Lower Duncan River Kokanee AUC Analysis 2015

The suggested citation for this analytic report is:

Thorley, J.L. and Hogan, P.M. (2015) Lower Duncan River Kokanee AUC Analysis 2015. A Poisson Consulting Analysis Report. URL: https://www.poissonconsulting.ca/f/1629724501.


Since 2008 aerial surveys have been conducted in the Lower Duncan River (LDR) to count the number of kokanee spawners. The primary objectives of the current analysis report are to:

  • Estimate the aerial observer efficiency based on ground counts.
  • Estimate the annual kokanee spawner abundance from the aerial counts.
  • Estimate the peak spawn timing.


Data Preparation

The data were provided by LGL Limited in the form of an Access database and an Excel spreadsheet.

Statistical Analysis

Hierarchical Bayesian models were fitted to the count data using R version 3.2.3 (Team 2013) and JAGS 4.0.1 (Plummer 2015) which interfaced with each other via jaggernaut 2.3.1 (Thorley 2013). For additional information on hierarchical Bayesian modelling in the BUGS language, of which JAGS uses a dialect, the reader is referred to (Kery and Schaub 2011, 41–44).

Unless specified, the models assumed vague (low information) prior distributions (Kery and Schaub 2011, 36). The posterior distributions were estimated from a minimum of 1,000 Markov Chain Monte Carlo (MCMC) samples thinned from the second halves of three chains (Kery and Schaub 2011, 38–40). Model convergence was confirmed by ensuring that Rhat (Kery and Schaub 2011, 40) was less than 1.1 for each of the parameters in the model (Kery and Schaub 2011, 61).

The posterior distributions of the fixed (Kery and Schaub 2011, 75) parameters are summarised in terms of a point estimate (median), lower and upper 95% credible limits (2.5th and 97.5th percentiles), the standard deviation (SD), percent relative error (half the 95% credible interval as a percent of the point estimate) and significance (Kery and Schaub 2011, 37, 42).

Variable selection was achieved by dropping insignificant (Kery and Schaub 2011, 37, 42) fixed (Kery and Schaub 2011, 77–82) variables and uninformative random variables. A fixed variables was considered to be insignificant if its significance was \(\geq\) 0.05 while a random variable was considered to be uninformative if its percent relative error was \(\geq\) 80%. The Deviance Information Criterion (DIC) was not used because it is of questionable validity when applied to hierarchical models (Kery and Schaub 2011, 469).

The results are displayed graphically by plotting the modelled relationships between particular variables and the response with 95% credible intervals (CRIs) with the remaining variables held constant. In general, continuous and discrete fixed variables are held constant at their mean and first level values respectively while random variables are held constant at their typical values (expected values of the underlying hyperdistributions) (Kery and Schaub 2011, 77–82). Where informative the influence of particular variables is expressed in terms of the effect size (i.e., percent change in the response variable) with 95% CRIs (Bradford, Korman, and Higgins 2005).

Observer Efficiency

The aerial observer efficiency was estimated from ground counts using a Poisson model. Key assumptions of the observer efficiency model include:

  • Spawner abundance at a site does not vary between the ground and aerial count.
  • Ground observers are unbiased.
  • Ground counts are drawn from a Poisson distribution.
  • Aerial counts are drawn from an overdispersed Poisson distribution.
  • Aerial observer efficiency does not vary with abundance or channel type (side versus main channel).


The aerial spawner counts were analysed using a hierarchical Bayesian Area-Under-the-Curve (AUC) model (Hilborn, Bue, and Sharr 1999). Key assumptions of the AUC model include:

  • Spawner arrival and departure times are normally distributed (Hilborn, Bue, and Sharr 1999).
  • Spawner duration is constant across years.
  • Spawner abundance varies randomly by year, drawn from a normal distribution.
  • Peak spawn timing varies randomly by year, drawn from a normal distribution (Su, Adkison, and Van Alen 2001).
  • Spawner residence time is between 7 and 14 days (Acara 1970, @morbey_timing_2003).
  • Spawner observer efficiency is drawn from a normal distribution as estimated by the observer efficiency model, with a mean of 1.05, SD of 0.15 and lower and upper limits of 0.79 and 1.36 respectively.
  • The residual variation in the spawner counts is normally distributed.

