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:

Read the press release here:

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.

Revolutionising mass fish marking one otolith at a time

The use of farmed and restocked fish to supplement the worldwide human consumption of fish, recreational fishing stocks, and conservation efforts, is growing globally. But how well fish survive after release from hatcheries is still a mystery in many places. Hatcheries seldom mark or tag all fish prior to release, despite a range of mass-marking methods being available to mark farmed and restocked fish en masse. In a recent paper, Dr. Fletcher Warren-Myers and co-authors reviewed a range of thermal and chemical otolith (ear-bone) marking methods to assess their suitability as mass marking tools for hatchery-produced fish. These marking methods were compared in terms of (1) ease of application, (2) cost, (3) mark longevity, and (4) effects on fish welfare. His conclusion? Although some techniques will have limited use due to regulations, the majority of otolith mass marking techniques are simple, easy to apply, cost effective and highly suitable for long term monitoring of hatchery produced fish.

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