Aquaculture nets are increasingly recognised as a potential source of microplastic (MP) emissions to the marine environment, yet their contribution has not been quantified. This study assessed MP emissions from aquaculture nets manufactured with different materials and coatings, and subjected to different cleaning technologies, to identify practical reduction measures. Laboratory abrasion tests were conducted on new and used nets made from nylon, high density polyethylene (HDPE), and ultra-high molecular weight polyethylene (UHMWPE), with two coatings (standard and premium). New, uncoated nylon nets released over five times more MP than HDPE and UHMWPE nets, with coatings, particularly the premium formulation, further increasing MP release from nylon nets. UHMWPE nets showed no coating-related increase in MP, suggesting stronger coating integration and higher resistance to abrasion. A custom net pen constructed using different material and coating combinations was deployed at sea. Individual panels were cleaned using pressure washing, cavitation or robotic brushing, and any MP released during cleaning were collected by pumping seawater over stainless steel filters. The resulting MP levels were comparable to background levels, while microscopy revealed that robotic brushing caused less coating damage than pressure or cavitation cleaning. Field sampling during net cleaning at a salmon farm showed sporadic MP emissions, influenced by the presence/absence of lice skirts. At service sites, land-based washing produced detectable MP levels, but filtration systems effectively prevented marine discharge. By integrating laboratory and field data, this study identifies combinations of net materials, coatings and cleaning technologies that could minimise MP emissions and contribute directly to improving aquaculture sustainability and operational practices.

The elasticity and flexibility of nylon mooring ropes offer many advantages, including a potential to reduce dynamic loads and by that fatigue damage and extreme loads in mooring elements. However, there is very limited experience with use of nylon ropes in permanent mooring of floating structures. To ensure their integrity during a lifetime of operation in wind and waves, it is important to study short- and long-term stiffness properties, strength, and elongation of nylon ropes in varying sea temperatures. Nylon ropes for moorings are made of high tenacity PA6 multifilament yarn. These fibres hold a high tensile strength to weight ratio and are known to have good abrasion resistance. However, their non-linear stress-strain behaviour at different temperatures in the saturated wet state, is not fully documented. Mechanical testing of mooring ropes is expensive and time consuming due to their large dimensions and high strength, and limited access to relevant test equipment. Thus, when feasible, it will be beneficial to perform extensive testing and parameter studies on the nylon yarn used in rope production. This includes studying the effect of different water temperatures on stiffness and strength of nylon yarn. This paper presents a test set-up and procedure for tensile testing of yarn in a water tank with varying temperatures. Results from tensile testing of two different batches of PA6 multifilament yarn submerged in water temperatures of 4, 15 and 25 degrees C water are given. Both quasi-static stiffness and strength properties are presented based on multiple tests replicates. Results showed that mechanical properties of PA6 yarn are affected by water temperature, as both strength and stiffness increased as temperature was reduced. Yarn breaking load increased from an average of 126 N at 25 degrees C to 143 N at 4 degrees C, while yarn stiffness (given as a function of strain) at low strain increased from an average of 212 N at 25 degrees C to 259 N at 4 degrees C. The tests revealed minor differences in mechanical properties between different spools of yarn.

The expansion of aquaculture production into more exposed harsh and remote ocean environments presents both new opportunities and challenges. To manage the complexities of exposed operations, research into fish welfare, personnel safety, and facilitating technology is thus of key importance. This paper reviews recent research advances in the areas of safety, fish welfare, and technology, while the focus is on the Norwegian salmon farming industry, the results could benefit exposed fish farming internationally. Regarding fish welfare, the study summarizes the current knowledge status of salmon coping abilities and welfare indicators in strong currents and waves. On the safety front, there has been significant progress in operational safety management, accident analysis, and emergency preparedness, all of which are crucial for human personnel in these demanding settings. Human safety and fish welfare also rely on structures and equipment, and recent research results include advances in environmental load analysis, vessel design, simulations of fish farms. Notably, the development of contact-free, autonomous lifting operations, and hole detection methods represents a significant leap in maintaining aquaculture infrastructure. This multidisciplinary study underscores the need for integrated research approaches to address exposed aquaculture, emphasizing that while recent innovations have enhanced safety and robustness, ongoing research and new strategies are critical for safety and fish welfare in exposed aquaculture operations.

