Prospects and opportunities for mussel Mytilus trossulus farming in the southern Baltic Sea (the Gulf of Gdańsk)
Article Category: Original research paper
Online veröffentlicht: 31. März 2022
Seitenbereich: 53 - 73
Eingereicht: 30. Juni 2021
Akzeptiert: 18. Nov. 2021
DOI: https://doi.org/10.26881/oahs.2022.1.06
Schlüsselwörter
© 2022 Adam Sokołowski et al., published by Sciendo
This work is licensed under the Creative Commons Attribution-NonCommercial-NoDerivatives 4.0 International License.
In many countries around the world, the economic use of bivalve mollusks involves the exploitation of natural resources and farming (mariculture) using specific technical infrastructure. Shellfish-related activities are primarily concentrated in the shallow water zone of supralittoral and sublittoral (permanently submerged zone), where abiotic conditions, such as high availability of suspended organic matter and reduced hydrodynamics, favor the development and rapid growth of bivalves. Harvested bivalves are mainly used for human and animal consumption as a valuable component of a high protein diet (FAO 2016). Potential applications of bivalve soft tissue and shells also include small-scale manufacturing of everyday objects (e.g. handicrafts, jewelry, furrier's tools, musical instruments, fabrics and even means of payment) as well as the production of antifouling agents (
The growing interest in mussel farming has also been observed in the Baltic countries over recent years (Kotta et al. 2020 and references therein). In the Baltic Sea, owing to low salinity limiting the growth of many marine organisms (Sanders et al. 2018), bivalve farming for consumption by humans is not justified (
Figure 1
Location of the mussel farms in the Baltic Sea (based on the original map of de Grunt 2019, GIS data: © EuroGeographics for the administrative boundaries; Laboratory of GIS, University of Gdansk for the contour of the Baltic Sea).
The objective of this study was to assess the potential of mussel farming in the Gulf of Gdańsk for the purpose of human consumption, nutrient uptake and improvement of the quality of the coastal environment. Preliminary technical conditions (e.g. selection of substrate for seeding, basic parameters of structural elements) and selected environmental aspects such as farm location, its optimal depth and production time were also determined. Combining the obtained biological and chemical data with economic estimates allowed a comprehensive evaluation of the production and economic performance of mussel farming in the southern Baltic Sea. In addition, regulatory and administrative aspects were identified to present the current legal framework and local requirements for Baltic mussel cultivation.
In the Polish sector of the southern Baltic Sea (Polish Exclusive Economic Zone, EEZ), mariculture of mussels has not been attempted so far. The first experimental mussel aquaculture of
The biological and chemical results obtained allowed evaluation of the potential of mussel farming for the purpose of human consumption, nutrient uptake and overall improvement of the coastal environment quality. Preliminary technical conditions (e.g. choice of substrate for seeding, basic parameters of structural elements) and selected environmental aspects such as farm location, depth and production time were also determined. Due to increased eutrophication, small depth and limited water dynamics, the Gulf of Gdańsk (particularly its westernmost part – Puck Bay; the southern Baltic Sea) provides good conditions for mussel farming. The presence of research centers and the accumulation of industrial and agricultural activity in the nearby Tri-City agglomeration and the Pomeranian region create favorable opportunities for the utilization of the bivalve biomass produced. The gulf is the natural habitat of the mussel
Pilot mussel farming in the Gulf of Gdańsk was carried as a field experiment at three coastal sites located in the sublittoral zone with a water depth of 11.8 to 12.0 m: 1) adjacent to the Głębinka Strait (site GLE), 2) close to the discharge of sewage from Grupowa Oczyszczalnia Ścieków (group wastewater treatment plant) “Dębogórze” (site MEC), and 3) near the city of Sopot (site SOP; Fig. 2). At each site, one experimental unit was deployed between April 2009 and April 2012. A single experimental unit consisted of five 7 m long polypropylene braided hawser ropes (32 mm diameter) submerged in a vertical position about 5 m apart. Underwater buoys were attached to each rope to compensate for increasing mussel biomass (Figs 3 and 4). Each rope was equipped with a 40 kg concrete anchor and heavy sinking ropes between two neighboring anchors to mitigate the impact of waves. The design of the experimental units closely followed the commonly used submerged long-line system, where vertical culture ropes are connected by a horizontal carrying line. To prevent damage to the structure by random hydrological events (storm, ice cover) or destructive human activity (theft, devastation) and to constrain the growth of algae on the ropes, the farm structure was completely submerged 2.5 m below the water surface (Fig. 3; Sami Alias 2014). The ropes served as both collectors of spat and a substrate for the growth of sedentary mussel juveniles and adults.
Figure 2
Location of the experimental mussel farming sites in the Gulf of Gdańsk (source: Sami Alias 2014, modified). Dashed line border of Puck Bay within the gulf.
Figure 3
Diagram of the mussel farming structure in the Gulf of Gdańsk.
