Burrowing burbots of Lake Laberge
Doug Davidge first heard about the unusual holes in the bottom of Lake Laberge about three years ago. A fellow underwater diver had noticed large pits in the mud of the lake bottom, out where the water is 20 to 30 metres deep.
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Burbot living in the cool depths of Lake Laberge.
(photo: Doug Davidge)
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Davidge, who has been exploring the Yukon's underwater world for about 20 years, set out to investigate the mystery. He used his remote-control underwater camera to video tape the burrows.
At first Davidge could not explain the holes that he found. Some were quite large, as much as two metres in diameter and one metre deep. Even more interesting, some had tunnels leading off of the pits.
Davidge, who works as an environmental assessment officer with Environment Canada, has dived all over the Yukon in his free time, and has never observed these burrows in other lakes. He considered many different possible causes for the holes, but nothing quite made sense.
Beavers or otter could not have dug the holes in such deep water. Test bombs from the World War 2 era that still sit on the lake floor were another possibility, but the mystery holes did not really match the shape of the bombs.
But the pits Davidge has found are located near objects such as bombs or logs or rocks, which are just the sorts of places where burbot like to spend their time. Davidge now thinks that Lake Laberge is home to a truly unique group of fish -- burrowing burbot.
"Burbot are bottom feeders," he says. "They like burrows or hiding under objects such as logs or rocks." Burbot are also well known as voracious predators, and Davidge speculates that their secret hiding place might help them to surprise their prey.
Davidge assumes that the burbot could form the pits while swimming about favoured hiding places, but the origin of the tunnels is another matter. He has contacted other fish biologists about this discovery, but has found no other reports of burrowing burbots.
He did learn that the fish have saltwater cousins living in the North Pacific that excavate holes. As the Yukon River links these marine cod with the freshwater burbot in Lake Laberge, Davidge says there is always a possibility that the two species evolved from the same ancestral fish.
While many sport anglers in North America dismiss burbot as garbage fish, both Scandinavians and First Nations people hold them in higher regard. The livers in particular are considered to be a delicacy.
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Some burrows in the bottom of the lake have two entrances.
(photo: Doug Davidge)
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But the same quality that makes the livers desirable can also make them a health concern. Burbot accumulate both fat and toxins in their livers, and tests showed that burbot in Lake Laberge had particularly high levels of toxaphene contamination.
Between 1990 and 1993 fish from a number of Yukon lakes were tested for contaminants. The burbot in Lake Laberge were found to have the highest levels of contaminants of all the fish tested, so Health Canada warned people not to eat burbot livers from fish taken there.
Toxaphene, an organochlorine, is a synthetic chemical that has been banned in Canada for some time. It is assumed that this contaminant blew into the Yukon in the upper atmosphere.
Davidge says burrowing in the lake bottom probably does not improve the burbots' health either. "They live in the mud where the contaminants collect, so they would be exposed a bit more to them."
Since burbot are not ranked as a commercial species, they are not studied as much as fish such as salmon or trout. But it is known that they are one of the few freshwater fishes that spawn in mid-winter under the ice. This activity is said to take place in a writhing ball of 10 to 12 fish that tumbles over the spawning grounds.
Davidge says he has noticed other distinguishing qualities as well. "They are a curious fish. They will come and investigate the ROV (remote-operated vehicle) while I'm using it, while most fish will just swim away," he says.
Doug Davidge can be contacted through Environment Canada at (867) 667-3400.
