River Copasity

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The Chandler/Bjornn article was in the TAFS in 1988; vol. 117 issue 5.

According to the abstract the study of the fry early and late emerging fry were held in artificial channels with the earlier fish both larger and more robust than the later fish. When the two were held in the same channel the early fish dominated but the over all numbers were greater than when held together. The kicker in the study was that the fish were held 1 to 2 months. As we know that is only a small portion (less than 5% of the time) that juvenile steelhead spend in freshwater. In much of the steelhead world that early freshwater period of the newly emergent fry is rarely the smolt production (carrying capacity) bottle neck. That certainly the case on the north sound rivers; typically for the steelhead in that area the production bottle neck is more often the over-wintering habitat of either the first year fry and the second year parr.

WW raised a critical point in that in a basin like the Skagit the steelhead fry/parr are sharing the water with a variety of species (cutthroat, bull trout, coho, Chinook, pink, whitefish, etc.)all of which of individual habitat preference in which they do the best. As a result the fish partition the habitat with each species dominating the habitat in which they are best suited. In basins with less diversity of species or some species at depressed levels there is more potential for a species like steelhead (a higher carrying capacity).

On the north Sound rivers the wild steelhead spawning timing is determined by the constant tug-a-war between the pull to spawn early (emerge early) and produce larger fry and the pull to spawn later so that the newly emerging fry avoid the hazards of the snow melt run-off.

Curt

KerryS said:

At what point does the gene pool become full? Is it at 500 fish? Or is it at 1000? My questions have nothing to do with carrying capacity but more to do with survival of the most capable. Sometimes I wonder if there is to much emphasis put on the minimum number needed to maintain the population at the detriment of the population's ability to produce the most fit to adapt and survive.

Click to expand...

Hi Kerry,
Your question is directed at fields of conservation biology called minimum viable population and population viability analysis. In the case of steelhead, remember that the best evidence at present indicates that the resident rainbows are part of the same gene pool as ocean-going steelhead. Therefore, they are a reservoir of genetic diversity too.
Over the long-term, genetic diversity is probably the more important consideration than is "most capable". There is significant variability in all the various environments that steelhead experience during their lives. The most resilient population is one which contains enough genetic diversity that there are always some individuals that will thrive under the new conditions.
For example, biologists always tell the story of the amazing homing abilities of salmonids; the adaptive explanation is that this facilitates a population evolving to match the specific local conditions of a specific watershed. Yet, somebody needed to seed the rivers in the first place, especially with the dynamics of ice ages. Therefore, there are situations when straying is a positive characteristic. The offspring of the first fish to spawn into a pristine watershed will experience little competition and should experience solid growth and reproduction. A healthy steelhead/rainbow trout population will have some individuals that stay at home (residents) and others that migrate out (steelhead), some that spawn in the same watershed and others that are more adventurous.
Steve