Figure 4
Culture ropes at the MEC site in Puck Bay after two years of exposure.
Mussels were collected by retrieving one culture rope at each site using the SCUBA diving technique every four months in the first year of farming and at the end of the following two years. The ropes were then divided into four sections: 0.0–1.0 m, 2.0–3.0 m, 4.0–5.0 m and 6.0–7.0 m corresponding to water depth zones: 3–4 m, 5–6 m, 7–8 m and 9–10 m, respectively, and each section was frozen at −20°C.
After thawing, all bivalves were gently scraped from the entire surface around the rope in three randomly selected subsections (subsamples), each 10 cm high (total surface area of 100.5 cm2). The shell length (along the longest anterior-posterior axis) of each individual from each subsection was measured to the nearest 0.01 mm using a digital caliper. The organisms were then divided into groups of 5 mm length and counted. Next, the mussels were deshelled and the soft tissues were weighed to determine the individual soft tissue wet weight (shell-free wet weight, SFWW). The flesh and shell were air-dried for 72 h at 50°C to measure individual soft tissue dry weight (= shell-free dry weight, SFDW) and shell dry weight. Biomass of bivalves at each subsection was computed as a sum of individual weights of all mussels, which was then averaged for three subsections and recalculated per 1 m long rope section (total surface area of 1005 cm2). The annual growth rate of mussels was estimated as the mean shell increment for all individuals in each rope subsection per year. Filtration capacity of mussels (i.e. total volume of water filtered by mussels on one rope during a day and over a surface area of 2 ha) was calculated for the entire culture rope after two years of farming, assuming a filtration rate of 6.6 dm3 h−1 g−1 SFDW, i.e. the value determined for the population of mussels inhabiting coastal waters of Askö (Sweden) in the central Baltic Sea (Kautsky et al. 1990), and uninterrupted filtration throughout a day. The choice of the source data was justified by the similar saline regime at both locations (7.0 and 6.5 in the Gulf of Gdańsk and Askö, respectively) and the same unit. Direct estimates of mussel filtration in the Baltic Sea are scarce, and measurements were usually made under different laboratory conditions and the obtained data were expressed in different units, e.g. dm3 h−1 ind. −1 (Clausen and Riisgård 1996; Riisgård et al. 2013) or dm3 h−1 (Theede 1963). All these constraints may hinder direct geographical comparisons of physiological parameters of mussels across environmental variables and make it difficult to standardize the results. Elemental analyses of mussel soft tissue – Perkin Elmer Series II CHNS/O Analyzer for C and N, and wet high-pressure digestion followed by photometrical measurements (Grasshoff et al. 1999) for P – were carried out using individuals with a shell length within a range of 20.1–40.0 mm, which were randomly collected in triplicate at all sites after two years of farming. The content of C, N and P was expressed as % SFDW. In addition, basic environmental parameters (salinity, temperature, dissolved O2) were measured in the overlying bottom water (ca. 20 cm above the seafloor) with a portable WTW Multiset 340i meter equipped with TetraCon 325 and CellOx 325 electrodes.
Seawater salinity varied from 5.8 (April 2009 at MEC) to 8.4 (April 2010 at GLE), temperature from 5.5°C (April 2010 at MEC) to 20.7°C (August 2009 at MEC), and dissolved oxygen concentration ranged from 2.0 mg dm−3 (August 2009 at SOP) to 15.4 mg dm−3 (April 2010 at SOP), indicating normoxic conditions. Environmental parameters at all farming sites were within the range typical for the near-bottom zone in the shallow areas of the Gulf of Gdańsk (Sokołowski 2009) and Puck Bay (Sokołowski et al. 2015).
Despite the ice cover in the winter of 2009–2010 and a severe storm with extremely strong winds (gusts up to 130 km h−1) from the east (generating large waves), the submerged farm units survived and did not move on the bottom. Mussels were found in large quantities on culture ropes in all seasons, with abundance varying with water depth and farming site (Table 1). Massive colonization of ropes by mussels indicates high availability of larvae throughout the year and thus the ease of obtaining spat for farming purposes in the studied regions of the Gulf of Gdańsk (Fig. 5). During the entire farming period, the largest density of the smallest organisms (< 2 mm in length) on the ropes was observed in December 2009, i.e. after 243 days (8 months) of farming (Table 1). Polypropylene ropes thus proved to be a durable and effective substrate for spat collection, which is an important element for planning mariculture on a larger scale. The number of juvenile forms on collectors was substantially higher in the vicinity of Sopot (SOP; nearly 150 000 ind. m−1 rope, i.e. 21 326 ind. m−2) than at MEC (79 000 ind. m−1 rope, i.e. 11 232 ind. m−2), which is likely due to the higher density of mussel beds on the sea floor close to Sopot (Kruk-Dowgiałło & Szaniawska 2008) and subsequently a larger number of larvae in the water column in the open part of the gulf (Bielecka et al. 2000).
