A Bayesian Approach to Combine Multiple Sources of Escapement Data to Estimate Wind River Steelhead Abundance

Dan Rawding and Charlie Cochran

Steelhead Abundance EstimateGoal - provide an unbiased estimate with an estimate of uncertainty (confidence interval) for Wind River summer steelheadMotivation - 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 PortlandDrainage Area is 226 sq milesOnly summer steelhead can jump Shipherd FallsNov-Apr 100% ladderMay-Oct 25% ladderHatchery steelhead excluded since 1999Hatchery steelhead are less than 2% of steelhead escapement

Shipherd Falls TrappingTrap operation (Jun 1 – May 1); all fish taggedFluorescent 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 tagsEnsure resight during snorkelsTo estimate tag loss

Recapture & Resight MethodsPetersen 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 = 100Population estimate is the sum of the estimates

300 + 150 + 100 = 550

Unbiased EstimatesPetersen assumptions must be met for an unbiased estimate

Population is closedNo 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

ClosureTag recoveries

adjacent basins,PIT tag detections from mainstem Columbia River damstributary instream interrogators, and anglers

Probably a slight under reporting of tags leaving basin, since all out of basin recoveries are not complete99.3 to 99.7% of tagged fish stayed in the Wind River

Tag LossDouble Floy tag all fish at Shiphed FallsInspect 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 recoveriesUse logistic regression to estimate tag loss based on days after tagging and probability of losing two tagsUse median day between tagging and snorkel to estimate tag loss, and the probability of retaining at least one tags

Tag Loss

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Observed Tag Retention

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Predicted Retention of Atleast 1 Tag

Tag Induced MortalitySkamania Hatchery test500+ controls hatchery summer steelhead swim into hatchery raceway untouched500+ treatment held in raceway then netted, anesthetized, and double Floy tagged21 day survival was over 99% for controls and treatment. Test hypothesis that handling reduced survival & estimate survival of treatment relative to controlBayesian P-value = 0.20, so can’t reject the hypothesis that survival of handled steelhead is reducedSurvival = 99.5% w/95%CI (98.4% -100%)

Equal Mixing Aug & Sep Snorkel Surveys

Petersen estimate unbiased if systematic survey & complete mixingSummer steelhead hold in the mainstem Wind River before ascending into tributaries during freshets Oct –May FreshetsBest estimate is 93-100% of summer steelhead are in the mainstem Wind River during Aug & Sep snorkel surveysSystematic survey of mainstem Wind RiverComplete 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 MarkedIs 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 crewsIs 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

model; { R ~ dbin(C,p) p ~ dbeta(a,b) M ~ dbin(p,N) N ~ dunif(min,max) }

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

Nt

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Jumper Model

logU

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qR

M_ehtl

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b_ha_h

b_ma_m

p_ee_s

e

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p_1m_1

DAG for tag loss, handling, closure incorporated intoPetersen Estimate

Wind River Population Estimates

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2 of 27 estimatesSignificantly Different (=0.05)

Wind River Population Estimates

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SummaryPrecision (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

SummaryThere 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 comparisonsMark-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.