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Salmon River Ecology

      The day began with an early morning walk along the Credit River.

It was November and the salmon runs were either over or had not yet started. The river has a reputation as a great “salmon” river but most people know (in their heads if not their hearts) that that is not true. A great “salmon” river would not have to be stocked every year and would have natural regeneration. Maybe it’s a great salmon fishing river but it is no longer the great salmon river it once was.

The Credit River is, however, just a short hike away from my home and it is a walk I enjoy at least a few times a month. It brings back memories of other “salmon” rivers. Rivers like Brooks River in Alaska, Glendale Stream in British Columbia and many others whose names escape me. Now those are great “salmon” rivers! Their ecosystems are in tact or nearly so.

Out west scientists have been studying the ecology of a healthy salmon river for sometime now. They are spurred on by the economic importance of the salmon fishing industry and by the realization that hatchery reared salmon cannot restock wild populations. What we’ve learned since the 1980’s is that there is more to a salmon than is at first apparent.

“A sockeye is a sockeye” is not true. In the west in any given river where sockeye salmon spawn and die it was assumed that all of the fish were the same. Studies revealed that that is not the case. Sockeye salmon from different locations along the river were different form other Sockeye salmon. Those that had to travel far upstream had different physiological adaptations than those that spawned at the mouth. They were stronger and matured slower in the river while those that spawned near the mouth rapidly changed from silver ocean-going fish to the red-coloured breeding fish.

Biologists are now treating Sockeyes breeding in different areas as completely different populations. It is believed that these findings also apply to other West coast salmon. The Chinook salmon that spawn in the Credit River and other Great Lake’s tributaries are considered to be one population. They were (and many still are) hatchery raised. Those that do manage to breed in Ontario rivers just haven’t had time to differentiate and in fact most are still stocked. (There are a few streams and rivers where successful spawning has been observed. The species is now firmly entrenched ln the Great Lakes.)

Two hundred years ago these same tributaries (at least in Lake Ontario) were alive with spawning salmon. These were land-locked Atlantic salmon and we know very little about their biology because by the middle of the 1800’s they were extinct. We do know why they disappeared. Milldams on the rivers prevented them from migrating to spawning grounds. Erosion from nearby fields, now cleared of forests, caused the beds to silt up so that any eggs that were laid suffocated. The final blow was the accidental introduction of alewife into the lakes. Atlantic salmon that eat these fish pick up a toxin that prevents successful spawning. (The continued presence of alewife in the lake is why attempts to re-introduce Atlantic salmon in to Lake Ontario have so far failed.)

So what does it take to make a healthy ecosystem for salmon?

Dave Gonder is a Management Biologist with the Upper Great Lakes Management Unit. I put the question to him. Salmon need five basic things, he explained. “They need open access to spawning and juvenile habitat.”

The Credit River, my river, was never destined to become a “great salmon river”. The old milldams were already in place when the Province made the decision to stock the river. This allowed the MNR the opportunity to control the fish’s access to parts of the river. The intention was (and still is) to keep the upper Credit River free of stocked salmon and trout and let the native species (browns and speckles) have it to themselves. In short: Access Denied!

In the other Great Lakes especially Lake Superior there were no impediments to the migration of salmon up their natal streams. Salmon do successfully spawn in those streams. They also spawn in the Nottawasaga Watershed where Gonder works.

            The second thing is clear ground substrate in which to spawn. Gravel in other words. The size of substrate varies by species but it has to be large enough that the fertilized eggs can fall between the spaces and clean enough that it does not silt up and cover the eggs. The lower Credit is far to silty and muddy for salmon to breed and I naively figured that that was why the salmon can’t breed there.

            However, when Dave talked about the third thing that salmon need (the order of these is mine, not his) I realized it came back to that access point. He explained that one of the things that the Nottawasaga watershed had going for it was that much of its water emerges as groundwater from the Niagara Escarpment. “This keeps it cool. Salmon need cool clear water to spawn in. The Credit gets its water from the same Escarpment.