The potential future of open ocean aquaculture (OOA) lies in its ability to provide low-impact, sustainable seafood production offshore, offering a solution to meet the rising global seafood demand while addressing climate change and reducing the pressure on coastal areas. However, the development of OOA structures is costly, which emphasises the crucial importance of thorough concept evaluation during the design phase, where any risks associated with the new structure during its operation must be addressed. This involves not only ensuring structural reliability in challenging offshore environments but also ensuring that fish thrive in the enclosed space provided by the structure. A comprehensive, integrated evaluation of novel OOA concepts and design solutions may require special analysis tools and methods, which are found in neither today's offshore engineering nor traditional coastal aquaculture technology. This paper presents methods for a comprehensive analysis of a novel, submersed, flexible enclosure system developed for open-ocean finfish aquaculture in New Zealand. Herein, the enclosure structure is simplified and generalised by considering it as a horizontal fabric cylinder with ends covered by nets. The structure has outer ring structures to stiffen the enclosure and help maintain its cross-sectional shape. A single-point-mooring (SPM) system with either catenary-like or taut moorings allows the structure to freely rotate with the flow while being submerged at depths below which wave actions are reduced. The performed analyses consisted of three major steps needed to evaluate the OOA structure from different perspectives. In step (i), a computational fluid dynamics (CFD) modelling of steady flows through the structure with porous end covers was carried out to assess the internal flow reduction, which depends on the structure's shape and solidity of the net. Here, a CFD approach based on measured hydrodynamic properties of net material was employed. This modelling is necessary for ensuring optimal flow conditions for fish contained within the structure. It also provides water-velocity data needed for predicting the dissolved oxygen (DO) transport. In step (ii), time-domain simulations of the moored OOA structure in dynamic environments with waves and varying currents were conducted. Here, extreme conditions were modelled to evaluate the reliability of the structure and its mooring system under possible design loads. To model the flexible enclosure, a new finite element model based on so-called rotation-free shell elements was developed within the framework of FhSim, which is the simulation software at SINTEF Ocean for modelling flexible marine structures in waves and currents. In step (iii), the FhSim model was used to predict the motion of the structure in a harmonic tidal current. Building upon the results from the steady-state CFD analysis, these transient simulations provided the basis for predicting the DO dynamics within the enclosure depending on water temperature and respiration rates of fish (considering Atlantic salmon as an example). This helped identify when the dissolved oxygen levels within the enclosure may drop and how long it may take for them to recover during the tidal transitions, which is important for fish welfare. It was observed that the motion of the structure connected to an SPM mooring system may have a large impact on internaloxygen levels, offering insights for optimising the mooring design. Overall, the proposed methodology represents an integrated solution for concept evaluation during the design phase of OOA structures, potentially enabling the optimisation of new designs to handle offshore environmental loads while promoting fish welfare.

Abstract Effective mooring of floating offshore wind turbines (FOWTs) is challenging, and conventional mooring designs often require line components with quite large diameters. This is mainly a consequence of large mean environmental loads from wind, namely the wind thrust force, combined with a relatively stiff mooring system. Nylon ropes have the potential to reduce stresses in mooring lines due to their flexible and elastic behaviour. However, there is very little experience with use of nylon for permanent mooring of floating structures. Of particular concern is the long-term endurance and behaviour of nylon ropes in the ocean environment. Furthermore, new material models are required to simulate mechanical properties of nylon ropes, and design analysis tools must be further developed to accommodate these new models. The paper starts by presenting state-of-the-art in the topic of nylon ropes with focus on mooring applications. It reviews and documents advantages of nylon ropes in mooring of FOWTs with results from two case studies, and identifies knowledge gaps. Then it describes the NYMOOR research project, which addresses some of the knowledge and technological gaps. We start with the project objectives. The assumed hypotheses are formulated based on the best knowledge in the topic. Then the project methodology and the research activities are described in detail.

