Biological Opinions - Bureau of Reclamation

Figure 12.9. Proportion of surveyed streams where coho salmon were detected (Good et al. 2005). The
number of streams surveyed is shown next to data.
12.2.5.4 Diversity
The primary factors affecting the diversity of SONCC ESU coho salmon appear to be low
population abundance and the influence of hatcheries and out-of-basin introductions. Although
the operation of a hatchery tends to increase the abundance of returning adults (70 FR 37160;
June 28, 2005), the reproductive success of hatchery-born salmonids spawning in the wild can be
significantly less than that of naturally produced fish (Araki et al. 2007). As a result, the higher
the proportion of hatchery-born spawners, the lower the overall productivity of the population, as
demonstrated by Chilcote (2003). Williams et al. (2008) considered a population to be at least at
a moderate risk of extinction if the contribution of hatchery coho salmon spawning in the wild
exceeds 5 percent. Populations have a lower risk of extinction if no or negligible ecological or
genetic effects are demonstrated as a result of past or current hatchery operations. Because the
main stocks in the SONCC coho salmon ESU (i.e., Rogue River, Klamath River, and Trinity
River) remain heavily influenced by hatcheries and have little natural production in mainstem
rivers (Weitkamp et al. 1995; Good et al. 2005), many of these populations are at high risk of
extinction relative to the genetic diversity parameter.
In addition, some populations are extirpated or nearly extirpated (i.e., Middle Fork Eel, Bear
River, Upper Mainstem Eel) and some brood years have low abundance or may even be absent
in some areas (e.g., Shasta River, Scott River, Mattole River, Mainstem Eel River), which further
restricts the diversity present in the ESU. The ESU’s current genetic variability and variation in
life history likely contribute significantly to long-term risk of extinction. Given the recent trends
in abundance across the ESU, the genetic and life history diversity of populations is likely very
low and is inadequate to contribute to a viable ESU.
311