Model Code

The JAGS model code, which uses a series of naming conventions, is presented below.

Observer Efficiency

Variable/Parameter Description
Aerial[i] Aerial count for ith survey
bAbundance log(eAbundance) intercept
bEfficiency eEfficiency intercept
eAbundance[i] Expected spawner abundance for ith survey
eAerial[i] Expected aerial count for ith survey
eEfficiency[i] Expected aerial observer efficiency for ith survey
Ground[i] Ground count for ith survey
sAbundance SD of residual variation in log(eAbundance)
sDispersion SD of overdispersion
Observer Efficiency - Model1

  bEfficiency ~ dunif(0, 3)

  bAbundance ~ dnorm(5, 5^-2)
  sAbundance ~ dunif(0, 5)

  sDispersion ~ dunif(0, 5)
  for (i in 1:length(Aerial)){

    eEfficiency[i] <- bEfficiency

    eAbundance[i] ~ dlnorm(bAbundance, sAbundance^-2)
    Ground[i] ~ dpois(eAbundance[i])

    eAerial[i] <- eAbundance[i] * eEfficiency[i]
    eDispersion[i] ~ dgamma(1 / sDispersion^2, 1 / sDispersion^2)
    Aerial[i] ~ dpois(eAerial[i] * eDispersion[i])


Variable/Parameter Description
bAbundance log(eAbundance) intercept
bAbundanceYear[i] Effect of ith year on log(eAbundance)
bDuration eDuration intercept
bEfficiency eEfficiency intercept
bPeakTiming ePeakTiming intercept
bPeakTimingYear[i] Effect of ith year on ePeakTiming
bResidenceTime Spawner residence time
Count[i] Spawner count for ith survey
Dayte[i] Centred day of the year for ith survey
eAbundance[i] Expected annual spawner abundance for ith survey
eCount[i] Expected spawner count for ith survey
eDuration[i] Expected SD of the duration of spawner arrival timing
eEfficiency[i] Expected aerial observer efficiency for ith survey
ePeakTiming[i] Expected timing of annual peak spawner abundance for ith survey
eSpawners[i] Expected number of spawners for ith survey
sAbundanceYear SD of effect of year on log(eAbundance)
sCount SD of residual variation in eCount
sPeakTimingYear SD of effect of year on ePeakTiming
Area-Under-The-Curve - Model1

  bEfficiency ~ dnorm(1.05, 0.15^-2) T(0.79, 1.36)

  bAbundance ~ dnorm(10, 5^-2)
  bPeakTiming ~ dnorm(0, 5)
  bDuration ~ dunif(0, 42)

  sAbundanceYear ~ dunif(0, 5)
  sPeakTimingYear ~ dunif(0, 28)
  for(i in 1:nYear){
    bAbundanceYear[i] ~ dnorm(0, sAbundanceYear^-2)
    bPeakTimingYear[i] ~ dnorm(0, sPeakTimingYear^-2)

  bResidenceTime ~ dunif(7, 14)

  sCount ~ dunif(0, 10000)
  for(i in 1:length(Count)){

    log(eAbundance[i]) <- bAbundance + bAbundanceYear[Year[i]]

    ePeakTiming[i] <- bPeakTiming + bPeakTimingYear[Year[i]]

    eDuration[i] <- bDuration

    eSpawners[i] <- ( phi((Dayte[i] - (ePeakTiming[i] - bResidenceTime/2)) / eDuration[i])
                     -phi((Dayte[i] - (ePeakTiming[i] + bResidenceTime/2)) / eDuration[i])
                    ) * eAbundance[i]

    eEfficiency[i] <- bEfficiency

    eCount[i] <- eSpawners[i] * eEfficiency[i]
    Count[i] ~ dnorm(eCount[i], sCount^-2)


Model Parameters

The posterior distributions for the fixed (Kery and Schaub 2011 p. 75) parameters in each model are summarised below.

