A Bayesian Approach to Combine Multiple Sources of Escapement Data to Estimate Wind River Steelhead Abundance Dan Rawding and Charlie Cochran

Steelhead Abundance Estimate Goal - provide an unbiased estimate with an estimate of uncertainty (confidence interval) for Wind River summer steelhead Motivation - Hemlock dam was removed during the summer 2009, so we want to determine if mark-resight (snorkeling) estimates are a robust alternative to mark-recapture (trap) estimates from the recoveries at Hemlock dam. Test Petersen assumptions and correct to obtain an unbiased estimate

Wind River is located 55 miles east of Portland Drainage Area is 226 sq miles Only summer steelhead can jump Shipherd Falls Nov-Apr 100% ladder May-Oct 25% ladder Hatchery steelhead excluded since 1999 Hatchery steelhead are less than 2% of steelhead escapement

Shipherd Falls Trapping Trap operation (Jun 1 – May 1); all fish tagged Fluorescent Floy tags (Jun 1–Nov 15) ; rotate tag colors so repeats jumping falls can be identified White tag for summer steelhead fish (Nov 15 - May 1) Blue tag for winter steelhead (Nov 15 to May 1) Double Floy tags Ensure resight during snorkels To estimate tag loss

Recapture & Resight Methods Petersen Estimate is N = m*c/r, where m= marks, c = captures, and r = recaptures Trout Creek Two trap design based on all marks released at Shipherd Falls and trapping at Trout Creek Winter Snorkel Mark-resight abundance estimate for fluorescent tags plus a census of all fish passed from Nov 15- early May.

Jumper Method Aug & Sep snorkel estimate (200 & 300) Sep – Aug snorkel estimate (100) Jumper Rate = (Sep - Aug estimate) / trapped fish (100/20)= 5 # of trapped fish until end of jumper period * jumper rate(30*5) =150 # of trapped fish after jumper period = 100 Population estimate is the sum of the estimates = 550

Unbiased Estimates Petersen assumptions must be met for an unbiased estimate Population is closed No tag effects No tag loss Tagging does not effect catchability/survival All tags are identified and reported Equal Catchability all fish have the same probability of being marked; or all fish have the same probability of being captured in the second sample; or marked and unmarked fish mix completely

Closure Tag recoveries adjacent basins, PIT tag detections from mainstem Columbia River dams tributary instream interrogators, and anglers Probably a slight under reporting of tags leaving basin, since all out of basin recoveries are not complete 99.3 to 99.7% of tagged fish stayed in the Wind River

Tag Loss Double Floy tag all fish at Shiphed Falls Inspect fish at Trout Creek trap to estimate single tag loss, and then estimate the probability of losing both tags, and apply to Trout Cr. Estimate. Annual probability of retaining at least one tag is ~97% What about snorkeling? Obtain tag recovery information from Trout Cr, trap, Shipherd Falls re-ascension & repeat spawners, and angler recoveries Use logistic regression to estimate tag loss based on days after tagging and probability of losing two tags Use median day between tagging and snorkel to estimate tag loss, and the probability of retaining at least one tags

Tag Induced Mortality Skamania Hatchery test 500+ controls hatchery summer steelhead swim into hatchery raceway untouched 500+ treatment held in raceway then netted, anesthetized, and double Floy tagged 21 day survival was over 99% for controls and treatment. Test hypothesis that handling reduced survival & estimate survival of treatment relative to control Bayesian P-value = 0.20, so can’t reject the hypothesis that survival of handled steelhead is reduced Survival = 99.5% w/95%CI (98.4% -100%)

Equal Mixing Aug & Sep Snorkel Surveys Petersen estimate unbiased if systematic survey & complete mixing Summer steelhead hold in the mainstem Wind River before ascending into tributaries during freshets Oct – May Freshets Best estimate is % of summer steelhead are in the mainstem Wind River during Aug & Sep snorkel surveys Systematic survey of mainstem Wind River Complete mixing test of the upper & lower snorkel survey section showed complete mixing of tagged an untagged fish in 29 of 32 surveys (  =0.05)

Snorkel Efficiency & Proportion Marked Is snorkel efficiency (ratio of recaptures to marks) the same? 14 of 18 consecutive day surveys no difference (  =0.05) Interpretation: there is some evidence that some crews see tagged fish at higher rate than other crews Is the proportion of marked fish (ratio of recaptures to observed fish) the same? 18 of 18 consecutive day surveys no difference (  =0.05) Interpretation: even though there is some evidence of varying snorkel efficiency, there is a lack of evidence that tagged and untagged fish are being observed at different rates. Population estimates are consistent between consecutive day surveys. Pool snorkel surveys from consecutive days to improve abundance estimate

Bayesian Mark-Recapture Model Beta prior (a=b=1), Uniform prior (min=T, max=2000), p=snorkel efficiency, Nt = Pop. Est, T=tags, R=recaptures,C=captures

Jumper Model

DAG for tag loss, handling, closure incorporated into Petersen Estimate

2 of 27 estimates Significantly Different (  =0.05)

Summary Precision (CV) of individual Trout Cr, Jumper, and Winter Snorkel survey estimates ranged from 11% to 22% from 2001 onward. Jumper estimate was the most precise in 8 of 10 years with CV from 11% to 17%. When all method were combined the precision (CV) improved to 7% to 10%. Multiplying the number of tags released by 96% to 99% is needed to obtain an unbiased mark-recapture estimate to adjust for closure (<1%), handling mortality(<1%), and tags loss (probability of retaining at least one tags <3% from tagging to prespawning) violations

Summary There was no difference in the proportion of marked fish between 18 consecutive snorkel surveys but there was some evidence of different snorkel efficiency on consecutive surveys (4 of 18) Mark-recapture (trap) and mark-resight (snorkel) estimates were similar in 25 of 27 comparisons Mark-resight estimate is a consistent alternative to the mark-recapture estimates in the Wind River. Summer steelhead mark-resight (snorkeling) may be a more cost-effective alternative to mark-recapture (trapping) in larger system where holding a trap is expensive and problematic.