Over 100 million years ago primitive fish known as salmoniformes were thriving in the icy seas of the north Pacific Ocean. From these fish, evolutionary lineages developed that ultimately gave rise to trout, salmon, and char.

Prehistoric onchorynchids (salmon and trout) looking much like a modern cutthroat or rainbow trout, were invading rivers and lakes by 3.5 million years ago (eg Rhabdofario lacustris).

It is believed that these primitive trout-like fish would have spawned in cool, clean-flowing waters that flowed into the Pacific Northwest. Fertilized eggs would have been deposited into gravel redds, where they would have remained over winter.

The following spring, fry would have emerged from the gravel to begin their life as a predator of aquatic and terrestrial animal life. After spending osome time in freshwater the juveniles would have migrated downstream to the ocean to feed on ocean fare such as euphausids, baitfish, crab larvae and squid.

After reaching maturity on the high seas they would have returned as adults to spawn, and perpetuate the cycle. The anadromous life history, whereby spawning takes place in freshwater environments and maturity is achieved in the food-rich oceans has proved to be a very successful
strategy. No wonder it has persisted basically unchanged for millions of years.

Compared to the ocean, there are fewer predators in a coastal stream. On the other hand, there is much more food available for salmon and trout in the ocean. Viewed in this light, it makes sense for fish to spawn in freshwater streams, and it makes sense for their offspring to spend at least part of their life feeding in the oceans.

Subsequent adaptation to the water habitats available to these prehistoric onchorynchids led to the eventual development of modern species: coho, sockeye, chinook, chum, pink, sea-run cutthroat trout and steelhead.

All share the same basic shape and life history as their primitive ancestor. It is a generally accepted axiom that no two species can occupy the same ecological niche indefinitely in the same habitat. So how is it that all species of onchorynchid can use the same stream?

In actual fact each species has evolved unique adaptations which allow them to effectively exploit slightly different resources at different times and places. While there is overlap in habitat use and feeding habits, each species does seem to have preferred habitat and feeding requirements, providing a natural sorting order.

The end result is that the available freshwater habitat produces more biomass than it would if only one onchorynchid species spawned in the system. It is this ability to evolve adaptations which make optimum use of all available microhabitats that has allowed onchorynchids to thrive (the current salmon crisis notwithstanding.)

Amazingly, there seems to be no end to the variety of ways the different species differ. There are differences in size and appearance of juveniles and adults; there are differences in water depths and stream velocities used for spawning and juvenile rearing; differences in eating habits and in preferred food organisms; differences in duration of residence in freshwater and in saltwater; differences in timing of spawning and migrations; and there are differences in how the species interact with one another.

Some of the differences are obvious, some not. The variations are designed to keep the species ecologically isolated from one another in space and/or time.

Each species, with its own spawning time and preferred spawning area, will not usually disrupt the others or crowd the desirable spawning beds. In the Bella Coola River, for example, chinook salmon are the first adult oncorhynchid of the year to move into the river to spawn.

They arrive sometime in May, and they are the largest (averaging 10 kg, getting up to 25 kg). Chinook, which spawn in September, prefer the deepest and fastest water. Sockeye are the next salmon species to enter the system.

Typically they arrive in June and July. Spawning takes place from the end of September to the middle of October. Sockeye are a much smaller salmon (averaging 2.2 - 3.1 kg), and they require a lake as rearing habitat for their young.

Pinks and chums begin arriving to the Bella Coola system as early as July, spawning from mid-August to October with the peak in September. They are the smallest Bella Coola salmon, usually weighing 1.5 to 4 kg. They tend to spawn in the shallower river gravels of the system, which does not put them in conflict with the chinook or sockeye that arrived before them.

Chum salmon are fairly large, usually weighing 3.6 to 4.5 kg. Peak chum spawning activity is the end of August, but it can continues into November. Chum like to concentrate at the mouths of the smaller tributaries and creeks in the lower 20 miles of the valley. They are particularly noticeable near tidewaters of coastal streams.

Coho begin to enter the Bella Coola in August, but the peak time of entry is late September and early October, and they continue to arrive in December. Spawning begins and peaks in October, and continues into February.

Steelhead, as a species, seem to have specialized in invading headwaters of cold and unproductive glacial streams. They are the most adventuresome of the V onchorynchids, and one can imagine them being the first explorers when the glaciers melted, creating a myriad of new rivers.

In the Bella Coola, at least three distinct steelhead runs are recognized. The "summer run", which begins in late May or early June, consists of sexually immature fish that overwinter in holding pools of the upper Bella Coola River before moving up into the Atnarko River to spawn between March and May.

The second steelhead run begins in October or November and continues throughout the winter months. They spawn later in spring, usually May and June. A third spring run of fish begins to move in March, peaks in April, and continues to enter the system in May.

These are sexually mature fish which head on up to spawn in the Atnarko River after holding briefly in the pools of the upper Bella Coola River.

Adult steelhead use much the same habitat as do adult chinook salmon, but there is no major conflict because the species spawn at different times of the year - Bella Coola salmon spawn in the fall, whereas Bella Coola steelhead spawn in the late spring.

Coastal cutthroat trout also use the Bella Coola River system. As a general rule, they spawn in the small, slow moving tributaries or sloughs associated with the lower valley tributaries. The adults enter the river in the fall months, and spawning does not take place until May and June.

The behavior of fish in the Bella Coola system is typical of salmonids in watersheds throughout the Pacific Northwest.

Following patterns of behavior established over a millennium, they maximize the habitat with remarkable effectiveness.