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Insight into real-world complexities is required to enable effective response from the aquaculture sector to climate change

['Lynne Falconer', 'Institute Of Aquaculture', 'University Of Stirling', 'Scotland', 'United Kingdom', 'Trevor C. Telfer', 'Angus Garrett', 'Seafish', 'Edinburgh', 'Øystein Hermansen']

Date: 2022-05

The following subsections provide an overview and summary of the key findings linking climate stressors to impacts for the different stages of production and the wider supply chain, as outlined in Figs 3 and 4. Overall, 45 impacts were identified across all stages, and 154 ‘Documented links’ and 58 ‘Potential links’ between climate stressors and impacts. The impacts are not single branches of knowledge, and instead focus on topics that have one or more biological, environmental, technological, socio-economic, or other aspects, highlighting the need for understanding across a range of disciplines to address the real-world challenges that Norwegian salmon aquaculture faces.

Between and within stages, the impacts are not equal, and may differ in both effect and magnitude. For some climate stressors and impacts, more information was available than for others. Temperature was one of the stressors where it was easier to establish ‘Documented links’ with impacts due to the number of studies available and industry reports. In contrast, there were few studies on ocean acidification, and there was uncertainty about how this may affect salmon aquaculture. There were some identified impacts that were common across multiple stages, such as damage to land-based infrastructure, while others were specific to an individual stage. Breeding and Hatchery and Juvenile stages are similar due to their production environments, with operations primarily in land-based flow through or recirculating systems, so they would have a similar exposure to climate stressors.

Gamete quality and early development had ‘Documented links’ to 3 climate stressors (Temperature, Extreme temperatures and heatwaves, Deoxygenation). Gamete quality is dependent on a number of factors including broodstock management and environmental conditions in the hatchery [ 71 ]. Temperature at egg stage affects muscle growth and condition factor later in life [ 90 ]. Sub-optimal conditions, such as low oxygen, during early development may have long-term effects through epigenetic changes [ 72 ]. Hypoxia can affect early development, lead to delayed hatching, reduced growth, deformities, and mortalities [ 73 ]. Gamete quality and early development had a “Potential link” to 1 climate stressor (Precipitation/runoff). Though most research has been done on temperature, there are other aspects of water quality that affect gamete quality [ 91 ], and changes in precipitation and runoff may affect the inlet water [ 92 ].

The broodstock management impact refers to broodstock when held in land-based facilities. Broodstock management had ‘Documented links’ to 2 climate stressors (Temperatures, Extreme temperatures and heatwaves). Water temperatures can affect broodstock performance [ 84 ], and stress can affect offspring survival, growth, and malformations [ 85 ]. King et al. [ 86 ] showed there was a significant reduction in fertility and survival of ova from broodstock held at higher temperatures. Higher temperatures have been shown to inhibit milt production and ovulation [ 87 , 88 ]. Broodstock management had ‘Potential links’ to 3 climate stressors (Ocean acidification, Deoxygenation, Precipitation/runoff). Although most studies focus on temperature, other environmental factors affect reproductive development and spawning, and changes in water quality may affect the health and welfare of the broodstock [ 89 ]. Broodstock salmon may however be held in sea cages until they are ready to spawn, and those salmon would experience the same impacts as the fish in the Growout stage, highlighting the importance of not looking at impact in a siloed way and the need to consider the entire production system.

Breeding programs are an important part of salmon aquaculture as they are used to improve key production traits [ 78 , 79 ]. While they are often considered a possible adaptation response, climate change may also impact breeding programs. There were ‘Documented links’ to 3 climate stressors (Temperature, Extreme temperatures and heatwaves, Deoxygenation). Breeding program managers need to know what the desired traits are and what the potential future conditions could be. Most breeding programs have focused on faster growth [ 78 ] and growth is strongly influenced by temperature and oxygen [ 80 , 81 ], so these climate stressors will affect decisions on breeding strategies. However, it is also important to understand the trade-offs in trait selection, as recent work on rainbow trout has suggested that resistance to hypoxia and resistance to temperature are linked to two genetically different traits [ 82 ]. Breeding programs also had ‘Potential links’ to 2 climate stressors (Ocean acidification, Precipitation/runoff). Although ocean acidification and changes in precipitation/runoff are linked to several impacts in later stages, it is unclear if breeding programs would focus on these impacts, so these were considered ‘Potential’. Overall, the uncertainty of future conditions is a challenge, as chosen breeding goals may be inappropriate for the complexities the industry will face in the future [ 83 ].