            It dawned on me, rather slowly I’ll admit, that the Nottawasaga River where I spent my youth cottaging on was not the portion of the river the salmon and steelhead use. Its way too muddy but further up it was a different story.
Agriculture, home building, industry and other land uses such as logging in BC can added to sediment load of a river and these activities can cause declines or prevent salmon spawning success. It is equally true that it depends where these activities are having their impact. The other thing that must be remembered is that while our species destroyed much of the historic salmon runs, we’ve also done much to restore them.

This is especially true in British Columbia where much has been done to rehabilitate streams and rivers.

The fourth thing salmon need is space. In their report “Pacific Salmon and Wildlife” Jeff Cederholm, Washington Department of Natural Resources (et al), makes the point that “a typical anadromous salmon life history has five main stages: (1) spawning and incubation, (2) freshwater rearing, (3) seaward migration, (4) ocean rearing, and (5) return migration to freshwater.” To successfully go through all five stage salmon need room and lots of it.

On the both coasts rivers and either the Pacific or Atlantic Ocean meet these requirements. In Ontario the Great Lakes provide a suitable stand in.

The fifth thing they need is an intact ecosystem. They need “food”.

While in the ocean/ lake stage of their lives the salmon require relatively pollution-free waters and ample forage fish to feed on. These needs are more or less met in all three environments. As mentioned above the alewife is a problem for Atlantic salmon but Dave pointed out that it is also a problem for Chinook salmon as well. Yet these salmon manage to breed. Gonder told me that a study had been done that suggested that it depended on when the fish ate the alewife, their numbers and what else the fish ate.

A link between the alewife and the success salmon reproduction has been identified. Alewives contain the enzyme thiaminase that breaks down thiamine. Thiamine is a naturally occurring chemical that increases the survival rates of salmon fry. By feeding on alewives the salmon were developing thiamine deficiencies. When the females spawned, their eggs lacked thiamine and either failed to hatch or produced weakened fry.

To me there is a sixth thing that you need to have present and that is a major predator. Certainly it is missing from the lower Great Lakes (other than humans) but in the west sharks, sea lions, orcas and seals all compete for the salmon in the Pacific Ocean while in the Atlantic it is primarily seals. Commercial fisheries are major consumers of wild stocks in both coastal locations.

A truly great salmon river needs to have an intact ecosystem. The great ones that I’ve worked as a photographer were places where I had a good chance of seeing a grizzly bear or a bald eagle. There was a real sense of adventure just being there because I wasn’t the biggest predator on the river. But I am just as keenly aware that the food chain must be complete too.

The Pacific ecosystem is more intact simply because we have not been taking the fish stocks for as long as we have on the East coast. Food chains on the west are still fairly lengthy. The report lists over 150 species that have a relationship with West Coast salmon.

Many of these species interact with the salmon in the ocean but just as many interact with them while they are in freshwater habitats such as streams, rivers, marshes and lakes. Turtles eat the eggs; dragonfly nymphs consume the fry, as do bass and other native fish. Growing salmon return the favour and eat many of the species that once ate them (not turtles however). It is this struggle to survive that makes a great fish.

Cederholm’s report makes it clear to that the habitat is just as important. Tree-lined waterways shade the river and help keep it cool. Fallen trees create quiet pools for the fish to rest. In BC and Alaska it has been recently discovered that salmon carried into the forest by bears provide important nutrients for the trees as well.

As we understand more about what the salmon need we come closer to being able to create and maintain great salmon rivers. There was a time, not that long ago, when all we thought we needed was the salmon. Those we could get by simply restocking the fish. We now know that these fish lack the genetic diversity and the determination of wild fish. The Credit will never be home to those fish, nor should it. It is comforting to know that in the Nottawasaga and other tributaries on the Great Lakes this process is ongoing. The work being done in the West to restore Pacific salmon habitat and in the East to restore Atlantic salmon rivers is encouraging.

It is nice to know but it is also nice to be able to experience something like that here in Ontario.

Note: A version of this article first appeared in Bob Izumi's Real Fishing Magazine in 2000. It is used with the permission of editor.