Fishy findings: parasites in the eyes of Yukon burbot

Burbot from a Yukon lake. Photo: Yukon Gov’t/O.Barker

Burbot, also known as lingcod, mariah or loche, are a bottom-dwelling fish that feed on fish, insects and crustaceans. Burbot are found in many lakes and rivers throughout Canada, and are the target of both recreational anglers and a set-line fishery. There is considerable interest and value in maintaining and supporting healthy populations of burbot, particularly in popular fishing areas.  In the Yukon, recent reports of declines in burbot numbers and fish size in some lakes prompted Environment Yukon biologists to assess burbot populations in two lakes.  Burbot captured during the surveys were examined for any obvious lesions before they were released, and several burbot were also collected for necropsy and histopathology to identify any health issues.

One of the key findings was the presence of cataracts in the lenses of approximately 35% of burbot. Further examination of sacrificed fish confirmed that the cataracts were associated with the presence of larval digenean parasites, tentatively identified as the genus Diplostomum.  Diplostomids have a complex, three-host life cycle involving

1. a free-living larval stage that infects the first host, an aquatic snail
2. release of a  free-living larval stage which penetrates the skin and gills of the second host, a fish such as burbot, where unencysted larvae move to the eye and remain within the lens, humor, or under the retina of the infected fish and
3. consumption of the infected fish by a pisciverous (fish-eating) bird, the third host, with the adult parasite developing in the bird’s intestine.

Burbot with cataract. Photo: Yukon Gov’t/J.Harms

Diplostomids can infect a wide range of fish species, and have been reported in fish in numerous locations throughout North America and Europe.  Infections with Diplostomum species have been associated with decreased lens size, cataracts, inflammation, and vision reduction or blindness. The impaired vision of infected fish may lead to increased susceptibility to predation by birds and enhance transmission of the parasite. However, burbot prefer to dwell in deeper parts of lakes and may not be as affected by decreased vision as other species of fish. Additionally, burbot in deeper water may act more as an ecological sink for the parasite, since fish-eating birds may not have the opportunity to consume them. In Yukon, the effect of this parasitic infection on the health of burbot, and the distribution of the parasite in other species of fish, remains to be determined, and will continue to be investigated as a component of the health assessment of burbot.