Figure 5
Culture ropes after four (a) and 12 months of exposure (b).
Average length (mm) and growth rate (mm year−1) of mussels on ropes as a function of exposure time, depth zone and location of mussel farming site in the Gulf of Gdańsk in 2009–2012 (Sami Alias 2014, modified)
| Exposure time (year) | Depth zone (m) | MEC | GLE | SOP | |||
|---|---|---|---|---|---|---|---|
| Mean shell length | Shell growth rate* | Mean shell length | Shell growth rate | Mean shell length | Shell growth rate | ||
| 1 | 3–4 | 6.7 | 6.7 | 5.8 | 5.8 | 4.2 | 4.2 |
| 5–6 | 6.2 | 6.2 | 5.4 | 5.4 | 3.7 | 3.7 | |
| 7–8 | 7.0 | 7.0 | 5.4 | 5.4 | 3.0 | 3.0 | |
| 9–10 | 4.7 | 4.7 | 4.4 | 4.4 | 3.2 | 3.2 | |
| 2 | 3–4 | 9.0 | 4.4 | 11.3 | 5.5 | 10.0 | 4.9 |
| 5–6 | 9.6 | 4.7 | 14.2 | 6.9 | 9.0 | 4.4 | |
| 7–8 | 9.2 | 4.5 | 11.1 | 5.4 | 7.2 | 3.5 | |
| 9–10 | 12.4 | 6.0 | 10.6 | 5.1 | 9.7 | 4.7 | |
| 3 | 3–4 | 7.8 | 2.6 | 13.1 | 4.3 | ||
| 5–6 | 11.8 | 3.9 | 8.5 | 2.8 | |||
| 7–8 | 7.2 | 2.4 | 13.1 | 4.3 | |||
| 9–10 | 9.8 | 3.2 | 7.3 | 2.4 | |||
Due to the varying duration of the growing period in subsequent years of exposure (from 367 to 376 days), the shell growth rate was recalculated for a standardized period of 365 days across all sites
After the first year of farming, the mussels reached an average length of 3.0 mm in the deepest section of the rope (SOP) to 6.7 mm in the shallowest section (MEC). After two years of exposure, the average shell length ranged from 7.2 mm to 14.2 mm in the depth zones of 7–8 m (SOP) and 5–6 m (GLE), respectively, while after three years, the average mussel shell length ranged from 7.2 mm to 13.1 mm in the deepest and shallowest zone (SOP), respectively. The youngest specimens demonstrated the largest growth dynamics (from 3.0 mm year−1 to 6.7 mm year−1) and the growth rate decreased with the age of bivalves, reaching only 2.4 mm year−1 in the third year of farming (Fig. 6; Sami Alias 2014). The growth rate of mussels growing on polypropylene ropes in the Gulf of Gdańsk was similar to that of bivalves inhabiting the sea floor at a similar depth in the gulf (Cuena Barron & Wołowicz 1980). It exceeded the annual growth rate of bivalves cultured in the coastal waters of central-eastern Sweden (2.2–3.1 mm year−1; Kautsky 1982) and western Finland (3.4–3.8 mm year−1; Antsulevich et al. 1999), suggesting that mussels grew faster in the southern Baltic Sea. The elevated growth rate of mussels likely reflects the high eutrophication status of this area, which receives considerable loads of nutrients from the Vistula River and agricultural runoff (HELCOM 2018b). In comparison to the sheltered areas of the western Baltic (30 mm year−1; Loo & Rosenberg 1983) or the tidal zone in fully saline estuaries of western Europe (50 mm 18 months−1; Bayne & Worral 1980), the growth of mussels in the gulf was, however, markedly slower, which inevitably results from the brackish nature of this water basin.
Figure 6
Wet biomass, i.e. soft tissue and shell (a) and mean shell length of cultured mussels (b) after different growing periods in the coastal zone of various regions of the Baltic Sea. For comparative purposes, biomass data were normalized to a common unit of kg m−1 of rope, belt or net. The insert map indicates the location of the farms; source of the graph: Minnhagen (2017) based on data from: Engman (2009); Wennström & Engman (2014); Lindahl (2012); Olofsson et al. (2014); Ek Hening & Åslund (2012); Sami Alias (2014); Schröder et al. (2014); Bucefalos project (2015); Moltke Lyngsgaard et al. (2017).
