Tasmanian research scientists studied farmed Atlantic salmon to see how the fish would respond to climate change.
Subscribe now for unlimited access.
$0/
(min cost $0)
or signup to continue reading
The research team from the Institute for Marine and Antarctic Studies tagged the salmon with sensors to study how their behaviour changed when they experienced warmer temperatures and falling oxygen levels in their aquaculture cages in Macquarie Harbour.
Lead author Dr Kilian Stehfest and his team conducted the research during the hottest summer on record, and found the salmon tended to concentrate in the middle depths to avoid low levels of oxygen at the bottom of their cages and warm surface waters at the top.
“We ended up recording data during the most extreme summer on record [in Hobart]. Macquarie Harbour had a really warm surface layer with high oxygen levels, but was cooler at the bottom with lower oxygen levels,” Dr Stehfest said.
“We could see how the fish trade off optimum temperature and oxygen levels between temperatures higher than 20 degrees [at the surface] and lower oxygen levels [at the bottom of the cages],” he said.
Salmon stay cool during hot summer
The researchers noticed changes in the fish behaviour as their study continued.
“As the summer got hotter the fish tried to avoid both [the water surface and the cage bottom] and ended up being squeezed in the middle,” Dr Stehfest said.
At the height of summer the salmon concentrated at depths of four to six metres in the cages, which means only around two metres of their 17-metre-deep cages were available to them.
“This behaviour peaked during the summer months as dissolved oxygen levels fell near the bottom and surface temperatures rose, leading to considerable contraction in the vertical habitat available to the fish,” Dr Stehfest said.
“If the fish are squeezed we assumed an increase in density. If fish are squeezed by the environment that is relevant when setting stocking densities.
“These results highlight that local environmental conditions, and the tolerance of fish to them, need to be considered when determining the stocking densities at a given aquaculture site,” he said.
Interestingly, despite the change in behaviour for the salmon, the fish still spent a large amount of time in waters with oxygen levels that Dr Stehfest said would be considered sub-optimal based on studies from other species, which suggested the tolerance of Tasmanian Atlantic Salmon might be higher than that reported for other stocks.
New behaviour as climate changes
Dr Stehfest’s research, which was published in the international journal Scientific Reports, showed this squeezing is likely to be a significant response to warming from climate change.
“Warm water holds less oxygen and warmer surface temperatures makes the water less dense, so more stratified,” Dr Stehfest said.
“If there is less mixing, then replenishment is reduced at the bottom, which gives us a glimpse into the future if we lose oxygen from oceans.
“Atlantic salmon, both farmed and wild-caught, are a globally important commercial species. It is crucial that we determine how they will respond to extremes of dissolved oxygen and ocean temperatures as the climate warms,” he said.
How the sensors worked
The sensors were attached to a fin on the back of the fish via a monofilament fishing line and the tag was slightly positively buoyant so it hovered above the fish.
Dr Stehfest said the sensors attached to the fish were trialled in a tank under veterinary supervision, and when tagged fish were harvested at the end of the study they were observed to be in good body condition.
“We had an extensive trial on the tagging method in our laboratory and compared behaviour, feeding and growth over several months,” Dr Stehfest said.
“There was no significant difference in health between the tagged and untagged fish, which was important for animal ethics and to ensure no change in behaviour,” he said.
The sensors transmitted an acoustic signal via a mobile network so researchers could see where the salmon were, what the temperature was, the depth they were at and the environment in real time.
“While other studies have been done in laboratories or used equipment that shows how caged salmon behave as a group, our research is novel in that we used new acoustic telemetry tags to measure oxygen levels and temperature in real time in the immediate environment of the fish in on-farm conditions,” Dr Stehfest said.
“We were really fortunate to work with beta versions from Canadian technology [company] Vemco, that specialise in acoustic telemetry collecting data under water to measure oxygen, temperature and depth.
“This is the first time sensors have been used in this way, paving the way for similar uses in other species and situations to track how individual fish respond to different environmental conditions,” he said.
The research team studied salmon farmed by Huon Aquaculture in Macquarie Harbour.
The $700,000 project was funded by Sense-T, which is a partnership between the University of Tasmania, CSIRO and the Tasmanian government, and the federal government.
Its aims were to:
- Reduce feed wastage by ensuring fish are fed when environmental conditions were suitable
- Collect new or difficult to obtain information about the harbour environment and the impact of salmon farming on variables, such as oxygen levels
- Improve the salmon industry's efficiency and productivity, providing real-time data to support decision-making
- Highlight areas where the industry can improve environmental practices.
Sense-T’s contribution was $638,000.
While the salmon component of the project is now finished, Dr Stehfest and his team have moved on to study tagged skate species.