Suppression of non-native Brown Trout and Brown Bullhead in a Modoc Sucker stream (northern California)

Authors: Stewart B. Reid

Year: 2010 (xlii)

Abstract

The Modoc Sucker, Catostomus microps, a small species found in northeastern California and south-central Oregon, was federally listed as endangered in 1985. A recent conservation and status review determined that status of the species has improved substantially and the principal remaining threat to the Johnson Creek population is predation by non-native fishes, specifically the piscivorous Brown Trout, Salmo trutta (USFWS 2009). Brown Trout have been present in Johnson Creek since the 1930’s, but have increased in number and relative abundance. The present project was initiated in 2008 to attempt eradication of Brown Trout and Brown Bullhead, Ameiurus nebulosus, (the only two non-native fishes present) from Johnson Creek using non-chemical, targeted removal methods (hand-net and restricted field electro-shocker) to not adversely affect Modoc Suckers. Methodology is based on experience with successful eradication of non-native Largemouth Bass from a nearby drainage also containing Modoc Suckers. There is an existing gabion barrier 0.9 mi up from the mouth of Johnson Creek that limits reinvasion, as do low summer and fall flows which dry up the lowest reach. In 2009, the first full year of the project, Brown Trout (7-29 cm) were removed from Johnson Creek, and by the end of the year only 22 Brown Trout (Year 1+) were estimated to remain in reaches containing Modoc Suckers, although higher numbers remained upstream. Rainbow Trout, Oncorhynchus mykiss, a native species generally found with Modoc Suckers and out-competed by the more aggressive Brown Trout, were present in all reaches, except the lowest, warmest reach (below barrier). Generally, Rainbow and Brown trout were of similar sizes, but Browns were considerably more abundant and included a few isolated larger individuals (25-36 cm). Brown Bullheads were found in moderate numbers, with a limited distribution (17 pools). Over half were removed, and only ten pools contained bullheads (mostly juveniles) at the end of the summer, with most (~65%) in just three large pools. Brown Trout were determined to spawn in Johnson Creek during October, when immigration from downstream is not possible due to dry downstream reaches. In 2010, removal of Brown Trout continued, with greater emphasis on higher reaches, which act as a source to lower reaches containing Modoc Suckers. By October 2010 only five adult Brown Trout (14-20 cm) were estimated to remain in reaches containing Modoc Suckers, with about 20 remaining in upstream reaches. There was apparently little, if any, recruitment of Brown Trout in the Modoc Sucker reaches. Rainbow Trout populations are still low but now exceed adult Brown Trout both above and below the highway. Brown Bullheads were restricted primarily to a single pool just above the barrier. The project’s first two years demonstrates that removal of Brown Trout and Brown Bullhead from the Johnson Creek drainage is feasible using a low-impact methodology over a period of 3-5 years. In 2011, removal of both Brown Trout and Brown Bullhead and census of Rainbow Trout will continue, focusing on complete removal of non-natives from Modoc Sucker reaches.