When moving from a very sheltered aquaculture site to a very exposed oceanic aquaculture site, the energy increases proportionally in a continuum. Lojek et al. (in review) considered the primary influential parameters (water current, wave height, wave period, wavelength and water depth) which influence the species, structure, technology, methods, and operational aspects of any aquaculture endeavour and investigated six possible indices which cover these variables. Added to advanced computer modelling, assisted by detailed and constant environmental monitoring, it may be possible to refine site selection, structure selection and design, species selection, equipment and logistic requirements and health and safety requirements. This manuscript has selected two indicative indices: Specific Exposure Energy (SEE) index and Exposure Velocity (EV) index from the potential equations provided by Lojek et al. (in review) and compared them with known operational aquaculture sites highlighting present structural capability and limitations. The two indices are also utilized to reflect on their suitability for assessing sample sites with respect to biological, technological, operational or maintenance aspects of aquaculture activities. The indices have shown themselves to be useful tools in the general assessment of the energy that will influence the species and structure selection at potential aquaculture sites. This information can help prospective fish farmers characterize their sites concisely and accurately to consultants, regulators, equipment vendors, and insurance brokers.

The terms “offshore” and “open ocean” have been used to describe aquaculture sites that are further from the coast or in higher energy environments. Neither term has been clearly defined in the scientific literature nor in a legal context, and the terms are often used interchangeably. These and other related terms (for example “exposed”, “high-energy”) variously refer to aspects of a site such as the geographic distance from shore or infrastructure, the level of exposure to large waves and strong currents, the geographic fetch, the water depth, or some combination of these parameters. The ICES Working Group (ICES, 2024) on Open Ocean Aquaculture (WGOOA) therefore identified a need to define the terminology to reduce ambiguity for these types of aquaculture sites or more precisely, to: (1) promote a common understanding and avoid misuse for different classifications; (2) enable regulators to identify the characteristics of a marine site; (3) allow farmers to be able to assess or quantitatively compare sites for development; (4) equip developers and producers to identify operational parameters in which the equipment and vessels will need to operate; (5) provide insurers and investors with the terminology to consistently assess risk and premiums; and (6) circumvent the emergence of narratives that root in different cognitive interpretations of the terminology in public discourse. This paper describes the evolution of the use of the term “offshore aquaculture” and define the most relevant parameters to shift to a more definitive and robust term “exposed aquaculture” that can inherently relay clearer information. Adoption of this more definitive definition of “exposed” will allow the user to define a site with more than just distance from shore. Key differences and the importance of these terms are discussed that affect various interest groups. Follow-up articles in this compilation from scientific members of the WGOOA as well as other scientists outside ICES are incorporated that develop a set of definitions and a rigorous exposure index.

This work attempts to define metrics for hydrodynamic exposure, using known oceanographic variables to provide a universal site assessment method for mariculture structures. Understanding environmental conditions driving open-ocean mariculture siting is crucial in establishing consistent ocean governance, minimizing adverse environmental impacts, and facilitating economically sustainable farm operations. To provide a metric of oceanic conditions and associated requirements for structural design and operation of aquaculture systems, six Exposure Indices (EI) are proposed that consider physical energy levels related to hydrodynamic forces at a site. Four of the proposed indices consider only environmental conditions, while the other two also consider the dimensions of the gear that is exposed to the external loads. These indices are: Exposure Velocity (EV), Exposure Velocity at Reference Depth (EVRD), Specific Exposure Energy (SEE), Depth-integrated Energy Flux (DEF), Structure-centered Depth-integrated Energy (SDE), and a Structure-centered Drag-to-Buoyancy Ratio (SDBR). While these indices are derived with a focus on aquaculture structures, they may also have applications for estimating biological stressors and operational challenges. The proposed exposure indices were evaluated for a range of known aquaculture sites around the world. A sensitivity analysis was conducted that quantified the relationship between the exposure indices and storm event return period. At a regional scale, hindcast numerical data for the German Bight combined with calculations of 50-year extreme values were used to calculate and map each proposed index spatially. Resulting maps showed that exposure is not simply a function of distance from shore. The six indices show plausible performance regarding the objective assessment of aquaculture sites. The authors herein present the indices to the aquaculture and ocean engineering communities for discussion, application, and potential adoption of one or more of the proposed indices.