In order to determine the potential of mussel aquaculture for the purpose of human consumption, nutrient mitigation and improvement of marine environment quality, an increase in bivalve biomass was assumed to be of key importance as it indicates the potential of mussels to gain weight, purify water and remove biogenic compounds. At all study sites, the largest total mussel dry wet weight (i.e. soft tissue and shell, from 487 g m−1 rope at MEC to 1642 g m−1 rope at GLE) was recorded in the 3–6 m depth zone after 767 days of exposure. The highest efficiency of mussel production in the Gulf of Gdańsk was observed after two years of exposure. It is worth noting that prolonged mussel farming (up to three years) did not lead to a further increase in biomass and even caused a partial loss of the total mussel wet weight (Table 1), presumably as a result of the fact that large mussels were overgrown by small ones and the largest individuals fell off the culture ropes (Gagnon 2019). Bivalve biomass decreased gradually with water depth, reaching the lowest values in the near-bottom zone of 9–10 m depth (54 g m−1 rope after one year at SOP), where food availability is limited compared to the shallow zone (Wołowicz et al. 2006). Due to favorable thermal conditions and good light penetration, small planktonic algae develop most intensively in the shallow water layer throughout the growing season (March–October) providing mussels with high-energy and nutrient-rich food (Gosling 2004). The shallow euphotic zone down to ca. 6 m water depth should be therefore considered the most efficient for mussel biomass production in the gulf, despite potential impact of wave action and ice cover. Particularly good trophic conditions (i.e. high concentration of nutrients and increased primary production in the water column) occur in the western part of the gulf, where much greater growth of mussel biomass was observed (MEC and GLE). Since this part of the gulf is inhabited by numerous and diverse species of other hard-substrate fouling epifauna (e.g. barnacles
Harsh meteorological conditions in the southern Baltic Sea, including severe and extreme weather events such as increased storm frequency and severity in autumn and spring pose a threat to farming infrastructure and marine operations. Special focus on these risks and risk mitigation strategies is therefore a preliminary condition for mussel farm planning and management in this area (Ahsan & Roth 2010). Similarly to other coastal regions of the Baltic Sea, the formation and drift of sea ice and high-energy short waves (Lindahl 2012) can cause serious damage to farming infrastructure in the Gulf of Gdańsk. The optimal technological solution includes therefore farming modules that can be submerged (or optionally submerged) below the water surface permanently or over the wintertime. Based on our experience from the presented pilot study, the so-called “long-line” system, which supports submerged suspended cultures, can be recommended to reduce risk from damage and to ensure durability. Another technical solution with potential application is the commercial SmartFarm system (Smart Farm AS), which is widely and successfully used on different scales in mussel farms throughout Europe (e.g. Przedrzymirska et al. 2018; Hylén et al. 2021).
The traditional submerged long-line system consists of an anchoring system (anchors, anchor lines), lifting elements (buoys, floating lines and floaters), as well as single culture ropes (collectors) connected by a supporting line. Anchors are used to anchor the ends of the supporting line to the bottom. The harvesting of mussels requires raising the culture ropes on the vessel deck. The system is secured in the water column by stabilizing surface and submerged plastic buoys, whereas the collectors are attached to the horizontal supporting line and they are suspended in a vertical position parallel to each other (Fig. 7). The system does not include any rigid structure on the water surface or in the water column and presents minimal resistance to the effects of weather and sea. The much weaker movements of the structure under adverse hydrological conditions cause less wear on anchor lines and shackles (Johns & Hickman 1985). The system recommended in the Gulf of Gdańsk consists of single modules, each 200 m long, with a distance of 10 m between modules, which allows the installation of five modules over a surface area of 10 000 m2 (1 ha). The length of culture polypropylene ropes is 7 m to cover the water depth optimal for mussel growth (3 to 6 m) and their diameter is 32 mm. The ropes are spaced 0.5 m apart, resulting in 400 ropes in each module. The modules are anchored with 3.0 t concrete anchors (or 2 x 1.5 t anchors) at each end parallel to the direction of the sea currents prevailing in the area of interest. Buoys of 35–40 dm3 ensure the buoyancy of the system and are used as the main line floats and corner boys (3 buoys attached to anchor lines) following the recommendations of Bonardelli (2013) and Bonardelli et al. (2019). Basic technical information on the culture module in the long-line system is presented in Table 2. Assuming that the expected total biomass of mussels farmed is 4.5 t (this study) and that mussels lose 75% of their weight due to buoyancy in water (Bonardelli et al. 2019), 28–33 main line buoys should provide sufficient buoyancy for production of mussels over two years. When submerging the modules at more exposed sites, it is recommended to install also intermediate compensation floats with counterweights to stabilize and keep the supporting line horizontal at a proper depth during the entire farming period. It may also be advisable to increase the distance between the modules with increasing water depth (e.g. distance = 1.5 x water depth) and to increase the weight of an anchor to 5 t under open sea conditions as suggested by Bonardelli (2013) and Bonardelli et al. (2019).