Inlet water quality had ‘Documented links’ to 4 climate stressors (Temperature, Extreme temperatures and heatwaves, Deoxygenation, Precipitation/runoff). Temperature and oxygen are important parameters for hatcheries and early stages [ 70 – 73 ], and it is important that conditions are kept within optimal ranges, and gradual temperature increases as well as heatwaves could affect inlet water quality. Changes in rainfall may also affect inlet water quality, for example runoff could lead to increased suspended solids [ 70 ], or metal contamination [ 74 ].

Availability of inlet water had ‘Documented links’ to 3 climate stressors (Temperature, Extreme temperatures and heatwaves, Precipitation/runoff). Temperature and precipitation will affect water availability in water sources (e.g., rivers, lakes, groundwater). Results from modelling studies suggest the duration of droughts in some parts of the country will increase in the future due to increasing summer temperatures leading to increased evaporation [ 35 , 64 , 69 ]. Increased droughts could lead to reductions in water levels and streamflow, which may affect the availability of water for hatchery systems. Some areas will experience increased precipitation and runoff, as well as increased snowmelt, which may result in increased water levels [ 35 ], depending on the location and the time of year. Both RAS and flow through systems would be affected by water availability, though RAS have much lower water demands since they reuse a high proportion of their water [ 70 ].

There were 7 impacts identified within Breeding and Hatchery ( Fig 3 ), all of which were linked to multiple stressors. Damage to infrastructure had ‘Documented links’ to 4 climate stressors (Sea level rise and extreme water levels, Storms, Extreme temperatures and heatwaves, Precipitation/runoff). Breeding and hatchery facilities are located close to the coast ( Fig 1 ), and studies have shown that many Norwegian coastal areas are susceptible to flooding from storm surges due to sea level rise [ 40 , 41 ], which may cause damage to buildings and wider infrastructure. Storms are also known to cause destruction within affected areas, e.g. disruption of power supplies and damaged buildings and infrastructure [ 66 ]. Heatwaves, or sudden temperature changes, and/or precipitation can lead to avalanches, rockfalls, and landslides which can result in infrastructure damage [ 67 , 68 ].

There were 5 impacts identified within Juvenile production ( Fig 3 ). As juvenile production facilities are similar to hatcheries (land-based flow through or RAS), there were 4 impacts (Damage to infrastructure, Availability of inlet water, Water quality of inlet water, Energy use) exposed in a similar way to the same climate stressors, so to avoid repetition, see Section 3.1.1. The fifth impact was a broad grouping ‘Biological impacts’ which was used as an overview of the potential biological considerations, and there were ‘Documented links’ to 4 climate stressors (Temperature, Extreme temperatures and heatwaves, Deoxygenation, Precipitation/runoff). These stressors are associated with the conditions in the juvenile production environment that are important for further growth and development, not only in this stage, but also later life. Increased temperatures and poor water quality may stress fish, impair development, and result in deformities [ 93 – 97 ]. Metal contamination in inlet water from precipitation/runoff affects the development of smolts and causes issues for seawater transfer [ 74 , 98 , 99 ].

3.1.3. Growout.

The Growout phase had 22 identified impacts, which was the highest number compared to other stages (Fig 3). This is unsurprising since cages are open to the environment, directly exposing the fish to climate stressors. As a result, some of the impacts are more detailed than the other categories, but still include broad groupings due to the level of information that is available. The Growout stage assessment focused on coastal cage systems. Existing and future development and implementation of new production technologies such as closed containment systems, offshore cages and land-based RAS will affect the overall vulnerability in the Growout stage to climate stressors, but how and to what extent is uncertain.

Damage to land-based infrastructure would have the same links to stressors as in the previous production stages (Section 3.1.1. for an overview). Damage or failure of cage infrastructure had a ‘Documented link’ to 1 climate stressor (Storms). Storms have been shown to damage cages [100], sometimes leading to escape events [101]. Human safety issues and disruption of activities also had a ‘Documented link’ to 1 climate stressor (Storms). Rough conditions make it difficult to access farm sites and can lead to unsafe working conditions [102]. Holen et al. [103] showed the highest number of injuries occurred in autumn and winter months, when weather conditions are challenging.

