms (Ralph and Troake, 1980), community structure (e.g., Wolfson et al., 1979; Forteath et al., 1982; Stachowitsch et al., 2002), biomass-to-size ratios (e.g., Richardson and Seed, 1990; Richardson et al., 1990; Qvarfordt et al., 2006), depth zonation (Rule and Smith, 2005), regional differences (García, 1991), and growth in relation to food availability and temperature (Page and Hubbard, 1987). Kerckhof et al. (2021) found substantial hard substrate settlement at OWFs in the southern North Sea. Another offshore-related study described the settlement on the research platform FINO, 45 km off the Island of Borkum in the German Bight (Joschko et al., 2008; Orejas et al., 2005). The percent coverage of the platform piles and the colonization at various depths were investigated. However, there are no data available on the density of settlement and the numbers of larvae. Hutchison et al. (2020) observed populations of blue mussels on North American OWFs, and reported spat abundances on buoys in St. Lawrence Gulf in Canada. On these suspended substrata, they observed a gradient in spat number from the inner sheltered regions to the outer more exposed waters of the Gulf. Off the coast of Santa Barbara, mussel spat was collected from an oil structure for commercial purposes (Lindsey, 1985). Unfortunately, there was no information on the settlement of this farm. Nearshore spat settlement approximately two nautical miles off the coast of New Zealand () ranged from 844 to 2.562 individuals m−1 and was attributed to the adjacency to the mussel farms one nautical mile away.
Whether larval abundance will increase in response to the expansion of OWF foundations remains unclear and requires further investigations. According to the data in Table 1, the current settlement success is low at some locations and partly within the lower optimal range. The lower settlement in the offshore area can also have its advantages: while seed production takes place in classic mussel culture on longlines, the substrates are then thinned out in order to be re-settled at a lower density (no competition for space and food) (Hickman, 1992; ). Here, the second step can be skipped because the density of the offshore farm corresponds approximately to the density after thinning prior to step 2. Furthermore, the absence of various macroparasites in the offshore area could also be advantageous as these can disrupt mussel growth or cause an increased mortality rate at high infestation rates. found that mussels farmed in offshore sites were completely free of macroparasite infestation and therefore experienced better farming conditions.
A limitation of the present study is that only one mooring system per site could be deployed. Although multiple collectors were installed within each frame, these collectors cannot be considered fully independent replicates because they were exposed to the same hydrodynamic and operational conditions. Consequently, unrecognized site-specific artifacts, such as changes in frame orientation, fouling intensity, collector exposure, or mechanical malfunction, may have influenced the settlement estimates. Although all rotating couplings (e.g., swivels and chains) were still in working condition after the moorings had been retrieved, this fact should nevertheless be mentioned. Given the considerable logistical and financial challenges associated with offshore deployments, replication at the mooring level was not feasible within the scope of this study. The results should therefore be interpreted as exploratory indicators of relative settlement potential across sites. Future investigations would benefit from replicated mooring systems to better quantify spatial variability and strengthen statistical inference.
Conclusions
Although the settlement of Mytilid larvae was low in 2003, the results described above are restricted to one season. Incomplete data acquisition due to bad weather conditions and the loss of moorings influenced the quality of the results. Nevertheless, these challenges provide valuable information regarding the technical constraints associated with offshore aquaculture in the North Sea and provide an insight into the type of structures required to survive harsh environmental conditions. The present findings should therefore be considered exploratory and hypothesis-generating. Future studies employing replicated mooring systems at selected sites would allow a more rigorous assessment of settlement variability and improve confidence in the estimates of offshore seed mussel production potential.
The natural distribution of Mytilid larvae in the water column can be regarded as one key factor for the success of such offshore mussel farming. If recruitment remains consistently low offshore, seed production is unlikely to be economically viable, unless larval supply increases substantially in the future. The environmental data indicate that food resources are generally available throughout the year to supply food for filter feeders. Further studies on larval distribution and settlement dynamics are required to contribute to a better understanding of the aquaculture potentials of offshore areas. In particular, a larval drift model would need to be developed to successfully establish mussel farms, including site selection and the timing of when to deploy spat collectors to allow as many post-larvae as possible to attach to the artificial substrate.
Statements
Data availability statement
The raw data supporting the conclusions of this article will be made available by the authors, without undue reservation.
Ethics statement
The manuscript presents research on animals that do not require ethical approval for their study.
Author contributions
BB: Conceptualization, Data curation, Formal Analysis, Funding acquisition, Investigation, Methodology, Project administration, Resources, Software, Supervision, Validation, Visualization, Writing – original draft, Writing – review & editing.
Funding
The author(s) declared that financial support was not received for this work and/or its publication.
Acknowledgments
The study was part of the projects called “Offshore Aquaculture” (grant number FV 142), “MytiFit” (grant number FV 168) and “AquaLast” (grant number FV 174), all financed by the Ministry for Construction, Environment and Transport in Bremen (Germany). The ship time to conduct the transect between the City of Bremerhaven and Helgoland was financed by the steering group on the operation of medium-sized research vessels as well as by AWI. The author would like to thank Susanne Spahic (AWI), Gerd Liebezeit, Uwe Walter and Inge Walter (formerly of Research Center Terramare - FTM), as well as Sabine Strieben (AWI), who contributed their very knowledgeable technical assistance during the investigations in 2003. The author also gratefully acknowledges the dedicated help of the crews of the Research Vessels RV Heincke, RV Uthörn, Aade, Diker (all AWI), and Remzy (FTM) and the Helgoland research diving crew. Finally, the author thanks the reviewers who contributed their time and critical expertise helping to improve the manuscript.
Conflict of interest
The author(s) declared that this work was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.
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Summary
Keywords
larval distribution, multi-use, mussel farming, Mytilus spp., North Sea, offshore wind farm, offshore/exposed aquaculture, spat settlement
Citation
Buck BH (2026) Mytilid larval appearance and settlement at offshore wind farm sites in the German Bight—a trial to estimate multi-use potentials for bivalve seed collection and grow-out cultivation. Front. Aquac. 5:1823282. doi: 10.3389/faquc.2026.1823282
Received
04 March 2026
Revised
29 June 2026
Accepted
29 June 2026
Published
29 July 2026
Volume
5 - 2026
Edited by
Charles Weirich, NOAA National Sea Grant Office, United States
Reviewed by
David Rudders, College of William & Mary, United States
Hafsa Janah, University Abdelmalek Essaadi - Polydisciplinary Faculty Larache, Morocco
Tomás Isola, CONICET IIDEPyS, Argentina
Updates
Copyright
© 2026 Buck.
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*Correspondence: Bela H. Buck, Bela.H.Buck@awi.de
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