Figure 7
Long-line farm system (source:
Selected technical parameters of the culture module in the long-line system
| Parameter | ||
|---|---|---|
| Length of supporting line/head rope | 200 | m |
| Length of growing rope (collector) | 7 | m |
| Diameter of head rope | 32 | mm |
| Diameter of growing rope | 32 | mm |
| Buoys buoyancy | 35 | dm3 |
| Number of buoys | 30 | |
| Total length of growing ropes | 2800 | m |
In many coastal and offshore areas around the world, mussels are mainly cultivated for human (and animal) consumption, contributing significantly to the nutritional quality of human diets (Chi et al. 2012; Suplicy 2020). Mussel meat is a cheap source of high-quality protein, including essential amino acids (EAA) in proper proportions, and is therefore considered to provide multiple dietary benefits (Astorga España et al. 2007). Due to salinity-induced osmotic stress, mussels in the Baltic Sea rarely reach marketable size (> 35 mm) within a reasonable farming time (Westerbom et al. 2002). In this study, only 1.0 to 1.7% of the mussels grown on the culture ropes reached shell length greater than 30 mm and only 0.2% of the mussels were > 35 mm long. Mussels from the southern Baltic have therefore limited potential for gastronomic use and direct human consumption (albeit still possible). Other ways of exploitation and end uses of mussels have been therefore developed in the Baltic countries with potential importance for animal nutrition and improvement of marine environment quality. These include the replacement of fishmeal by mussel flesh as poultry feed containing valuable amino acids, oils and proteins for meat and egg production, the use of mussel flour in the production of fish feed (Árnason et al. 2015; Kraufvelin & Díaz 2015; Suplicy 2020 and references therein), the use in animal feed production as high-quality calcium-rich food (Lindahl & Kollberg 2008; Jönsson 2009) and for plant cultivation as a good alternative for production of fertilizers (Spångberg et al. 2013). The production of mussels involves a large quantity of shells that need to be managed. It is worth noting that mussels can be processed along with their shells into feed for some animals (especially birds). The shells themselves can be used as a liming agent in acid soils, which also increases soil fertility. Other potential applications of shells include their use as biofiltration medium, water pH buffer, calcium supplements, bactericides, fillers, construction material, artificial bones, dehalogenating agents, adsorbents and catalysts (Jović et al. 2019 and references therein). As primary consumers, mussels occupy a low position in the marine trophic chain and their use in the production of fish feed is considered reasonable also in terms of sustainable management of marine resources (Muminović 2010; Lindahl 2013). This aspect may be of particular importance in the Baltic Sea, where stocks of many commercial fish species have been overexploited in recent years for fishmeal production (HELCOM 2018c).
The direct effect of mussel farming on the water column is primarily the improvement of water transparency through reduction of organic suspended particulate matter (phytoplankton and small detritus). Due to their natural filtration activity, mussels take up suspended particles from the ambient water and incorporate organic matter into their body. Bivalves thus reduce organic matter and the associated elements and chemical compounds, nutrients, silt, bacteria and viruses, and increase light transmission which, in turn, improves the condition of submerged vegetation (Shumway et al. 2003). In this study, the total volume of water filtered by bivalves on one culture rope during a day was estimated at 157 449.6 dm3. When expanding the scale of long-line mussel culture up to a total surface area of 20 000 m2 (i.e. comprising 4000 ropes, each 7 m long, grouped in 10 modules), mussels would filter ca. 18 893 952 m3 of water in 30 days, which corresponds to 426 384 m3 ha−1. A high rate of water filtration by bivalves results in effective removal of suspension from the water column, which may be of particular importance during warm summer periods with intensive blooms of phytoplankton algae, including species producing toxic compounds (e.g. cyanobacteria such as
Through filtering and taking up suspended particles, mussels assimilate and incorporate considerable amounts of carbon (C), nitrogen (N) and phosphorus (P) into their body structures (Jansen et al. 2011). These nutrients are stored in mussels and are removed from the marine environment when mussels are harvested (Petersen et al. 2016). Based on the results obtained at site MEC, which showed the largest increase in wet and dry biomass of mussels after two years of exposure, i.e. 11.2 kg of wet weight (soft tissue and shell) rope−1, the maximum potential for using large-scale mussel farming for nutrient removal from water was assessed. The following initial culture conditions were assumed in the calculations: i) the mussel farm is constructed using the long-line system and consists of 10 modules, each 200 m long, over a total surface area of 20 000 m2 (2 ha); ii) each module contains 400 vertical culture ropes, each 7 m long, spaced 0.5 m apart; iii) modules are located every 10 m in parallel; iv) the growth rate, biomass gain and filtration of mussels are the same on all culture ropes regardless of their position in the module, and v) the exposure period is two years. The estimated total wet mussel biomass (soft tissue and shell) reached 44.8 t (22.4 t ha−1). The production efficiency of
Summary of biological parameters of mussels
| Parameter | (this study) | (Taylor et al. 2019) | (Kotta et al. 2020) | |||
|---|---|---|---|---|---|---|
| Puck Bay (Gulf of Gdańsk) | Limfjorden | Kumlinge (Åland archipelago) | Sankt Anna (Östergötland archipelago) | Kiel (Bay of Kiel) | ||
