Bull Trout population monitoring in the Oregon portion of the mid-Columbia recovery unit Progress Report: 2020 - 2021

The Bull Trout recovery plan calls for the use of a threats assessment tool for evaluating the threats to the species in recovery units for 5-year status reviews and potential delisting. To some extent, those threats evaluations and status assessments will be dependent on demographic and ecological information related to characteristics of a “recovered” recovery unit. Demographic and ecological data for most Bull Trout populations in the Oregon portion of the mid-Columbia recovery unit have not been collected consistently or extensively. A monitoring strategy for Bull Trout populations in the mid-Columbia recovery unit was recently developed (Howell et al. 2018). Our aim is to implement parts of that strategy by undertaking such activities as PIT tagging juvenile Bull Trout in rearing areas to take advantage of data generation provided by existing PIT tag detection sites downstream (e.g., in the Imnaha River), conducting electrofishing surveys to confirm results obtained from eDNA sampling efforts (presence/absence and distribution), and conducting electrofishing, snorkeling, and spawning ground surveys in streams supporting Bull Trout that are surveyed annually (abundance or relative abundance and distribution), or have not been surveyed in many years. In addition, we will participate in cooperative investigations (with the Oregon Department of Fish and Wildlife [ODFW] and Pacificorp) expanding on past work conducted in the Wallowa River drainage upstream from Wallowa Lake involving the genetic and demographic characteristics of Bull Trout, Brook Trout, and Bull Trout x Brook Trout hybrids.

In 2020 and 2021, we conducted electrofishing surveys in 20 streams in northeast Oregon (Appendix Tables 1 and 2; Appendix Figures 1 – 18). Sampling reaches were typically spaced 1 km apart within potential Bull Trout spawning and early rearing areas identified through various modeling efforts (e.g., Isaak et al. 2015); however, in streams where we failed to collect Bull Trout, but in which Bull Trout eDNA had previously been detected, we subsequently electrofished at the midpoints between sites to increase sampling intensity. While conducting the surveys, we measured the wetted stream width at the beginning, middle, and end of each reach and enumerated and estimated the fork length (FL) of the fish that were captured. Most of the sampling reaches were 50 m. To assist with an Oregon Department of Fish and Wildlife (ODFW) research project aimed at developing a model to predict Bull Trout occupancy (M. Meeuwig, ODFW, personal communication), we electrofished six 250-m (five contiguous 50-m) sampling reaches in the Catherine Creek drainage.

In Little Sheep Creek and its tributary Cabin Creek (Appendix Figure 19), we electrofished to capture and PIT tag Bull Trout in August 2021. We PIT tagged the Bull Trout to obtain a mark-recapture estimate of the number of individuals >120 mm FL (i.e., to obtain an estimate of the adult population size) and to determine whether the population includes any migratory individuals and if two culverts aligned side-by-side near mouth of Little Sheep Creek form a fish passage fish passage
Fish passage is the ability of fish or other aquatic species to move freely throughout their life to find food, reproduce, and complete their natural migration cycles. Millions of barriers to fish passage across the country are fragmenting habitat and leading to species declines. The U.S. Fish and Wildlife Service's National Fish Passage Program is working to reconnect watersheds to benefit both wildlife and people.

Learn more about fish passage barrier. We collected and marked Bull Trout throughout their entire distribution in Little Sheep and Cabin creeks (Appendix Figure 19). Captured Bull Trout were anesthetized with tricaine methanesulfonate (MS-222) and measured for fork length. We injected a PIT tag (8.5 mm long) into the dorsal sinus of those that were >120 mm. After the fish had recovered from the anaesthetic, they were released near their capture site. The marking event occurred from 16 - 18 August. On 24-26 August, we again electrofished the study reaches in Little Sheep and Cabin creeks to collect Bull Trout and measure and enumerate marked and unmarked individuals.

In three streams in which Bull Trout eDNA previously had been detected at multiple sites—Chicken and East Fork Indian creeks and the Grande Ronde River—we failed to collect any Bull Trout during the initial electrofishing survey (one 50-m sampling reach every 1 km). When we subsequently electrofished at the midpoints between sampling reaches, we captured seven Bull Trout in a single reach in East Fork Indian Creek, but again failed to capture any Bull Trout in Chicken Creek and the Grande Ronde River. Fork lengths of the Bull Trout in East Fork Indian Creek ranged from 70 to 120 mm, indicating multiple age classes, and, presumably a population, were present. We confirmed through electrofishing that Onion, Granite, and Boundary creeks, in which Bull Trout eDNA had previously been detected, did, in fact, support Bull Trout (Appendix Table 1 and 2; Appendix Figures 1 – 18). In streams where Bull Trout were present, densities ranged from 0.002 to 0.072 fish/m² (Appendix Tables 1 and 2). Brook Trout were captured in Onion, Crane, and Hurricane creeks and the Grande Ronde River. Their densities ranged from 0.005 to 0.027 fish/m² (Appendix Tables 1 and 2). An apparent Bull Trout x Brook Trout hybrid was captured in Hurricane Creek. Hybrids were not observed in the other streams supporting both Bull Trout and Brook Trout.

We PIT tagged and released 176 Bull Trout ranging in fork length from 124 to 235 mm in Little Sheep and Cabin creeks. We captured, but did not PIT tag, one individual that was greater than 300 mm FL, indicating it might have been a migratory adult. During the recapture event, 130 Bull Trout >120 mm FL were captured, 50 of which were recaptures. Thus, we estimated (simple Petersen estimate) 457 +/- 50 Bull Trout >120 mm FL were present in Little Sheep and Cabin creeks. Whereas Bull Trout were relatively abundant and distributed widely in Little Sheep and Cabin creeks in 2021, they were found in only three of the lowermost GRTS sites (n = 17) sampled in those streams in 2010 (Hudson et al. 2014). Hudson et al. speculated recent fires might have degraded the habitat in the upper portions of Little Sheep and Cabin creeks and affected the distribution of Bull Trout. If that was the case, it appears the habitat has recovered sufficiently to support Bull Trout.