Fouling of cage infrastructure had “Documented links” to 3 climate stressors (Storms, Temperature, Extreme temperatures and heatwaves). Storms could affect access to sites and the ability to clean nets, which is a common approach for biofouling control in Norway [104]. Fouling development is often associated with temperatures, with highest abundance in summer months and peak summer temperatures [105,106]. Fouling of cage infrastructure had “Potential links” to 3 climate stressors (Ocean acidification, Deoxygenation, Precipitation/runoff). Dobretsov et al. [107] reviewed studies and found mixed reports on the effect of ocean acidification on biofouling communities. Oxygen levels influence ecosystem structure, but effects on biofouling communities are complex, especially as species can be producers as well as consumers, of oxygen [107,108]. Changes in rainfall/runoff may affect nutrient levels in the water column which could affect biofouling populations, however there are still knowledge gaps about the role of nutrients from the surrounding area, as well as the role of aquaculture production, in biofouling development [109,110].

Dispersion and assimilation of wastes had ‘Documented links’ to 4 climate stressors (Storms, Temperature, Deoxygenation, Precipitation/runoff). Salmon farming releases wastes into the environment including uneaten feed and faeces, which can accumulate in the area underneath and surrounding the fish cages [111–113]. Storms can lead to sediment resuspension [114], although this will also depend on the depth of the site. Settling rates of wastes can vary depending on a number of factors including water viscosity (e.g. temperature and salinity) [115]. Many Norwegian salmon farms are located in fjords, and studies have shown that warming is leading to reduced oxygen levels in some areas [55]. Deoxygenation can reduce assimilative capacity [55,116]. Precipitation and runoff will contribute organic matter to the fjord or coastal environment and the associated microbial activity could affect overall assimilative capacity of the environment [117]. Dispersion and assimilation of wastes had a ‘Potential link’ to 1 climate stressor (Extreme temperatures and heatwaves). While links between temperature and dispersion and assimilation of wastes have been established, the link with extreme temperatures and heatwaves is less clear.

Stocking density had ‘Documented links’ to 3 climate stressors (Temperature, Extreme temperature and heatwaves, Deoxygenation). The maximum allowable biomass (MAB) at a farm is defined by regulations and licences [118], nevertheless optimal density varies depending on life stage, water quality, feed strategy, management practices and design of the system [119,120]. Salmon adjust their position in the cage depending on temperature and oxygen, and may crowd in certain areas [13,121]. High densities can have negative consequences for health and welfare [121–123]. Consequently, increased temperature and decreased oxygen could lead to farm managers reducing stocking density to improve health and welfare, and overall growth and production. Stocking density had ‘Potential links’ to 2 climate stressors (Storms, Ocean acidification). During storms, high stocking densities could mean greater risk of collisions, but there is limited information or demonstratable examples that link storms to stocking density. It is unclear if ocean acidification would affect salmon behaviour (see behaviour impact), and thus, it is uncertain if there would be implications for stocking density.

Feed intake and utilization had “Documented links” to 5 climate stressors (Storms, Temperature, Extreme temperatures and heatwaves, Deoxygenation, ‘Precipitation / runoff’). Feed is usually supplied through automated feeders, but if conditions are too stormy, then distribution of feed could be affected, and/or salmon behaviour and feed intake changed [124]. The link between feed intake, utilization and temperature is well documented [80,125,126]. During a heatwave event in Tasmania, salmon reduced feed intake and stopped feeding [127]. Maximal feed intake is also affected by oxygen and temperature and may also be size-dependent [81]. The microbiota in the gastrointestinal tract play an important role in fish nutrition, growth and health [128], and temperature, salinity and other aspects of water quality can all influence the diversity of species within the gut [129,130]. Feed intake and utilization had a ‘Potential link’ to 1 climate stressor (Ocean acidification). Cominassi et al. [131] showed that restricted feed intake exacerbated effects of increased temperatures and ocean acidification, but research is needed on whether ocean acidification would impact feed intake and utilization.