This paper presents a comparative study assessing the wear tolerance of rope materials in demersal fisheries, specifically seine ropes and dolly ropes. Fourteen different rope materials were assessed in this study, including conventional and alternative commercially available synthetic polymers, and biodegradable materials including natural fibre ropes and custom-made polyester monofilaments. The sample materials were subjected to controlled wear from a rotating abrasive drum. Tensile testing was performed to determine and compare mechanical properties of the samples before and after exposure to wear. A wear tolerance coefficient has been suggested, i.e. a comparative unit between the different rope material samples and a standard blended polyester/polyethylene rope material as reference. The tested nylon ropes showed the lowest reduction in breaking strength post wear and thus the highest wear tolerance of all tested materials. Conventional and biodegradable polyester ropes and monofilaments also performed well compared to the standard reference rope. The performed tests did not only consider the effect of different raw materials, but the combined effect of material and structural properties. A rope’s tolerance to wear may be affected not only by the mechanical properties of the raw material, but also fibre thickness and cross section, and rope thickness, structure and lay of rope. This study demonstrated the potential of using biodegradable polymers with higher tolerance to wear than conventional non-degradable plastic materials as a circular solution to reduce microplastic pollution caused by demersal fisheries worldwide. Application of alternative commercially available ropes and hard-lay rope structures may increase the tolerance to wear and by that reduce plastic waste.

While highly successful in terms of profitable seafood production, salmon (Salmo salar) aquaculture may also be a source of potential negative environmental externalities. In an attempt to address these challenges through supporting the development of new technology, the Norwegian government has introduced a new class of aquaculture licenses labeled as development licenses. As a result, new technological solutions were proposed to reduce negative externalities through (1) expansion to open ocean areas not yet used for aquaculture and (2) reduced emissions from inshore production systems. This paper presents an analysis of the technological concepts proposed in applications for development licenses. The applications for development licenses provide a unique perspective on what technological directions existing marine aquaculture companies envisage that marine aquaculture may take in the future. The analysis indicates that units will become larger and stronger, as well as being specially designed to suit a variety of environments, creating a more heterogeneous industry. Large offshore structures such as semi-submersible platforms and other strong, rigid structures with permeable enclosures (nets) have been particularly successful in this application process, receiving relatively many development licenses. In sheltered fjord areas, many concepts involving closed enclosures (bags and tanks) have been suggested and awarded licenses.

New types of fish farms are often larger and structurally more complex than conventional fish farming structures, and associated challenges concerning safety and costs increase correspondingly. Thus, increased precision in structural design is required, with estimation of hydrodynamic loads on nets as an important topic. Today, both load coefficients for nets and measured netting dimensions are given with relatively high uncertainties. New knowledge for netting materials with high solidities as well as scaled netting commonly applied in model tests are included in the presented study. Results from towing tests and the development of a new mathematical expression for local drag coefficients (for netting twines) indicate that drag coefficients are not only dependent on solidity and Reynolds number, but may also be affected by the velocity reduction and the local velocity at the twines.

The Norwegian aquaculture industry expands towards sites with a harsher current and wave environment than before, while utilizing larger and more complex designs. This increases the need of precise modelling of hydrodynamic loads on nets to ensure a safe design that minimize risk of failure and avoids over-dimensioning and corresponding increase in costs. Established methods may overestimate drag forces, especially for high solidity nets. In this paper, a new formulation for drag and lift forces on nylon multifilament aquaculture nets has been implemented in a numerical analysis software. The formulation was derived from towing tank tests of net panels with a wide range of solidities. The numerical code has been applied to estimate drag and lift forces on netting cylinders (representing a simplified, scaled net cage model) with four different solidities, similar to previously published physical model tests in a flume tank. The results from the numerical simulations and physical model tests were then compared to validate the new load model. Resulting drag forces from the numerical simulations compared well with measured drag forces from the model tests, especially for the higher solidity netting materials, while numerical estimates of lift and measured total lift forces were dissimilar for some velocities and nets. Possible error-sources and uncertainties have been identified in both the numerical load model and physical tests. A parameter study on the magnitude of drag and lift forces with varying net inclination angle was conducted and indicated that net panels and net cylinders may affect the flow differently. Applying formulas derived from net panel tests may therefore not be straightforward. The study also showed that the estimated total drag forces were dependent on the lift formula and vice versa, due to changes in net cage deformation.