| farming period | 2009–2011 | 2017–2018 | 2010–2012 | 2016–2018 | 2011–2012 | |
| farming technology | long-line | long-line | nets | spat catching ropes | ropes with collector bands | |
| total wet biomass (soft tissue and shell) and SFDW (kg m−1) | 1.60/0.142 | 0.72/– | 3.40/– | 2.5/– | ||
| total biomass production (wet soft tissue and shell; t ha−1) | 22.4 | 41.0–90.4 | 16.0* | 20.4* | 16.7* | |
| mean content of C/N/P in SFDW** (%) | 48.6/9.8/1.1 | –/3.2–3.8*** /0.6*** | –/4.4*** /0.6*** | –/5.6–7.5*** /0.6*** | ||
| content of elements in SFDW per 1 m rope (g m−1): | C | 80.5 | ||||
| N | 16.2 | 3.7 | 23.3 | 22.3 | ||
| P | 1.8 | 0.3 | 1.8 | 1.6 | ||
| yield of elements in one t of harvested mussels (kg): | C | 43.1–43.7 | ||||
| N | 8.3–8.7 | 13.7 | ||||
| P | 1.0 | 0.9 | ||||
| maximum total removal of elements (kg ha−1): | C | 1159 | ||||
| N | 234 | 600–1270 | 83 | 140 | 148 | |
| P | 26 | 40–100 | 6.4 | 10.8 | 10.8 | |
converted for a standard farm area of 1 ha;
shell free dry weight;
source data (% soft tissue wet weight) were converted to % SFDW using an SFDW/SFWW conversion factor of 0.159% after Rumohr et al. (1987); – no data
Another potentially beneficial effect on the marine environment is the accumulation of mussel shells detached from culture ropes and other organic elements such as shells of other mollusks (e.g. snails), calcareous shells of crustaceans (e.g. barnacles) or fragments of plants underneath the mussel farm. Shell clusters on the sea floor offer a complex spatial structure and create new benthic microhabitats and trophic conditions that facilitate the development of mobile (e.g. snails, crustaceans, polychaetes, demersal fish) and sessile (e.g. mussels, sponges) benthic fauna (Wilding & Nickell 2013; Kotta et al. 2020). In consequence, there is a local increase in species diversity and abundance of macrobenthic communities, which in turn attract other animal species such as predatory fish and birds (Morrisey et al. 2006; Varennes et al. 2013). High mussel densities associated with aquaculture can thus increase the biodiversity of an area (Byron et al. 2011), and mussel farms can act as floating artificial reef systems for both pelagic and benthic fish species (Wang et al. 2015).
The settlement and growth of a large number of bivalves on culture ropes, deployed on a relatively small area, can have adverse effects on the surrounding environment. By filtering water, mussels capture suspended particles, some of which are removed as rapidly depositing pellets of pseudofeces (before transport to the mouth) or feces (after passing through the digestive system). They form aggregates rich in organic matter and glued together by polysaccharide compounds (mucus), which sink to the bottom (Gosling 2004) and subsequently increase the organic matter pool in surface sediments in the immediate vicinity of the farm (Kaspar et al. 1985; Burkholder & Shumway 2011; Suplicy 2020; Wikström et al. 2020). As a result of increased sedimentation, organic matter accumulates in the sediments where it undergoes aerobic transformation, a process that enhances nutrient regeneration from the seabed and requires oxygen to oxidize soluble and refractory organic matter (McKindsey et al. 2011). Intensive decomposition of biodeposited organic matter can seasonally result in increased phosphate, silicate and ammonia fluxes, as well as oxygen depletion and hypoxia (oxygen concentration ≤ 2 cm3 dm−3) in the overlying bottom and interstitial water (Carlsson et al. 2009; Stadmark & Conley 2011). Mineralization of fresh biodeposists can thus lead to localized impacts on resident benthic organisms through reduction of oxygen concentration and alteration of geochemical conditions, e.g. accelerated release of ammonia and phosphate from sediments (Grant et al. 1995). For example, increased concentrations of ammonia and phosphates in the bottom zone impede the development of benthic vegetation (Vinther et al. 2008). Particularly susceptible to organic matter loading is the seagrass
Mariculture in Poland is considered a developing sector and one of the nine maritime economic sectors with the greatest potential in many coastal areas in the future (Brodzicki et al. 2013). The overall main legal requirements for the establishment of mussel farms are derived from European legislation such as the Water Framework Directive (MFD), the Marine Strategic Framework Directive (MSFD), the Nature 2000 Framework and their individual national implementations, amended by special national laws (for instance fisheries law or special private regulations to use marine areas;
Since a large part of the Gulf of Gdańsk lies within the Pan-European Natura 2000 protected areas network (Special Protection Area PLB220005 Zatoka Pucka and Special Area of Conversation PLH220032 Zatoka Pucka i Półwysep Helski) and the Coastal Landscape Park (Nadmorski Park Krajobrazowy), special regulations for the development of mussel farming should also be considered when planning to locate mussel farms. In the case of establishing submerged culture structures within Natura 2000 areas, the implementation of the so-called good environmental practices for effective integration with areas of the greatest natural value plays an important role. These include adapting aquaculture to the requirements of the European directives on the protection of biodiversity, ensuring the development of sustainable and environmentally friendly aquaculture or supporting production systems that promote diversification of economic activities and lead to their “environmentally friendly” status (Gil 2009). In addition, pursuant to the Act of 18 July 2001 Water Law, the entire Gulf of Gdańsk was classified as coastal waters, which in accordance with Directive 2000/60/EC imposes an obligation that any activities carried out in this area do not deteriorate the ecological status of its waters.
