A River Never Sleeps

By Harvey Thommasen

As do all animals, salmon and trout can suffer from infections caused by fungi, bacteria, viruses, and parasites. Anyone who’s ever watched Pacific salmon on their spawning beds will have seen advanced infections spreading over the skins of the once bright fish.

The following is a list of the infections the average angler can expect to see.

1) Fungal Infections:
Saprolegnia (right) is the most common type of fungal infection found in spawning salmon. Saprolegnia appears as a white, cotton-like growth which coats the skin and fins of fish. It usually appears on areas that have been scratched or cut. Most healthy fish can survive a minor infection of saprolegnia, occasionally it spreads to cover the entire body of the fish.

2) Bacterial Infections:
Furunculosis (right) is said to be the most common type of bacterial fish infection. It is caused by a bacterium called Aeromonas salmonicida. It begins as a round or oblong shaped lesion which then spreads in an outward direction. The pustules are usually beneath the skin (subdermal).

3) Viral Infections:
In general viral infections are more commonly seen in younger fish so they are not likely to be noticed by the average angler. They are however, very important in the hatchery setting. In a crowded hatchery, diseases can spread quickly through a fry population. Viral infections such as Infectious Haematopoietic Necrosis (IHN) have a mortality rate of 95% or more, and can have a devastating impact on hatchery populations.

Whirling disease (left) is another kind of disease which affects fish, and can wipe out hatchery productivity or spread through the wild with devastating effect, as it has in Montana. Whirling disease is caused by the myxosporean protozoa, Myxobolis cerebrelis. Free-swimming, mature myxosporean protozoa latch onto young fish, and inject "spores" into them. These "spores" preferentially attack cartilage and nerves which results in deformed fish, and a tail-chasing swimming behaviour that sends the fish whirling through the water.

Whirling disease was first identified in Europe in the early 1900's. Brown trout were among its favourite hosts. When brown trout were introduced to North American hatcheries (Pennsylvania and Nevada) in the mid-1950's, Whirling disease came along as well.

The disease thrived in the hatchery setting, and as hatchery fish were being introduced into North American streams, so too was whirling disease. Juvenile rainbow trout seem particularly susceptible, but cutthroat trout, and salmon are also killed by the Whirling disease protozoan. At the present time Whirling disease is known to have spread throughout a total of 19 states. Some streams have been devastated by Whirling disease. In the famous Madison River in Montana, Whirling disease was responsible for a 90% decline in the rainbow trout population, and a 58% decline in brown trout numbers.

Many other streams in the Pacific Northwest in the United States are experiencing similar declines. Though it has not been reported in Canada, it may well be on its way. Sculpins are carriers of the organisms and a favourite angler bait. Anglers who use sculpins could conceivably be spreading the disease. Similarly, birds can carry this protozoan in the mucky mud they transport from one water habitat to another. Slowly, but surely, the disease will probably make its way into northern Canadian waters.

Salmon and trout are also attacked by a number of different kinds of parasites, like sea lice, lampreys, and fish tapeworms, which can weaken fish, or kill them.

Sea lice (Lepeophteirus salmonis), are of increasing concern because populations of the parasites can soar in and around salmon farms.

Sea lice (above) are external parasites that, in small numbers, are commonly found on fresh fish that have just run into the river from the ocean. Sea-lice cannot live more than 48 hours in freshwater, after that time they atrophy and fall off the fish, so if you find them, it’s a sure sign the salmon has recently arrived from the ocean. If the water temperature is less than 12 C, they may stay on a day or two longer.

It is not uncommon to catch perfectly healthy Pacific salmon with two or three sea lice attached to them. But in Scotland and Norway, researchers have raised concerns about massive sea lice infestations on Atlantic salmon. It appears the sea lice have spread from fish farms to wild stock, with terrible results. Fish farm advocates deny the sea lice infestations are caused by their operations.

Fish tapeworm (right) (Diphyllobothrium latum) are harder to spot, and pose a potential health threat to humans.

After the tapeworm eggs are ingested by freshwater copepods, the larvae hatch internally. When the infected copepods are eaten by fish, the larvae develop further into a second larvae form, known as a procercoid. This procercoid migrates into the flesh and then transforms into a plerocercoid. This plerocercoid is infective to humans and other animals which eat undercooked fish flesh. It takes 3 to 5 weeks for the plerocercoid to mature into an adult, egg-laying tapeworm. Symptoms vary from person to person - some people have no symptoms, other can get diarrhea, abdominal pain, and even bowel obstruction.

Fungal infections are commonplace on spawning Pacific salmon. But anyone seeing signs of infection on trout or who suspect bacterial or viral infections on any fish, should notify government fisheries authorities.