Thinking Out of the Box: Exploring Strategies to Reduce Sea Lice Infestations in Salmon Farms
Fisheries and Oceans Canada (DFO) researchers and their research partners are investigating alternate solutions for reducing sea lice infestations that can plague salmon farms. Salmon farms can provide particularly good conditions for the growth and transmission of sea lice, including host availability, ideal salinity and temperature. If an infestation is severe enough, it may stress and eventually kill the fish.
The global industry ranks sea lice as the “number one issue” for salmon farming and it continues to be the subject of collaborative scientific research and conferences around the world. Since 2010, research scientists Dr. Shawn Robinson and Steven Leadbeater at DFO's St. Andrews Biological Station have been studying these parasites on Canada’s East Coast through the Aquaculture Collaborative Research Development Program (ACRDP) and the Program for Aquaculture Regulatory Research (PARR). Sea lice have posed challenges for salmon farmers in New Brunswick, Newfoundland and the state of Maine, leading to millions of dollars in lost production.
Sea lice and high-density culture
Two species of sea lice have been the most prevalent on salmon farms—Lepeophtheirus salmonis, which is common on Atlantic Salmon and to some extent Pacific Salmon, and Caligus elongatus, which was a major problem in the 1990s.
“It’s a typical host-parasite relationship that tends to develop in a farm environment whether it is terrestrial or aquatic,” says Robinson. “Like most parasites, sea lice are highly specialized, so when they have easy access to a lot of hosts, they’re capable of reproducing quickly and ensuring their young survive.”
Sea lice that have settled on fish mainly feed on the mucus coating their surface(i.e., skin). If there are too many parasites, sea lice can also feed on the skin and blood of the fish. Too much skin damage can stress and weaken the host fish by making it harder to maintain their fluid balance, and can also lead to secondary infections.
“Based on the frequency of outbreaks over the last two decades, it is apparent that the life cycle of sea lice meshes well with salmon farm conditions,” says Robinson. “A better understanding of these interactions can help inform the development of strategies to break the cycle of infection. One idea is to determine when the sea lice are at their most vulnerable stage and find a way to intervene at the appropriate time.”
Sea lice have been traditionally managed by good animal husbandry practices—area management, fallowing between “crops” of fish, and low density stocking—and by using in-feed and chemical and non-chemical bath treatments. Despite some success, including reduced outbreaks for a period of time, the sea lice on the East Coast have been developing resistance to some of these treatments. In addition, chemical treatments are only authorized for use under strict conditions to minimize potential negative impacts on other crustaceans in nearby ecosystems, including lobster and amphipods. These issues have prompted the search for alternative integrated pest management strategies, similar to the agriculture industry.
“We’re exploring alternative management strategies, including an internal approach that investigates what’s going on within the salmon, and an external approach that examines sea lice from an ecosystem perspective,” says Robinson.
Salmon broodstock and probiotic research
One area of research is connected to salmon breeding programs. Canadian aquaculture companies have benefited through collaborative research with several institutions—i.e., DFO Genome Atlantic, the University of Guelph, and Laval University—to explore genetic markers for sea lice resistance.
As part of this work, Steven Leadbeater embarked on laboratory research in 2013 to study the natural resistance to sea lice of 50 different families of salmon broodstock provided by Cooke Aquaculture in St. George, New Brunswick. Broodstock are a group of mature fish used in aquaculture for breeding purposes. About 1,000 fish were challenged under a variety of conditions that included exposure to sea lice. The number of sea lice that managed to attach to salmon was counted to determine whether some broodstock families were more resistant than others. These findings will be combined with probiotic-type research to assess whether any of the same salmon families have bacteria or bacterial bio-markers in their gut or mucus that provide some level of resistance to sea lice infestation.
“Mucus and faecal samples from each broodstock are being analyzed at Laval University. If the bacterial analysis and sea lice counts indicate that some salmon families had both fewer sea lice and contained certain species of bacteria, we will be able to explore the possibility of applying a type of beneficial (probiotic) —perhaps in feed—to fish that don’t naturally have that resistance,” says Leadbeater. “Such a finding would also introduce the potential for industry to selectively breed families of salmon that naturally support bacteria known to lower sea lice landings and infections.”
Ecology and sea lice research
Ecological research on sea lice to date has almost exclusively investigated the stage during which they attached to the fish. Surprisingly, there has been little research on the larval planktonic stage, where sea lice are the most abundant.
“Female sea lice can produce 300 to 500 eggs at a time in two egg strings, with 5 to 8 broods a year. It takes about 40 to 50 days to grow from the larval stage to a reproductive adult, so if conditions are suitable for larvae to encounter a host, there is potential for rapid population growth,” says Robinson.
A distribution study of planktonic sea lice larvae in the Bay of Fundy by Robinson’s team found virtually all of the larvae occurred around salmon cages, while none were present in comparative locations away from operational salmon farms. They were also distributed throughout the water column (top to bottom), not just in surface waters as the scientific literature of earlier larval studies reported.
“These results suggest that if the problem is internal to the farm, then solutions for sea lice infestation may also be generated at the farm level,” says Robinson.
Since planktonic sea lice larvae are attracted to light, Robinson set out to test whether light-based traps—combined with an understanding of sea lice larvae dynamics—could help to control sea lice levels at commercial salmon aquaculture sites. Underwater LED lights were used to attract sea lice toward traps to filter them from the water column. While the traps caught sea lice larvae, they did not catch enough to affect the overall density of sea lice on salmon throughout the summer. There was a silver lining, however.
“We discovered that the light traps made an excellent tool for remotely monitoring sea lice loads in various locations—in salmon farms as well as possibly remote sites where wild salmon could congregate.” says Robinson.
There is a strong possibility that the treatment and harvesting operations in salmon farms help to maintain high sea lice densities in salmon farms. To address this issue, Cooke Aquaculture approached Robinson to assist with the testing a thermal de-lousing system they had developed—a mobile warm water shower that could be used on-site to remove sea lice from. All of the water used in the system is recycled, reheated and filtered to capture the sea lice eggs on a screen so they don’t reinfect the salmon cages after the fish are treated.
“Preliminary results indicate that thermal delousing is quite effective,” says Robinson. “The company plans to continue field trials to determine the ideal water temperature, fine-tune the engineering of the equipment, and measure its overall effectiveness. Our team will continue to provide them with critical scientific advice and analyses during these trials.”
“The evolutionary and ecological relationships between host species and their parasites have developed over millions of years. These can be very complicated and sophisticated, which means our approaches to aquaculture have to be equally sophisticated and well-planned. That is the challenge for the future,” says Robinson.
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