How submerged cages affect salmon welfare and behaviour

Submerged cages are an exciting new method being tested by the salmon aquaculture industry to avoid salmon lice infestations within farms. However, salmon do not cope with long-term submergence as they need to refill their swim-bladders regularly to maintain buoyancy. Assoc. Prof. Tim Dempster together with researchers from the Institute of Marine Research, Norway tested how salmon coped in submerged cages that were lifted to the surface weekly to allow surface access. Three submerged cages were positioned at 10 m depth were deployed for 8 weeks, and measures of welfare and behaviour were compared against three standard cages. They found that submerged fish swam 1.4 to 3.4 times faster, schooled tighter, and their swim bladder volumes declined gradually from the beginning to the end of each week. When cages were lifted and the surface became available, negatively buoyant fish immediately exhibited jumping and rolling behaviour. However, they found no evidence of acute buoyancy problems during submergence, and growth rates and welfare scores were similar to standard cages. Their research suggests farming salmon in submerged cages with weekly surface access is a viable method to prevent lice infestations.

 

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Methods to prevent and treat biofouling in shellfish aquaculture

Biofouling in bivalve aquaculture can cause economic losses for the industry. Therefore, strategies such as avoidance, prevention and treatment to minimise biofouling are key. The type of rope used to collect spat or grow bivalves can prevent or reduce fouling by harmful species, but this method largely remains untested. Additionally, a range of eco-friendly treatment measures also exist, but their effect on common biofoulers are unknown. Researchers from the University of Melbourne tested biofouling accumulation and spat collection for seven commercially used ropes with ambient and heated seawater, acetic and citric acid, and combinations of both applied across a range of exposure times to two commercially grown shellfish and three biofouling species. They found that rope type and treatment type were successful on some biofouling species without adversely affecting shellfish.

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Can cold water be used to remove lice?

Many Norwegian Atlantic salmon farms are using warm water thermal delousing to control salmon lice in farms. However, treatments can lead to poor welfare outcomes for fish. Kathy Overton alongside researchers from the University of Melbourne and Institute of Marine Research tested if reverse thermal delousing by rapidly reducing water temperatures to very low treatment temperatures could reduce salmon lice without any negative side effects for fish. To do this, they tested the effects of transferring salmon from 15°C to cold water at different temperatures and durations. They found that treatments of −1°C water for 10 min and 1°C for 240 min treatments reduced mobile lice loads, but created more skin and eye damage than controls. While cold water treatment reduce mobile lice numbers, the cold shock reaction in salmon (illustrated in the video below) was identified as a major hurdle in industry-scale application.

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Struggling to breathe: small salmon struggle with hypoxia more than larger salmon

Dissolved oxygen is fundamental to the fitness and survival of fish. When there is not enough oxygen available in the water, hypoxic conditions occur which can have significant implications for the growth, feed intake and survival of fish. Monitoring dissolved oxygen saturation is one of the most important environmental factors analysed in Atlantic salmon aquaculture. To mimic the reduced oxygen levels experienced by fish in crowded streams and in commercial salmon aquaculture farms, researchers from the Institute of Marine Research, including SALTT-lab alumni Dr Tina Oldham, researched how fish of different sizes coped with hypoxic conditions. They tested how metabolic rate and swimming performance of Atlantic salmon in three size classes (0.2, 1.0 and 3.5 kg) were affected by exposure to 45-55% dissolved oxygen saturation. They found that while hypoxia did not affect standard metabolic rate, it caused a significant decrease in maximum metabolic rate and resulted in reduced aerobic scope. Further, swimming speed for small (0.2 kg) salmon was reduced by 23%, whereas large (3.5 kg) salmon were able to have slightly lower or similar swimming speeds compared to standard conditions. Their research illustrated that moderate hypoxia reduces the capacity for activity and movement in Atlantic salmon, with smaller salmon most vulnerable to hypoxic conditions.