Growth had “Documented links” to 6 climate stressors (Storms, Temperature, Extreme temperatures and heatwaves, Ocean acidification, Deoxygenation, Precipitation/runoff). Storms can create challenging conditions for salmon. If current speeds are too strong then salmon spend more energy on swimming, which could lead to reduced growth potential [124,132,133]. Temperature is the main environmental factor influencing salmon growth [80]. Extreme temperatures and heatwaves also affect growth [127], although it will depend on the duration and magnitude of the event. Oxygen is also an essential factor, with low oxygen levels negatively affecting growth [81]. Few studies have considered salmon aquaculture and ocean acidification, although in a short-term experiment, McCormick and Regish [134] found an increased growth rate in fish held in higher CO 2 conditions. Most other studies have considered CO 2 for land-based systems, and shown that as CO 2 increases, the growth rate will decrease [135,136]. Thus, although this was considered a ‘Documented link’, as there is a link between the stressor and impact, further research is required into the effects of ocean acidification on salmon growth. Salinity is not considered a major factor that affects growth rate, although there is a link, Handeland et al. [137] indicated there may be some long-term growth advantages of lower salinity. As mentioned in Feed intake and utilization, environmental factors such as temperature and salinity can affect gut microbiota, with implications for salmon growth [129,130].

Behaviour had ‘Documented links’ to 5 climate stressors (Storms, Temperature, Extreme temperature and heatwaves, Deoxygenation, Precipitation/runoff). Storms affect fish behaviour as fish adjust their position in cages in response to waves and currents [124]. Salmon individuals and groups have shown avoidance to high and low temperatures, indicating active behavioural thermoregulation [138,139]. During a heatwave event, salmon were observed moving to deeper and colder waters within the cage [140]. Salmon also move in response to oxygen levels [138,141,142], and have salinity preferences [13]. Oppedal et al. [13] noted that turbidity, as a potential consequence of runoff, has been suggested as a factor influencing salmon swimming behaviour but no studies were found that demonstrate this. Behaviour had a ‘Potential link’ to 1 climate stressor (Ocean acidification). The impact of ocean acidification on fish behaviour is a source of debate amongst researchers [143], and there is a need for further work on salmon.

Stress had ‘Documented links’ to 5 climate stressors (Storms, Temperature, Extreme temperature and heatwaves, Deoxygenation, Precipitation/runoff). Bad weather conditions and storms, resulting in high waves and fast current speeds can increase stress in salmon kept in sea cages [144]. Increased temperatures and decreased oxygen levels are also known to stress salmon [13,142]. Transcriptomics revealed stress responses in salmon following periods of extreme temperatures and heatwaves, and upon periods with low levels of oxygen [145–147]. Runoff could lead to increased suspended solids and decreased water quality, which could stress the salmon [92,148]. Stress had a ‘Potential link’ to 1 climate stressor (Ocean acidification). As highlighted previously, there are few studies on ocean acidification and salmon, and it is unclear if ocean acidification would be linked to stress.

Health and welfare had ‘Documented links’ to 6 climate stressors (Storms, Temperature, Extreme temperatures and heatwaves, Ocean acidification, Deoxygenation, Precipitation/runoff). This is a broad impact category as stressors may affect many specific aspects of health and welfare, each of which could be explored at length, but since the intention of this study is to demonstrate the range of considerations across salmon production, a summary category highlighting some of the major issues, and demonstrating complexity of this area was considered more appropriate. Storms and high current speeds can lead to wounds and fin damaged due to collisions [144]. Temperature is known to be linked to health and welfare, for example the development of cataracts [95] and cardiac health [149]. Furthermore, Sundh et al. [150] showed that reduced or fluctuating levels of dissolved oxygen and high temperatures cause primary and secondary stress responses and that the intestinal barrier function is reduced. It is difficult to know the implications of multiple stressors, for example, skin is strengthened after transfer to sea, and this could be temperature related [151], but lower pH reduces the barrier strength of skin [136] and increased temperature reduces the barrier strength and the alters the morphology of the skin [123,152]. Gill health is a priority for salmon aquaculture, since gills are responsible for many critical physiological functions [153] and are in direct contact with the water, and therefore susceptible to environmental conditions, water quality changes, and infection [154–156]. In acid rain experiments, Finstad et al. [157] showed that plasma chloride levels were elevated in fish from the high acid and moderate acid groups compared to reference groups. However, there is a need for more research into the effects of ocean acidification on salmon health and welfare, particularly regarding gill health [158].

