Sometimes, when you go fishing, you catch a little more than you bargained for.

At least a dozen macroparasites have been identified living on or in Yukon fish, says Whitehorse fish health consultant Rem Ricks. Macroparasites are parasites large enough to be seen with the naked eye.

The number or presence of parasites isn't surprising, says Ricks.

"Virtually any wild animal has parasites. They generally don't cause problems unless the host is compromised in some other way such as illness or stress. "

A few years ago, Ricks put together a pamphlet on fresh-water fish parasites for the Yukon Department of Renewable Resources. It was designed to help people identify any parasites they might find in or on fish caught in Yukon lakes and streams.

The parasites aren't a human health concern, Ricks says. Although fish parasites in some parts of the world can create problems for humans, the ones found in Yukon fish do not.

"Thorough cooking will destroy almost all of them," he says.

Ricks finds them fascinating. One of his favourites is an external parasite called Salmincola. The small crustacean, up to a centimetre long, attaches itself to the surface of the fish, often near the base of the pectoral or caudal (tail) fin, where it grazes on surface tissue.

The Salmincola adult is held in place by a spherical structure called a "head-bulla" that is embedded permanently in the fish's tissue. However, Salmincola larvae, when they hatch, are free-swimming and can survive several days while they search for a fish host.

Once they find the host, they latch on with temporary attachments, develop into males and females, and mate. The males then disappear, and the females move around the fish to find a suitable place for permanent attachment. There they live, grow, transform through various stages, and produce egg sacs containing the next generation of Salmincola.

External parasites like Salmincola are less common than internal parasites, Ricks says. The majority of macroparasites found in Yukon fish are worm-like creatures that live in the fish's intestinal tract, muscle, or occasionally in its swim bladder.

The pamphlet Ricks produced lists four species of tapeworms found in Yukon fish, but the biologist recently identified a fifth species, Ligula intestinalis, in fish from a couple of small lakes in the southern Yukon. It's an unusual parasite because its larval form is almost as long as the adult tapeworm, he says.

"I took two of them out of a three to four pound sucker, and both of them were over a metre long."

Like many parasites, some tapeworms have a life cycle that involves more than one host. Some of them develop as larvae in fish but spend their adult stage in birds. The large size that the Ligula larva reaches in its fish intermediate host may well help it make the transition, Ricks says, because the large larva can incapacitate the fish.

"The fish just puffs up like a puffer. The thing can barely swim," he says. "It flops about in the water, and that makes it easy prey for the bird that eats the fish and becomes the final host."

Probably the most common visible form of macroparasite in the Yukon is the encysted form of the Trianopherus tapeworm. It shows up as a pale grain-like capsule embedded in the muscle of fish like trout or whitefish. The cysts might be aesthetically unpleasing but, once the fish is cooked, they won't do humans any harm, Ricks says.

"They certainly won't cause you any problems -- just a little extra protein."

For more information about parasites in Yukon fish, look for the Yukon Renewable Resources pamphlet, "Common Parasites of Yukon Fresh Water Fishes."