NF Siletz Adult Resting Count Survey
Project Background and Description
An adult resting count survey was commissioned in the Siletz river above the falls (RM 64.7) by the Corvallis chapter of Trout Unlimited to supplement our collective understanding of how summer steelhead are utilizing the upper Siletz basin. Very contemporary (2014 -2015) sightings of adipose clipped summer steelhead above the Siletz falls by recreational snorkelers and actual sightings of adult summer steelhead clearing the natural barrier at the falls by ODFW staff had suggested that trap counts may not accurately represent both the total escapement of wild summer steelhead above the falls or be accurately portraying the presence of adult hatchery steelhead above the falls (assumed to be at or near zero).
The survey was conducted on September 22 and 23 by Bio-Surveys, LLC with assistance from 2 volunteers from the Corvallis Trout Unlimited chapter. The intent of the survey was to complete a 100% census of adult summer steelhead and adult spring chinook (including jacks) that extended from the falls on the mainstem Siletz to an unknown upstream point that described the full range of adult distribution for both species.
To achieve a 100% census, every pool was snorkeled in a downstream inventory. In addition, deeper glides exhibiting cover were also walked through or snorkeled to cover the potential that some adults may have been activated by 2 significant fall rain events and shifted into other than resting pool habitats. Both increased flows and cooler stream temperatures were observed than would have been present 1 month earlier.
The surveys encompassed 2 consecutive days. Day 1 completed approximately 8.9 stream miles that extends from the falls to the confluence of Warnicke Cr (3 snorkelers). Day 2 completed the 2.6 mile stream segment from Warnicke Cr upstream to the end point of the survey (1 snorkeler) at a bridge crossing in Section 23 (N44 56 34.81 / W123 45 05.62). In addition, 0.5 miles of Boulder Cr was snorkeled to look for resting adults in a cooler tributary. Warnicke Cr was not snorkeled and the surveyor described the confluence as a low flow contributor exhibiting limited potential for adult passage during low summer flow regimes. The SF of the Siletz that enters the mainstem at RM 67.5 was also not surveyed.
Project Goals and Objectives
The survey intended to produce an estimate of escapement to the upper NF Siletz for both summer steelhead and spring chinook adults. An escapement estimate is already produced annually at the trap and counting facility managed by ODFW at the Siletz Falls. In addition, the survey intended to estimate the pre-spawning ratio of hatchery to wild adult summer steelhead. Additional, information was gathered on the general range and relative abundance of each species of adult prior to spawning.
The distribution of summer steelhead adults was confined to an 8.7 mile stream segment of the mainstem Siletz and NF Siletz that extended from the falls to a point approximately 400 meters below the confluence of Warnicke Cr. The SF Siletz was not included in the inventory and may have contained a small number of resting adults. The pool at the confluence of the SF Siletz contained a high concentration of both steelhead and spring chinook and all steelhead and most spring chinook were observed holding in the cool water arm of that pool that extended up the NF and not below the confluence of the SF Siletz. Boulder Cr (a cool water contributor) was snorkeled for approximately 0.5 miles and no resting adults of either species was observed.
Table 1 combines all of the data for all of the surveyors for both inventory days. Surveys in the stream segment between the falls and the confluence of the SF Siletz (2.8 miles) contained 44.2% of the total of all ssthd observed. The visibility in deep pools within this stream segment was compromised by heavy tannins emanating from the SF Siletz that affected visibility and therefore our confidence in the final number. The surveyor was 100% confident that some unknown number of adults were not observed in at least 2 pools where the bottom could not be seen.
|Ad clip ssthd||Unidentified
|Total schin||Jack schin||schin carcass|
The species totals presented in table 1 therefore are minimum estimates of abundance with a high confidence that the actual number of resting adults was higher.
The data combined from all 3 surveyors suggests that 15.5% of all adult ssthd observed were of hatchery origin (adipose clipped). The range of hatchery percentages observed between the 3 individual surveyors was broad (5.4% – 24.7%). Two of the 3 snorkelers were classified as highly experienced and the 3rd was classified as a beginner. When we remove the data of the inexperienced snorkeler and query a subset of the data that represents a higher confidence estimate of hatchery abundance from just the 2 highly experienced snorkelers, an average value of 19.3% adipose clipped ssthd (N= 38 / 197) was produced.
Table 2 indicates the total count by species at the Siletz Falls trap operated by ODFW up to September 21, 2015 (the day before the snorkel inventory). The snorkel count of resting wild steelhead was 236 (including those observed of unknown origin). This is 86% of the actual number documented at the trap.
|Wild Summer Sthd Passed||Wild Spring Chinook Passed||Total Hatchery Summer Sthd not passed|
Because summer steelhead have been observed clearing the falls and adipose clipped steelhead have been observed in resting pools above the trap in previous years, the assumption is made that wild escapement above the falls may be higher than quantified at the trap and that hatchery / wild spawning interactions are occurring that may influence the genetics of the wild summer steelhead population above the falls.
Because of the compromised visibility below the confluence of the SF Siletz the snorkel inventory was not effective for the development of a population estimate of wild summer steelhead that may have been augmented by wild fish clearing the falls barrier and being unaccounted for at the trap. There are however some inferences that can be made from the observed success rate of the hatchery steelhead clearing the falls. A known 2,238 hatchery origin steelhead were either handled at the trap (2,196) or observed holding above the trap (42 naturally passed). Therefore, a minimum of 1.9% of the hatchery fish attempting passage at the falls were successful in navigating the falls under their own power. If this same success rate were applied to the number of wild steelhead that were handled at the trap, it would result in just an additional 5 steelhead clearing the falls naturally. This exercise only being pertinent for evaluating the scope of the issue as it relates to our confidence in the reported trap data (high).
Spring chinook comparisons were very close between the 100% snorkel estimate (284 / including observed carcasses) and the actual trap count data (281). This is an indication that very low pre spawning mortality rates are occurring for the spring chinook passed above the trap. During the time of the survey, spring chinook adults had backed out of the resting pools onto spawning beds and were actively spawning.