Viral diseases had ‘Documented links’ to 2 climate stressors (Temperature, Extreme temperatures and heatwaves). Viral diseases include cardiomyopathy syndrome (CMS), Pancreas Disease (PD), Infectious salmon anaemia (ISA), Infectious pancreatic necrosis (IPN) and Heart and Skeletal Muscle Inflammation (HSMI) [159]. There are known links between temperatures and some viral diseases, although the nature of the influence of temperature varies. Stene et al. [160] showed that seasonal increases in sea temperature triggered outbreaks of PD, whereas higher temperatures at sea transfer decreased the risk of IPN [161]. Additionally, for some diseases such as CMS, fast growth may be a contributing factor for developing a disease [162] and increased sea temperatures could result in faster growth in some areas, thus indirectly increasing incidences of CMS. Viral diseases were ‘Potentially linked’ to 4 climate stressors (Storms, Ocean acidification, Deoxygenation, Precipitation/runoff). The hydrodynamic conditions within an area and connectivity between farms will influence the spread of viruses such as PD [163], but it is unclear what effect storms would have on transmission. Environmental stress (e.g. suboptimal water quality) may also increase risk for diseases such as CMS and HSMI [162,164], and may also increase mortalities [164]. Furthermore, salmon suffering from disease may have reduced tolerance to suboptimal conditions, for example salmon suffering from HSMI have reduced hypoxia tolerance [165]. The immune response of fish is also affected by temperature and oxygen [166], which could affect susceptibility to viral diseases.

Bacterial diseases had ‘Documented links’ to 3 climate stressors (Temperature, Extreme temperatures and heatwaves, Precipitation/runoff). Bacterial diseases and problems include furunculosis, Yersiniosis, Mycobacteriosis, Moritella, Tenacibaculum [159]. There are clear links between temperature and bacterial diseases affecting salmon, for example higher temperatures are risk factors for furunculosis [167] and Tenacibaculosis [168], whilst low temperatures are a risk factor for winter ulcer disease which is caused by the bacterium Moritella viscosa [169,170]. Salinity is also an important factor that influences some bacteria including M. viscosa [170]. However, it is important to note that the development of vaccines, in combination with good hygiene, led to a significant reduction of outbreaks of some bacterial diseases such as furunculosis [159,171]. Bacterial diseases had ‘Potential links’ to 3 climate stressors (Storms, Ocean acidification, Deoxygenation). Storms could increase contact between salmon and/or increase spread of pathogens which may increase transmission of bacterial diseases, but there are many uncertainties about this. As low oxygen (and high temperature) affect the immune status of the fish [166], it may increase susceptibility for bacterial infections, but further research is necessary.

There are a number of parasites that affect Norwegian salmon aquaculture, and also cleanerfish [159]. Two of the most serious infections (sea lice, amoebic gill disease (AGD)) were included here. Parasitic infection (sea lice) had ‘Documented links’ to 4 climate stressors (Temperature, Extreme temperatures and heatwaves, Ocean acidification, Precipitation/runoff). The main species of sea lice that affects Norwegian aquaculture is Lepeophtheirus salmonis, although Caligus elongatus is also an issue at some locations. Temperature and salinity play an important role in the development, reproduction, and life history of sea lice [172–174], and infection pressure is predicted to increase in the future due to increased temperatures [175]. Extreme temperatures and heatwaves will affect sea lice as development is significantly faster at high temperatures [173]. Lice number is also affected by acidic condition with higher densities found at lower pH compared to a reference group [157], and studies suggest sea lice are tolerant of end-of-century conditions [176]. Parasitic infection (sea lice) had ‘Potential links’ to 2 climate stressors (Storms, Deoxygenation). The spread of lice is influenced by temperature and currents [177], so storms and resulting waves and strong currents could influence the spread of sea lice (e.g. Wright et al. [178] found new infestations after a storm event), but this will depend on site characteristics and connectivity between farms, so it is only considered ‘Potential’. It is unclear what effects deoxygenation would have on sea lice, although since oxygen levels can influence the immune system [166] then deoxygenation may affect susceptibility to sea lice infection.

