Chena River Chinook Salmon Study

Here’s a sample of the data being used to guide the selection of study reaches. These ArcGIS maps show the distribution of juvenile Chinook salmon in the Chena River in the spring (mid-June) and early fall (late August) of 2007. Each dot on the map represents fish counts in a 50-meter stretch of river. Big dots mean more fish, but dot size is not directly proportional to the fish count (or else some of the dots would have been huge). Counts were spaced at 5-km intervals.

Baited minnow traps and unbaited snorkelers were used to count fish. Interestingly, the snorkelers were far more successful in the spring than the minnow traps, and were almost useless in the fall. They were cold both times, and whined occasionally throughout.

Because these different counting methods are not equally effective, it’s more useful to compare the patterns of distribution between spring and fall than the actual number of fish found at each site. Remember, these are raw counts, not estimates of the total fish population at the site.

Spring (trap + snorkel counts):

Fall (traps only; snorkel counts were negligible):

In the lab, these data are being cross-referenced with other relevant numbers, like aerial redd counts. Those maps will get posted later. The most evident pattern is a downstream shift in distribution in the fall. A few of the several hypotheses to explain this pattern include:

1. The slightly warmer temperature downstream is more favorable for growth.
2. The lower water clarity downstream offers more protection from predators.
3. A major mid-summer flood carried most surviving fish downstream.

The study will hopefully reveal which of these (if any) explains the downstream shift.

Update August 1, 2008. New observations suggest there may not actually be a downstream shift.  We used the same methods and sampled similar habitat in the upper and lower river in 2007, but lower-river fish may be more inclined to utilize that habitat than upper-river fish.  We don’t have conclusive proof either way, but this possibility is noteworthy.