A River Never Sleeps

By Harvey Thommasen, with Photography by Nick Didlick

Spring brings warming of waters and movement of juvenile coho from overwinter areas, like back eddies and beaver ponds, to the main stream, where they begin to feed voraciously.

Coho juveniles that have survived the winter will begin to supplement their insect diet with fry of various oncorhynchid species - particularly pink, chum or smaller coho fry, which are emerging from spawning beds. As a general rule, coho fry spend one to two years in their freshwater residences before smolting, and then heading out to the ocean.

On emergence from gravel coho fry are typically 30 to 35 mm in length. By September most are 60 - 70 mm long. By March and April typical juvenile coho lengths are 80 - 100 mm.

On the Bella Coola River, on British Columbia’s Central Coast, a few of the smaller coho juveniles smolt after the first winter, but most spend an extra year in the system before migrating. During smoltification the parr marks fade, the sides become lighter and more silvery, and the dorsum becomes a more blue-black colour. These coho smolts stop defending their territories and begin to aggregate into schools of 10 to 50 fish. Within these schools, coho become braver and more adventuresome, swimming up to the surface of the stream, or out into the main part of the current. Most of the seaward migration actually takes place at night between 2300 hrs and 0400 hrs.

Fly fishermen need to be aware that trout and char will be feeding eagerly on both the smolts and the newly emerged fry. They may key on one group or the other, or they may feed freely on both. If you are on a coastal stream in the early spring and see big trout swirling in the shallows or on the surface, it will almost certainly be because they are feeding on salmon fry or smolts. But you may have to “match the hatch” by looking in back eddies to see what the trout are taking. Is it pink fry, coho fry - or bigger coho smolts?

In the Bella Coola River, the smolt migration occurs between mid-March and early June, with the peak in mid-April and early May. In warmer years this smolt migration appears a few weeks earlier, and colder years it occurs a few weeks later. The migration corresponds roughly with the main seaward migration of pink and chum salmon fry.

This is likely not a coincidence, as the coho smolts feed heavily on these fry, especially out in the Bella Coola estuary. Between one and ten percent of emerging coho fry actually make it to the estuary (ie the fry-to-smolt survival), average being 1% - 2%.

Floods, summer droughts, inability to secure a territory, and predation are all causes of mortality. Larger rainbow trout, cutthroat trout, Dolly Varden char, Rocky Mountain Whitefish, sculpins, mink, otter, and fish-eating ducks (mergansers) will all eat juvenile coho and downstream migrating coho smolts. When concentrated in small channel pools that are drying up, coho fry are also vulnerable to garter snakes, dippers, robins, crows, and herons.

In the estuary coho smolts encounter many of the same predators (cutthroat trout, Dolly Varden char, herons, mergansers, and mink) as well as a variety of new ones (lamprey, gulls, loons, seals, and dog fish).

Later in life, they will be eaten by sea lions, killer whales, and humans. Despite the variety of predators, the survival rate for coho once they hit the estuary is quite high compared to other salmon species.

Typical smolt-to-adult stage coho survival range from between 5-15%. These rates are 3 to 10 times the survival rate of other salmon species. In the estuary coho grow quickly on a diet of amphipods, euphausiids, fish larvae, and onchorynchid fry.

Coho smolts are particularly fond of pink salmon fry. Studies done at the Bella Coola estuary revealed that when coho smolts, and pink fry initially enter the estuary average coho smolt size is roughly 95 mm long and pink salmon fry are about 37 mm long. After approximately 40 days in the estuary, the pink salmon average 76 mm and coho about 130 mm. At this point, the pink salmon fry have essentially outgrown the ability of the predator to ingest it. Researchers believe that coho salmon smolts play an important role in preserving, and even reinforcing, those characteristics that make Bella Coola pink salmon such a fast growing commercial salmon species. That is, only fast growing pink salmon survive.

The quicker they can become too big for their prey to eat, the better off the stock is. Their selective predation on smaller fish could enhance the innate capacity for growth, reducing the time that the prey are vulnerable to mortality agents and producing larger fish going to sea. After a few months of feeding in the Bella Coola estuary, the schools of young coho move on out to coastal waters. Out in the ocean, the schools begin to break-up as coho begin to feed more and more as individuals. As coho grow larger they begin to eat more and more fish. In addition to chum and pink fry, they will begin to eat herring, anchovy, surf smelt, sand lance, sticklebacks, sardines, capelin, rockfish, sablefish, lantern fish, pacific saury, hake, walleye and pollock.

Depending on the local abundance of feed, and perhaps on their specific (stock) genetic make-up, Bella Coola coho will either spend their entire adult life feeding in local outside waters (eg Fitz Hugh Sound or Queen Charlotte Sound), or they will undertake longer open water northerly migrations. Tagging studies reveal that these open water, migratory coho will travel up to one thousand miles northwesterly before turning around and heading in a counterclockwise direction back to their home spawning streams 12 to 18 months later - usually the summer after spending one winter in the ocean. Traveling, and feeding in the upper 30 metres of the ocean, home-coming coho travel between 30 and 55 km/day.

Among all the salmon species, the coho is probably the most exciting and satisfying to catch. Unlike chinook salmon, coho consistently take a fly. Unlike pink, chum, or sockeye salmon, coho deteriorate slowly after entering freshwater.

This means coho remain fairly bright and aggressive for up to 2 to four weeks after they have entered the river. In contrast, pink and chum salmon deteriorate within days of entering the river. Provided there are coho in the river, and you have the patience to cast to their lies for several hours, the average flyfisher can expect to hook at least one coho in a day’s fishing. Flyfishers who have learned over the years where exactly the fish lie, and how exactly to present their flies, can do a lot better than that.