Oregon Chub, Biosolids, and Oakridge Sewage Plant

Oregon chub (Oregonichthys crameri) is a minnow endemic to the Willamette River drainage, with a current distribution limited to 25 natural and nine introduced populations (Scheerer et al. 2003). The species was listed as endangered in 1993 over its entire range. Oregon chub prefer slow-moving backwater habitats, such as sloughs, flooded marshes, and beaver ponds, that are common along the Willamette River and its tributaries. While some chub populations are stable or have increased in recent years, others are declining or have been extirpated (Scheerer et al. 2003).

In the Oakridge Slough in the Middle Fork Willamette River drainage, the chub population density is low and the population has shown a recent decline in abundance from about 500 fish in 1999 to less than 50 fish in 2002 (Scheerer et al. 2003). Oakridge Slough also has an approximately 1-m thick layer that consists of primarily suspended organic material in various states of decay overlying a hard substrate. Sloughs in the area with more stable chub populations typically have less than 10 cm of organic material accumulated over a solid substrate. Although reports from U.S. Forest Service indicate that natural eutrophication resulted in the material in the slough, the consistency and appearance of the material in Oakridge Slough was observed to be very different from nearby sloughs by Paul Sheerer, Biologist for Oregon Department of Fish and Wildlife.

We undertook this investigation to compare nutrient concentrations in the slough to concentrations upstream and to those at or near the sewage treatment plant. In addition, vegetation, biosolids, and detrital samples were collected from specific areas to evaluate levels of stable isotopes of nitrogen that could indicate a sewage source of nitrogen entering the slough.

Results indicate that nitrate nitrogen is elevated in Oakridge Slough above reference levels, and enriched isotopes of nitrogen present in the slough suggest the sewage is the source for the nitrogen. Although nitrate levels are not high enough to directly impact chub, chub habitat is limited due to the excessive amount of organic material in the slough. Specific recommendations are outlined in the final report to further address this issue and protect Oregon chub in the slough.

Complete Report
Oregon Chub Species Fact Sheet