What lurks in the loch: studying sea lice
Seeking out tiny larvae in the water column is a painstaking task, but one that could yield important data.
Sea lice represent a big problem for the salmon farming sector, but the creatures themselves are, of course, very small. This means that trying to study them in their natural habitat – the sea – is rather like trying to find the proverbial needle in a haystack.
Nonetheless, this is what marine biologist Dr Helen Reinardy has been doing. With her colleagues at the Scottish Association for Marine Science (SAMS), Dr Reinardy has been looking for sea lice larvae in the sea lochs on Scotland’s west coast.
She explains: “It is challenging to study larval stages in the water column. They are small, and part of a huge zooplankton community.”
There are also two separate types of lice to look for, Lepeophtheirus salmonis and the members of the Caligus family (Caligus elongatus is a particular problem for farmed salmon in Scotland).
L.salmonis pass through eight life stages, including two nauplii stages (the copepodid stage in which the louse becomes a parasite), two “chalimus” stages, two “pre-adult” stages and a fully mature adult stage. Depending on the temperature, the entire lifecycle can take between four and nine weeks.
The number of sea lice produced in each generation is relatively small compared to many other types of zooplankton.
Dr Reinardy says: “We know a lot about sea lice when they are on the fish but the larval stages are less well understood – their biology and how they relate to the environment, for example.”
“We are looking to increase our data sets on things like fecundity and mortality of the larvae.
“We are also interested in how deep they will go in the water column and what hydrodynamic conditions move them around. ‘How deep’ is at the top of our list, especially important when moving into ‘closed’ finfish aquaculture systems.
“The west coast of Scotland has very complex hydrodynamic conditions, as seen in a body of water like Loch Linnhe, with factors like tides, currents and changes in salinity. We are also interested in their sensitivity to light and how predation affects their numbers.”
While some studies have used sentinel cages – fish kept in cages to attract any lice that may be in the location – these primarily provide data on the later stages of the parasite, and raise welfare issues for the fish used. It can also be costly and time-consuming to maintain the sentinel cages across a wide range of locations.
Finding larvae in the water column requires a different approach. In the early 2000s, SAMS was part of the SPILLS (Salmon Parasite In Linnhe, Lorn, and Shuna) project, developing models and collecting data to try to gain an idea of how sea lice are distributed.
Dr Reinardy says: “SAMS developed a method of identifying sea lice under the microscope, but we did not find that many. Their distribution is typically patchy.
“From that experience we further refined our methods of collecting and sieving.”
A follow-on project last summer was more successful in finding sea lice larvae. The process involves collecting a wide range of zooplankton from the waters of the loch, and then identifying the sea lice under microscope back in the lab. It is a painstaking process and requires the researchers to be well trained in species identification.
SAMS has also been exploring the potential of new techniques and technologies to find sea lice larvae, for example holographic cameras, or environmental DNA (eDNA), which could also be used to help detect threats like harmful algal blooms and jellyfish.
Artificial intelligence, which could be used to sort through thousands of images, is another area of interest. Dr Reinardy says, however, that the SAMS experts will not be redundant for a long while, since experts are essential in identifying the zooplankton groups used to train the AI models.
More data would help to add weight to the models being used to help spatial planning by regulators.
Dr Reinardy says: “Sea lice models are highly useful to help understand the dynamics of different farm conditions, but we do need data to increase our confidence in their accuracy and keep refining the models as new technology comes along.”
The way forward
In November last year, SAMS hosted a workshop on sea lice, held in Glasgow, which brought together researchers, industry, regulators and policy makers. Organised by SAMS and supported by the Sustainable Aquaculture Innovation Centre (SAIC), the workshop heard how new technology is changing the way sea lice can be monitored and detected. The workshop, “Innovations in Sea Lice Monitoring”, was part of the annual Marine Alliance for Science and Technology Scotland (MASTS) Science Conference.
Dr Reinardy, chairing the event, said: “Sea lice is arguably the biggest issue in Scottish aquaculture and, with the adoption of the new sea lice regulatory framework this year, the issue needs to be better understood across research, policy, regulatory and industry sectors. As well as being a welfare issue for farmed fish, we need to better understand sea lice larvae distribution in the water to assess potential impacts on the surrounding environment.
“Through this workshop we’ve been able to get various stakeholders together to explore this issue in detail and to share the latest developments in research, including underwater laser holography, eDNA, artificial intelligence and fluorescent microscopy.”
Speakers included: Dr Kim Last of SAMS; Dr Thangavel Thevar, University of Aberdeen; Dr Lionel Camus, Akvaplan-niva; Professor Martin Llewellyn, University of Glasgow; Professor James Bron, University of Stirling; and Dr Alistair Duguid, Scottish Environment Protection Agency.
Dr Reinardy says: “There are moves towards more closed farming systems, but these systems will still need to source water from the sea. This makes it really important to understand the behaviour of sea lice larvae in the wild, specifically when and where they occur in the water column, which is challenging given their rarity.
“Research organisations like SAMS are working on better monitoring methods, but input from the various stakeholders at this workshop will be invaluable as we seek more collaboration to solve this major challenge.”
Peter Pollard, Head of Ecology at the Scottish Environment Protection Agency, said: “As Scotland’s lead environmental regulator responsible for managing interactions between sea lice from fish farms and wild salmon and sea trout, we welcome scientific innovation aimed at cost-efficiently and accurately detecting and monitoring infective-stage sea lice in the sea.”
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