Farmed fish escaping into the wild and other environmental concerns have curbed the expansion of the Norwegian aquaculture industry. Detailed knowledge of both direct and underlying causes of previous escape episodes is crucial to ensure successful development of new technology and targeted safety-measures at fish farms. This paper provides detailed descriptions of both technological, human and organisational factors relevant to escape of fish from Norwegian fish farms during 2010-2018. Fish farmers in Norway are obliged by law to report escape incidents to the Norwegian Directorate of Fisheries. A total of 305 reported escape incidents with Atlantic salmon (Salmo salar) or rainbow trout (Oncorhynchus mykiss) were confirmed from 2010 to 2018, involving in total 1.960.000 registered escapees. Analysis of 298 of these incidents shows that most registered escapees came from sea-based fish farms (92%), while 7% were from land-based facilities and 1% from transportation between sites. Most escape incidents were directly caused by technological factors, with holes in the net as the most common cause of escape. Bad weather or handling of weights and net prior to delousing have been associated with increased probability of escape incidents. In addition to direct and contributing causes, mostly technological, escape incidents may also have underlying causes related to human and organisational factors. These causes may have triggered the incidents or prevented barriers from being effective, with technical damage and escape of fish as result. Relevant human and organisational causes were explored through interviews with employees that have experienced escape incidents.

Drag forces on nets represent the largest contribution to hydrodynamic loads on traditional fish farms and will have a large impact on total loads on new designs utilizing netting as containment method. Precise methods for estimation of drag loads are needed. This article gives new knowledge on hydrodynamic forces acting on aquaculture nets. It presents results from towing tests, including updated drag and lift coefficients for Raschel-knitted netting materials used in nets for aquaculture, and quantify wake effect. The results include high solidity nets and high towing velocities. It was found that drag loads were close to proportional with the netting solidity for netting solidities ranging from 0.15 to 0.32. The wake effect is quantified through the average velocity reduction factor, which is given as a linear function of solidity. Much of the previously published data are close to the data found through these tests. However, for high solidity nets, the deviation is significant. Therefore, previously published data and models may overestimate drag loads for high solidity nets.

•The nylon gillnets caught 21% more fish (in numbers) than the bio gillnets.•Elasticity and breaking strength could explain the major difference in catch efficiency.•The number of times that the bio gillnets were deployed affected their relative catch efficiency.•Although less catch efficient than nylon gillnets, bio gillnets still show great potential for reducing ghost fishing.

This paper presents a comparative study of mechanical properties of biodegradable PBSAT (polybutylene succinate co-adipate-co-terephthalate) and conventional polyamide (PA) gillnets used in Norwegian fisheries. Field tests were performed to simulate abandoned, lost, or otherwise discarded fishing gear. Changes in mechanical properties of PBSAT and PA nets in two Norwegian coastal environments were studied. Samples of biodegradable PBSAT gillnets and PA gillnets were placed inside modified lobster pots at four different locations: two outside the island Hitra in the middle of Norway and two outside Tromsø in the north of Norway. For each pot, seawater temperature was logged each hour, and net samples were retrieved for analyses at 3 to 9 months intervals. Tensile strength testing was performed to determine and compare mechanical properties of biodegradable and PA monofilaments and gillnets. Comparative analyses were conducted, aimed at investigating the different behaviors of biodegradable material and conventional PA material, and the possible influence of seawater temperature on the degradation process of biodegradable PBSAT gillnets. Reduced tensile strength and elongation at break, and a slight increase in stiffness was observed for both PA and PBSAT monofilaments after the field trial at Hitra, indicating degradation of both polymer materials. After 25 months immersion in seawater, the PBSAT gillnets exhibited a significant reduction of tensile strength due to seawater exposure (35%), and the tensile strength of PBSAT gillnets was then 26% lower than the average strength of the PA net samples.

Fishing trials were carried out to compare the relative fishing efficiency of gillnets made of a new biodegradable resin (polybutylene succinate co-adipate-co-terephthalate, PBSAT) with conventional (nylon) nets. The fishing trials covered two consecutive fishing seasons (2016 and 2017) for cod (Gadus morhua) and saithe (Pollachius virens) in northern Norway. Results generally showed better catch rates for the nylon gillnets. The biodegradable PBSAT gillnets caught 50.0% and 26.6% fewer cod, and 41.0% and 22.5% fewer saithe than the nylon gillnets in 2016 and 2017, respectively. Even though the relative catch efficiency of the biodegradable gillnets was slightly better in 2017 than in 2016, the difference with respect to the catch efficiency of nylon gillnets may be too large for biodegradable gillnets to be accepted by fishermen if they were available commercially. Tensile strength measurements of the nylon and biodegradable PBSAT gillnets carried out before and after the fishing trials showed that the both types of gillnets had significant reductions in tensile strength and elongation at break, especially in 2017. Although less catch efficient than nylon gillnets, biodegradable PBSAT gillnets show great potential for reducing ghost fishing and plastic pollution at sea, which are major problems in these fisheries.