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.


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

Are submerged cages in Atlantic salmon aquaculture feasible?

Most Atlantic salmon aquaculture industries around the world keep their stock in surface-based cages, which can face issues such as poor environmental conditions and the presence of parasites such as salmon lice. This has generated interest in submerging cages underwater to try and prevent parasite infestations and improve conditions for fish. A submerged cage was recently deployed by a salmon farm in China. However, there are several obstacles that must be overcome before submerged cages can be deployed. Submerged cages can have adverse effects on fish buoyancy, which can alter swimming speeds and cause tilted swimming at night time. This in turn can reduce growth and cause vertebral deformities. Researchers at the Institute of Marine Research and University of Melbourne compared submerged and surface-based farming of Atlantic salmon over 42 days, to determine if continuous light can help increase swimming speeds at night and prevent tilted swimming. They found using continuous light increased swimming speeds, reduced tilted swimming and spinal deformities. Salmon lice infestations were also reduced by 72%. However, salmon growth in submerged cages was 30% lower compared to surface cages. Therefore, developing and engineering technologies to allow salmon to refill their swim bladders in submerged cages at commercial scale is an important area of research that should be further researched before they can be deployed at larger scales. To read the article, click here.

An example of a submerged cage (Fish Farming Expert, 2017)

Review of ‘The Field Guide to the Freshwater Fishes of the Kimberley’

A few months ago, members of the SALTT lab James Shelley and Matt Le Feuvre published a field guide to the Freshwater Fishes of the Kimberley. Recently, it was reviewed in Pacific Conservation Biology, which stated that ‘The Field Guide to the Freshwater Fishes of the Kimberley is a superb book and welcome addition to the natural history and biological conservation literature of Australia’. The field guide has comprehensive list of each freshwater species of fish found in the Kimberley, along with a photograph and map of their known distribution. Detailed descriptions of general features for each species is provided, as well as how you can recognise it.

If you’d like to read the review, please click here. You can also purchase the book for only $20 here.

Lice treatments and salmon mortality in Norwegian salmon aquaculture

Fish farms constantly struggle with parasites. Norway battles its main parasite, the salmon louse, with targeted control and preventative methods within farms. From 2012-2017, there were four main louse removal methods used: chemotherapeutant bathing (azamethiphos, cypermethrin, deltamethrin, hydrogen peroxide), mechanical treatment, thermal treatment, and general bathing (e.g. freshwater bathing). All farms in Norway report to two national-level databases, with one collecting data on registered delousing treatments and the other on monthly salmon mortality. By combining these two databases, we had access to over 40,000 lice removal events across 6 years to map salmon mortality rates to each delousing method.
We detected a rapid and recent paradigm shift in the industry’s approach to lice control, from chemotherapeutants dominating operations from 2012 to 2015 (>81%), to non-chemical mechanical and thermal treatments dominating in 2016 and 2017 (>40% and 74%, respectively). Thermal treatments caused the greatest mortality increases out of all delousing operations used from 2012-2017, with 31% of all treatments causing elevated mortality. This was followed by mechanical (25%), hydrogen peroxide (21%), and azamethiphos, cypermethrin and deltamethrin (<14%). Further, temperature, pre-existing mortality rates and fish size all influenced post-treatment mortality outcomes for all operations. Generally, as temperature increased, salmon mortality also increased across all treatment operations. Fish with high pre-existing mortality experienced increased mortality after treatment, and large fish were more susceptible to increased mortality than small. Our analysis illustrates the importance of national databases in identifying underlying mechanisms that can influence post-treatment salmon mortality. 

