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.

Read the full article here.


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.

Read the full article here.


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.

 

Read the full article here.


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.

To read the full article, click here.


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.

To read the article, click here.


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.


Temperature drives depth of salmon lice nauplii but not copepodids

Pinpointing the whereabouts of free-swimming salmon lice larvae is vital to successfully formulating lice prevention strategies that reduce contact between them and farmed salmon. Biophysical models are used to estimate salmon lice larvae locations, but model accuracy could be improved by more precisely coding how larvae change depth in response to environmental conditions. In a first set of experiments, we determined larvae swimming depth changes during salinity stratification, to fill in knowledge gaps from previous studies (see “salinity-mediated depth of salmon lice” article in Norskfiskeoppdrett nr 3/2018).

Another variable of interest is temperature. Field data from plankton net sampling has suggested that salmon lice larvae, particularly at naupliar stages, actively seek out warmer depths that optimise development, and attempts have been made to incorporate this information into lice dispersal modelling. However, experimental evidence for this behavioural response to temperature is lacking.

Temperature choice experiments

To uncover the temperature preferences of nauplii and copepodid salmon lice larvae, we produced vertical temperature gradients in 80 cm deep columns. The columns consisted of an inner column housing the larvae and an upper and bottom outer water jacket which could be filled with different temperatures so stable temperature stratification was created in the inner column. Using a salinity of 32 psu in the top and 34 psu in the bottom of the inner column, we were able to create both a warmer and a cooler top layer. The bottom temperature was set at 12 °C, and we varied the top temperature by -6, -4, -2, 0, +2, +4 and +6. Larvae were release at the bottom of columns and their depth distribution was recorded after 1 h (Figure 1).

Figure 1. Photo of Tom Crosbie marking of salmon lice larvae depth positions

Warmer or cooler surface conditions did not alter the depth distribution of infective copepodid larvae (Figure 2b). However, increasing surface layer temperature relative to underlying waters resulted in progressively fewer nauplii entering the surface layer (Figure 2a). Lowering the top layer temperature compared to bottom layer caused increasingly more nauplii to move into the top layer (Figure 2a).

Figure 2. The proportion of a) salmon lice nauplii and b) copepodids in the top layer of columns under varying temperature change. Exponential curves explaining the relationship between larvae in the top layer relative to temperature change are shown.


Implications

Despite the importance of temperature in controlling Atlantic salmon swimming depth, our results suggest this variable is of little or no effect to infective copepodid swimming depth. Infective copepodids may therefore rely more on other environmental (e.g. light and salinity) and host cues (e.g. semio-chemicals and flow) to find their fish hosts. From our results, no depth adjustments to copepodids should be made based on vertical temperature stratification alone in lice dispersal models.

In contrast, nauplii altered their swimming depths in response to vertical thermal gradients. The temperature-induced changes to nauplii depth we observed differed to the prevailing view that nauplii select warmer depths. Instead, we observed nauplii being pushed below a warmer surface layer.

Our column experiment results so far have shown that nauplii avoid surface waters as its density decreases with lower salinity or higher temperature relative to deeper layers. Transitioning into a surface layer of lower water density would require more energy for upward swimming. Nauplii may avoid water density transitions, staying in deep water to conserve energy stores for the energy-intensive host-finding copepodid stage.

We plan validate our results in future column experiments that test combined temperature and salinity stratification and different column depths. Collectively, the information will be coded into new and improved lice dispersal models that will continue to guide the way salmon lice infestations in farmed salmon are managed into the future.

 

Authors: Daniel Wright, Thomas Crosbie, Sussie Dalvin, Frode Oppedal, Tim Dempster


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