The economic performance of mussel farming was assessed using the experimental results of this study and the market retail prices of materials and services available for the individual consumer in 2020. Prices in PLN were converted to EUR at the average exchange rate published by the Polish National Bank for January–February 2020 (
Due to the lack of a market for mussels and trade from Baltic farms in Poland, the price of raw material remains difficult to estimate. The available market prices of mussels offered in Polish shops for human consumption, which come from the North Sea and the Atlantic, range from EUR 7 to over EUR 9 kg−1 (this study). Based on an extensive interview with experts, Ozolina & Kokaine (2019) estimated the market price of Baltic mussels to be on average EUR 4.0–5.0 kg−1 for frozen mussels and EUR 6.0–10.0 kg−1 for fresh mussels. In Denmark, small mussels are sold for EUR 100 t−1, i.e. EUR 0.1 kg−1. In this study, EUR 5.0 kg−1 and EUR 0.1 kg−1 were therefore assumed for fresh mussel for human consumption and for animal feed production, respectively.
The cost of a single farming module in the long-line system was EUR 8555 and the costs of individual components of the system and equipment used to harvest and sort mussels are listed in Table 4. The minimum total investment costs amounted to EUR 107 924 and included EUR 85 550 for modules and EUR 22 374 for equipment (i.e. stripping and declumping/sorting machines). As suggested by Schernewski et al. (2012), the expected lifetime of the equipment used to support growth and harvesting of mussels under the Baltic conditions (depreciation period) is 15 years. When assuming that modules will therefore be used for mussel farming in the Gulf of Gdańsk for eight two-year production cycles to reach the highest mussel production efficiency, and the equipment for 16 cycles (with harvesting every two years), the basic investment cost for one cycle was ca. EUR 12 092. To be able to harvest mussels every year, two independent farms should be operated simultaneously, one being established one year after the other, i.e. two farms are started within two consecutive years.
Estimated costs (EUR) of materials (including shipping) required for one 200 m long long-line module and equipment used for mussel harvesting in the long-line system
| Component | quantity | price |
|---|---|---|
| Polypropylene twisted rope (32 mm diameter) used as the main and anchoring lines (including provision for ties and loops) | 260 m | 491 |
| Polypropylene braided hawser rope (32 mm diameter) used as a growing line (including provision for securing ends) | 2860 m | 6111 |
| Polypropylene twisted rope (18 mm diameter) for tying up growing lines and buoys | 440 m | 265 |
| Polypropylene twisted rope (6 mm diameter) to prepare loop cords for setting attachment points on the mainline | 220 m | 15 |
| Submersible buoys with a buoyancy of 35 dm3 | 38 items | 970 |
| Self-made concrete blocks (anchors) of 1.5 t (including the cost of purchasing concrete, materials to build wooden molds, reinforcing bars, stainless steel rods for line attachment loops) | 4 items | 703 |
| TOTAL | 8555 | |
| Basic harvesting equipment (stripping and declumping/sorting machines) | 22374 |
Based on the experience of suspended culture aquaculturists from other Baltic coastal areas and pilot assessments conducted in this study, the annual labor effort for the deployment was assumed to be 150 man-hours (three men, 50 h each). Maintenance and monitoring of the farm during the two-year production cycle amounted to 130 man-hours (for one or two men depending on the tasks) and the workload for mussel harvesting was 1260 h (three men working simultaneously). After including an hourly rate of PLN 50 (including all elements of tax-deductible costs) into calculation, the cost of labor for one production cycle totaled EUR 18 054. The working time of a boat/vessel for all marine operations, such as deployment of the farms, their maintenance, inspection, as well as mussel harvesting and initial processing during the two-year production cycle was estimated at ca. 600 h, which resulted in renting cost of EUR 45 000 at a unit rate of EUR 900 for 12-hour vessel rental.