Salmon farm nutrients increase numbers of damaging sea urchins

With large networks of Atlantic salmon farming sea-cages spread throughout Norway’s fjords, the environmental impacts of increased organic and nitrogenous wastes surrounding farms has been questioned. The white urchin Gracilechinus acutus is an ecosystem engineer within fjords, and high densities of urchins can shift the ecosystem into urchin barrens, which can have cascading effects and change biodiversity. Dr. Camille White found that urchin barrens around aquaculture sites were 10 times more abundant and 15 mm smaller compared to urchins found at sites without sea-cages. In laboratory experiments, Camille also tested if urchin diets influenced by aquaculture waste affected reproductive outputs compared to natural diets. She found that while urchins fed aquafeed diets had gonad indices 3 times larger than urchins fed with a natural diet, their reproduction was compromised, with lower fertilisation success and lower larval survival. However, due to higher densities of urchins found at farming sites, the overall larval outputs at farming sites was five times higher than sites without sea-cages. Therefore, aquaculture waste can influence fjord ecosystems by stimulating aggregations of urchins, causing the formation of urchin barrens and altering natural ecosystems.

Read the full article here: https://www.int-res.com/articles/aei2018/10/q010p279.pdf

Read the press release here: https://www.fishfarmingexpert.com/article/salmon-farm-nutrients-increase-numbers-of-damaging-sea-urchins/

Short freshwater treatments against gill disease

Freshwater bathing is the go to treatment for amoebic gill disease (AGD) in many farmed fish species. Treatments are typically 2-4 hours long and kill the amoeba. This restricts how and when treatments can be delivered. Dr Daniel Wright from the SALTT lab and co-authors tested if using short, sub-lethal freshwater treatments were just as good at removing amoeba as long, lethal treatments. Sub-lethal (daily 30 min treatments) and lethal (daily 120 min treatment) treatments for 6 days both reduced amoeba compared to short daily 3 min freshwater treatments. In short, repeated sublethal freshwater treatments could be just as effective as longer, lethal doses of freshwater. Danny’s results could be transformative to the industry as they open up new ways to deliver treatments that are easier for farmers to do and less stressful for the fish.


Cryptic biodiversity in the freshwater fishes of the Kimberley endemism hotspot, northwestern Australia

The prevalence of unrecognised species is a real problem for estimating true biodiversity and hampers conservation planning. The remote and spectacular Kimberley region in northwestern Australia, with its rugged landscape and deep gorges, harbours some of the most diverse and unique animal and plant communities in Australia. Recently, many new cryptic species have been found on land, raising the question of whether the rivers and streams are also full of unrecognised species. We sampled fish from rivers right across the Kimberley and assessed different molecular genetic data for the Kimberley’s most species-rich fish family, Terapontidae. Clear evidence exists to describe 13 new fish species. Many of these new species are only found in a single river. Our findings show that the fish biodiversity in the Kimberley is severely under-represented, with significant implications for ecological research, conservation and management.


Freshwater Fishes of the Kimberley

A couple of our PhD students strayed into the amazing rivers of the Kimberley region for their research. Recently they released a beautiful new book ‘Freshwater Fishes of the Kimberley’. Congratulations to James Shelley and Matthew LeFeuvre.

For those interested in a copy, they are only $20 here.

Bigger is not always better: how cage size and depth influence dissolved oxygen in salmon farming

Bigger cages potentially allow for increased Atlantic salmon production and higher profitability, but only if water quality remains good. As cage size increases water exchange decreases, which may in turn cause low dissolved oxygen conditions within cages. To test this, PhD student Tina Oldham from the University of Tasmania compared how dissolved oxygen concentrations varied with cage size on a commercial salmon farm during the 2015/16 Australian summer heatwave. Overall, Tina found that dissolved oxygen levels in all tested cages were generally high and suitable for salmon feeding and growth, and lowest oxygen concentrations consistently occurred in the larger cages. Bigger, it turns out, is not always better.


Changing surface conditions in sea-cages to prevent parasite infections

Amoebic gill disease and salmon lice are among some of the greatest challenges the Norwegian Atlantic salmon farming industry faces. Manipulating surface conditions and pushing fish down beneath the surface layers with the highest infestation pressures could potentially control and prevent infestations. Dr. Daniel Wright from the Institute of Marine Research tested if a permanent freshwater surface layer in snorkel sea-cages can reduce amoebic gill disease and salmon lice levels compared to standard commercial cages. While freshwater surface layers were unable to prevent or reduce AGD and lice infestations, further research should test if other behavioural and environmental manipulations can be used to prevent parasites from infesting farmed salmon.


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