Parasitic infection (Amoebic Gill Disease–AGD) had ‘Documented links’ to 4 climate stressors (Temperature, Extreme temperatures and heatwaves, Deoxygenation, Precipitation/runoff). AGD is caused by the amoeba Neoparamoeba perurans [179]. Although the disease has been a problem in Tasmania since the 1980’s, it was first observed at several farms on the West coast of Norway in 2006, when temperatures were higher than average [180]. It was not seen again until 2012 but has now become a major issue for the Norwegian industry [159]. The two most important risk factors for outbreak of AGD are considered to be seawater temperatures and high salinity [159,181,182]. Reduced oxygen levels are also a risk for gill health [158], and AGD infected fish have been shown to have reduced survival in hypoxic conditions [183,184]. Oldham et al. [184] found that hypoxic conditions accelerate the progression of AGD but noted there are still uncertainties about effects of hypoxia on AGD susceptibility. Parasitic infection (AGD) had ‘Potential links’ to 2 climate stressors (Storms, Ocean acidification). Storms could play a role in movement of the amoeba but, as highlighted by Oldham et al. [179], there are still many knowledge gaps about N. perurans in the natural environment. More acidic conditions could have implications for gill health and AGD but further research is required into this [158].

Cleaner fish had ‘Documented links’ to 5 climate stressors (Storms, Temperature, Extreme temperature and heatwaves, Deoxygenation, Precipitation/runoff). Wrasse and lumpfish are used in salmon aquaculture as a biological control mechanism to reduce sea lice levels. One of the challenges of growing different species within the same environment is the different biology, life histories and environmental tolerances. For the cage environment conditions in which salmon are farmed, ballan wrasse are thought to be less robust than salmon [185]. Lumpfish are poor swimmers [186,187], and may therefore be affected by storms and increased current speeds. Wrasse are less effective at lower temperatures and become inactive in winter so lumpfish are considered a more appropriate species in many parts of Norway [188]. However, lumpfish prefer low temperatures [189,190], and cannot survive extended periods above 18°C [186]. Hvas and Oppedal [187] showed that lumpfish are less sensitive to low oxygen levels than salmon, though lumpfish are slower in responding to sudden changes than salmon. Cleaner fish also have salinity preferences, with lumpfish selecting considerably lower salinities than wrasse [191], with the latter having a low tolerance for freshwater [192]. Cleaner fish also had a ‘Potential link’ to 1 climate stressor (Ocean acidification). There are few studies on ocean acidification and cleanerfish. Sundin and Jutfelt [193] evaluated behavioural impact of increased CO 2 on the goldsinny wrasse (Ctenolabrus rupestris), one of the species used as cleaner fish, and found few effects, suggesting behavioural tolerance to high CO 2 levels. However, the authors note that other studies show there is variation in effects between fish species [193]. Thus, the link was considered ‘Potential’.

Disease prevention and treatment had ‘Documented links’ to 4 climate stressors (Storms, Temperature, Extreme temperatures and heatwaves, Precipitation/runoff). Storms can affect access to site and create difficult working conditions which may affect the ability to treat or use preventative measures, for example Wright et al. [178] tested snorkel technology at a cage site but an intense storm event damaged the equipment. Mortality rates for most delousing methods increases with rising temperature [194]. Some treatment methods should not be used above certain temperatures, for example hydrogen peroxide should not be used to treat AGD above 13.5°C [179,195]. Water quality and salinity may also affect disease prevention and treatment strategies, e.g. Oppedal et al. [196] found that salinity was a major factor affecting the efficacy of snorkel technology. Disease prevention and treatment had ‘Potential links’ to 2 climate stressors (Ocean acidification, Deoxygenation). As shown previously, ocean acidification and deoxygenation may be linked to different aspects of salmon health and so these stressors may also potentially be linked to disease prevention and treatment.

Mortalities had ‘Documented links’ to 4 climate stressors (Storms, Temperature, Extreme temperatures and heatwaves, Deoxygenation). Storm events can result in mortalities [197] and this is a high risk for younger and less robust fish following transfer to sea water. Waves and strong currents may lead to net deformation, reducing available space for the fish and leading to mortalities [198]. Mortalities occur at high temperatures [80,199]. In 2019 there was a multi-factorial mass mortality event at salmon farms in Canada and two of the drivers were prolonged increased temperatures above 18°C and reduced oxygen levels [200]. Mortalities had ‘Potential links’ to 2 climate stressors (Ocean acidification, Precipitation/runoff). As ocean acidification and rainfall/runoff are (or potentially) linked to other physiological and health impacts it is difficult to say there would be no effect, but there is insufficient information at present on direct impact on mortality.