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Identifying ‘firebreaks’ to reduce marine parasite spread

Parasite and disease outbreaks are a common issue for many aquaculture industries around the world, and efficient strategies to control the spread of them are scarce. The Atlantic salmon aquaculture industry is growing globally, with Norway producing the most salmon worldwide. However, salmon lice infestations hinder the growth of the industry and can have negative welfare outcomes. Salmon lice larvae are released from and transported among salmon farms by ocean currents, which create inter-farm networks of louse dispersal. Dr Francisca Samsing along with researchers from the University of Melbourne, Deakin University, and Institute of Marine Research investigated if introducing no-farming areas or ‘firebreaks’ could disconnect dispersal networks of salmon lice. Using a model to predict louse movement along the Norwegian coastline and analysis to identify potential firebreaks to dispersal, she identified one firebreak that split the network into two large unconnected groups of farms. She also found farms that should be removed during spring to prevent wild salmon migrating out into the ocean from getting bombarded with high infestation pressures. If applied to the industry, her model should help lower infestation pressure both at farms and in wild salmon populations.

 

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Effects of salmon farming density on wild cod reproductive fitness

Sea cage fish aquaculture attracts large aggregations of wild fish that opportunistically feed on farm waste. Over time, these fish can undergo physiological changes, and captive feeding trials indicate possible negative effects on their reproductive fitness. However, not much is known about the significance of this phenomenon for reproduction in wild fish over larger spatial scales. Dr. Luke Barrett with researchers from the University of Melbourne and the Institute of Marine Research investigated if coastal areas with intensive aquaculture impacts the fitness of wild fish. They collected Atlantic cod in southwestern Norway from two neighbouring areas with either a high or low density of Atlantic salmon farms, and compared a range of reproductive fitness metrics via a captive spawning trial. They found evidence that cod from the area with a high density of salmon farming produced smaller eggs which led to smaller larvae, indicating a possible reduction in reproductive investment among cod from the intensive salmon farming area.

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Does aquaculture create ecological traps for wildlife?

With aquaculture industries expanding around the world, there are growing concerns about their environmental impacts and effects on wildlife. Aquaculture farms are thought to either repel, act as a population source, or act as an attractive population sink (or ecological trap) for a variety of species. To assess the state of knowledge on the impacts of aquaculture on wildlife worldwide, researchers from the University of Melbourne led by Dr. Luke Barrett conducted a review and meta-analysis of empirical studies to better understand the outcomes of interactions between aquaculture operations and wildlife. Effects of aquaculture on wild populations depended on the wild taxa and farming system. Overall, farms were associated with a higher local abundance and diversity of wildlife, but this effect was mostly driven by aggregations of wild fish around sea cages and shellfish farms. Birds were also more diverse at farms, but other taxa, such as marine mammals, showed variable and comparatively small effects. While they identified evidence for widespread aggregation ‘hotspots’ in several systems, the authors also found that very few studies collect the data needed to assess impacts of aquaculture on the survival and reproduction of farm-associated wildlife. Such data will be crucial for determining whether the behaviour of aggregating around farms results in higher or lower population growth for farm-associated wildlife.

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Depth influences fish welfare when tagged

Electronic tags are tools used all over the world for studying aquatic animal behaviour. However, tags can have negative welfare outcomes and can also cause behavioural manipulation. While conducting different experiment, Dr Daniel Wright from the Institute of Marine Research discovered negative tagging effects on fish held in depth-modified cages. Fish were kept in unmodified cages and depth-modified cages which forced fish below or into a narrow seawater or freshwater filled snorkel. While all tagged individuals survived in the unmodified cages, survival was reduced to 62% in depth-modified cages. Further, survivors in depth-modified cages spent less time above 4 m compared to those in unmodified cages, and dying individuals tended to position in progressively shallower water. Overall, they found that the internal tag weight and volume affected buoyancy regulation, survival, and behaviour of tagged fish. Dr Wright recommends that future tagging studies on aquatic animals should carefully consider the buoyancy-related consequences of internal tags as well as the inclusion of data from dying tagged animals when estimating normal depth behaviours.

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Recent funding in Norway

Members of the SALTT lab and our colleagues have had resounding success in the recent Aquaculture funding round in Norway. Samantha Bui has her own project with a 2 year post-doc salary on ‘Parasite- and host-driven characteristics of infestation success in salmon lice (Lice-IS)’, Nick Robinson will lead a project called ‘Gene editing for elucidating gene function and refining genomic selection for CMS resistance in Atlantic salmon’ which will use the hot new CRISPR technology, and Lars Helge Stien was also successful for a project on ‘Optimising feed withdrawal for safeguarding fish welfare’ to investigate a very routine practice that we know very little about.


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