The total investment and operational/maintenance costs for a mussel farm in the Gulf of Gdańsk over a two-year production cycle amounted to EUR 69 670 (EUR 6258 in deployment costs, paid at the start of the farm establishment, was spread evenly over 8 production cycles) but the actual costs should be considered higher. Additional costs account for onshore labor (preparing the long-line system, sorting of mussels, etc.) and other items that are variable over time and difficult to estimate. They include interest on fixed capital and miscellaneous costs such as insurance premium, accounting services and administrative costs, road transport of structural elements and harvested bivalves, an onshore facility to store equipment and to perform onshore activities such as tying and repairing modules and other equipment, as well as office and licensing costs. Additional costs will also arise during farm decommissioning, and in addition to the costs of removing farm modules from the water, they may also include costs for disposal of the materials used. Depending on the size of the farm, the duration of farming and the method of financing (purchase, lease, rent, etc.), these additional costs can constitute up to 19% of the total costs (Buck et al. 2010). It is worth noting that the calculation did not include either potential costs of cleaning the mussels in filtered seawater pools or their storage in refrigerators prior to sale for human consumption, i.e. the procedures which are commonly employed for cultured mussels in many European countries (Lees et al. 2010). In addition, if mussels are not acceptable for human consumption or for use in animal feed production due to bacterial and/or chemical contamination, disposing of mussel biomass will be necessary. The disposal of bivalve waste will then generate additional costs, which vary from ca. EUR 25 to ca. EUR 100 per 1 t depending on the rendering plant and the classification of waste based on its composition and hazard. The largest part of the operating costs is the vessel rental, so for vessel owners, e.g. local fishermen who use boats for fishery and/or aquaculture, production costs may be substantially lower.
Given that only 1.0 to 1.7% of the mussels cultured in the pilot experimental study reached a shell length greater than 30 mm and 0.2% above 35 mm, the economic model assumes that 1.7% (ca. 0.76 t) of the total mussel biomass (44.8 t) is sold for direct human consumption and the remaining 98.3% (ca. 44.04 t) for animal feed production. The total revenue was then EUR 8204, i.e. EUR 3800 and EUR 4404 from sales for consumption and feed production, respectively. The total income from mussel farming in the gulf can therefore cover only ca. 12.0% of its total costs (cumulative investment and operational/maintenance costs), indicating negative budget balance (deficit) for such an investment. Even when assuming that local fishermen use their own boats for all marine operations, aquaculture is not profitable. For this reason, development of mussel farms in this basin can be justified only for the purpose of improving the quality of the marine environment. Extensive mussel farming for nutrient mitigation and reduction of primary production in the water column (so-called “mitigation culture”; Taylor et al. 2019) has recently received increasing attention in many Baltic countries and has been suggested as a promising abatement option (Gren 2019). This is particularly so as activities aimed at combating eutrophication and ameliorating the environmental status in coastal areas of special importance for local marine spatial planning (e.g. ecotourism) can be supported by additional funding programs such as the European Maritime and Fisheries Fund (EMFF) and the Natural Capital Financing Facility (NCFF; Schultz-Zehden et al. 2019). Another source of funding is in line with the polluter pays principle that requires polluters to take measures to reduce pollution, to measure pollution, and in some cases to pay taxes or charges for pollution and compensate for pollution impacts (ten Brink et al. 2009; Schultz-Zehden et al. 2019). As suggested by Lindahl & Kollberg (2009), significant financial support for mussel farming can also come from additional taxation of fertilizers that are used in agriculture or diversion of some agricultural subsidies. The estimated cost of removing 1 kg of nitrogen with mussels harvested in the Gulf of Gdańsk was ca. EUR 130, i.e. nearly eleven times more than the corresponding cost incurred by a conventional wastewater treatment plant in the water treatment process. For example, in the wastewater treatment plant “Dębogórze” north of Gdynia, where the average cost of treating 1 m3 of water is EUR 1.17 and the nitrogen removal efficiency reaches 90%, the cost of removing 1 kg of nitrogen was estimated at EUR 11.84. This contradicts the findings of Petersen et al. (2016) that mitigation mussel production could be a cost-effective measure for nutrient removal relative to much more expensive onshore measures. According to Schultz-Zehden et al. (2019), another alternative for funding mussel farming includes companies and persons that benefit directly or indirectly from the improved environmental quality, such as private foundations, individuals through crowdfunding, companies from profitable economic sectors or public authorities. The income from mussel farming can also be generated from selling carbon credits to industry, i.e. emission certificates for CO2 that is imbedded in shells of harvested mussels, thus contributing to the reduction of greenhouse gas concentrations in the atmosphere. For example, using verified Certified Emission Reductions (CERs), Schernewski et al. (2012) estimated a price of carbon dioxide capture in mussel shells at EUR 12 t−1, which can bring additional economic incentive.
Recognizing high potential of the Gulf of Gdańsk for bivalve aquaculture due to the presence of a natural mussel population on the one hand and the necessity to improve the state of the marine environment on the other hand, the pilot farming of