Harmful Algal Blooms (HABs) had ‘Documented links’ to 6 climate stressors (Storms, Temperature, Extreme temperatures and heatwaves, Ocean acidification, Deoxygenation, Rainfall/runoff). Temperature, pH, nutrient availability, deoxygenation, and regional and local hydrodynamics all affect the occurrence of HABs [201–203], and extreme storms and precipitation events have also been directly and indirectly linked to HAB events [204,205]. Due to the multi-factorial nature of HABs it is difficult to predict future occurrences, however the general consensus is that HAB occurrences are increasing in abundancy and frequency, and their range is expanding, with species being found in new areas [201]. In Norway, a range of algae species have been responsible for fish kills, most recently the Chrysochromulina leadbeateri bloom in 2019 that resulted in over USD$100 million losses [206]. As noted by Karlson et al. [206], a previous bloom of C. leadbeateri in 1991 also resulted in substantial losses, but as there was a long time period between the events and monitoring of conditions was inconsistent, it is difficult to identify trends. So, although the links between stressors and HABs are known, there are still many uncertainties in how HAB distributions and frequency of events will change, and it is very difficult to predict where and when blooms may occur.

Jellyfish had ‘Documented links’ to 5 climate stressors (Storms, Temperature, Extreme temperatures and heatwaves, Deoxygenation, Precipitation/runoff). Halsband et al. [207] highlight the lack of knowledge about distribution of jellyfish in Norwegian waters. At a global level there are also many uncertainties surrounding the potential impact of climate change on jellyfish. Mitchell et al. [208] suggest increasing frequency of severe storms in the North Atlantic may be a reason behind rising number of jellyfish blooms reported near salmon farms in Ireland. A review by Purcell et al. [209] noted that temperature and salinity affect abundance of some species, and many jellyfish are tolerant of very low oxygen levels. Studies elsewhere have shown jellyfish populations increase in hypoxic conditions while other species decrease [210], thus altering ecosystem structure. Extreme temperatures and heatwaves can affect jellyfish population dynamics, as shown by Chi et al. [211] for Aurelia aurita, a jellyfish found in Norway that is harmful to salmon [212]. Coastal runoff can contribute nutrients to the aquatic ecosystem, in some cases altering the food web which will have effects on jellyfish populations [209]. Furthermore, changes in precipitation and runoff can affect water clarity, and reduced transparency can benefit non-visual jellyfish over visual predator species [62]. Jellyfish had a ‘Potential link’ to 1 climate stressor (Ocean acidification). There is a lot of uncertainty and debate about the effects of ocean acidification on jellyfish [213], and it is unclear if it would impact jellyfish interactions with salmon aquaculture.

Other coastal users and activities had ‘Potential links’ to all 7 climate stressors (Sea level rise, Storms, Temperatures, Extreme temperatures and heatwaves, Ocean acidification, Deoxygenation, Precipitation/runoff). In addition to aquaculture, the Norwegian coastal zone is an extremely important area for many commercial and recreational activities, like fisheries, oil & gas, agriculture, surfing, and kayaking [214]. Changing environmental conditions in the coastal zone may affect the availability and suitability of areas for all activities, including aquaculture. Furthermore, there is increasing pressure on the coastal environment as stakeholders are demanding more space, including exclusive use of an area [214]. Changing requirements of other users may impact aquaculture as there may be increased conflict over space and fewer opportunities to expand, move or develop new sites. It is likely that stressors will be linked to impacts for other coastal users in some way, but each activity would have to do assessments, similar to the present one for salmon aquaculture, which is why all links were considered ‘Potential’.

Site availability, allocation, and restrictions had ‘Potential links’ to all 7 climate stressors (Sea level rise, Storms, temperatures, Extreme temperatures and heatwaves, Ocean acidification, Deoxygenation, Precipitation/runoff). At present, a relatively large share of Norway’s coastal waters is suitable for salmon aquaculture, as shown by the distribution of sites in Fig 1. However, biophysical changes to environmental conditions may affect the suitability of locations for aquaculture, as has been described in all impacts within this stage. Furthermore, sites and production restrictions are determined through political processes [215], and effects of climate change from all stressors on the sector and other users may affect the public and political perception of intensive salmon farming which could lead to changes in the allocation and regulation of licences, sites and operations. All of the links were considered ‘Potential’ as there is little information available to specifically link the climate stressors to site availability, allocation, and restrictions at present.

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