Observer Efficiency

Parameter Estimate Lower Upper SD Error Significance
bAbundance 4.8610 3.8450 5.9600 0.5580 22 0.001
bEfficiency 1.0607 0.6980 1.4954 0.2003 38 0.001
sAbundance 1.9720 1.2510 3.0790 0.4850 46 0.001
sDispersion 0.5569 0.3555 0.8776 0.1419 47 0.001
Convergence Iterations
1.05 20000


Parameter Estimate Lower Upper SD Error Significance
bAbundance 9.9900 8.8160 10.9470 0.5540 11 0.0010
bDuration 5.8400 1.2960 8.3460 1.6640 60 0.0010
bEfficiency 1.0578 0.8253 1.3204 0.1308 23 0.0010
bPeakTiming 0.0980 -0.7690 0.9430 0.4480 870 0.8104
bResidenceTime 10.9840 7.2900 13.8860 2.0560 30 0.0010
sAbundanceYear 1.1540 0.4640 2.8230 0.6050 100 0.0010
sCount 5965.0000 4651.0000 7595.0000 780.0000 25 0.0010
sPeakTimingYear 10.8000 4.7200 23.4200 4.8400 87 0.0010
Convergence Iterations
1.04 10000


Observer Efficiency

Figure 1. Aerial versus ground kokanee spawner counts and predicted ground count (with 95% CRIs) by year and channel.
Figure 2. Aerial versus ground kokanee spawner counts (on log10 scales) and predicted ground count (with 95% CRIs) by year and channel.


Figure 3. Kokanee spawner aerial counts with predicted aerial counts by date and year. Blue points indicate missing visibility values.
Figure 4. Predicted total kokanee spawner abundance by year (with 95% CRIs).
Figure 5. Predicted total kokanee spawner abundance to peak count ratio by year (with 95% CRIs). Data for 2015 removed as too uncertain to be informative.
Figure 6. Predicted kokanee start, peak and end spawn timing by year (with 95% CRIs).


The organisations and individuals whose contributions have made this analysis report possible include:

  • BC Hydro
    • Guy Martel
  • Okanagan National Alliance
    • Michael Zimmer
    • Natasha Audy
    • Skyeler Folks
  • LGL Limited
    • Elmar Plate
  • Amec Earth and Environmental
    • Louise Porto
    • Crystal Lawrence
    • Clint Tarala
  • Poisson Consulting Ltd.
    • Robyn Irvine


Acara, A. H. 1970. “The Meadow Creek Spawning Channel. Unpublished Report.” Victoria, BC: Fish; Wildlife Branch.

Bradford, Michael J, Josh Korman, and Paul S Higgins. 2005. “Using Confidence Intervals to Estimate the Response of Salmon Populations (Oncorhynchus Spp.) to Experimental Habitat Alterations.” Canadian Journal of Fisheries and Aquatic Sciences 62 (12): 2716–26. https://doi.org/10.1139/f05-179.

Hilborn, Ray, Brian G Bue, and Samuel Sharr. 1999. “Estimating Spawning Escapements from Periodic Counts: A Comparison of Methods.” Canadian Journal of Fisheries and Aquatic Sciences 56 (5): 888–96. https://doi.org/10.1139/f99-013.

Kery, Marc, and Michael Schaub. 2011. Bayesian Population Analysis Using WinBUGS : A Hierarchical Perspective. Boston: Academic Press. http://www.vogelwarte.ch/bpa.html.

Morbey, Y. E., and R. C. Ydenberg. 2003. “Timing Games in the Reproductive Phenology of Female Pacific Salmon (Oncorhynchus Spp.).” The American Naturalist 161 (2): 284–98. http://www.jstor.org/stable/10.1086/345785.

Plummer, Martyn. 2015. “JAGS Version 4.0.1 User Manual.” http://sourceforge.net/projects/mcmc-jags/files/Manuals/4.x/.

Su, Zhenming, Milo D. Adkison, and Benjamin W. Van Alen. 2001. “A Hierarchical Bayesian Model for Estimating Historical Salmon Escapement and Escapement Timing.” Canadian Journal of Fisheries and Aquatic Sciences 58 (8): 1648–62. https://doi.org/10.1139/cjfas-58-8-1648.

Team, R Core. 2013. “R: A Language and Environment for Statistical Computing.” Vienna, Austria: R Foundation for Statistical Computing. http://www.R-project.org.

Thorley, J. L. 2013. “Jaggernaut: An R Package to Facilitate Bayesian Analyses Using JAGS (Just Another Gibbs Sampler).” Nelson, B.C.: Poisson Consulting Ltd. https://github.com/poissonconsulting/jaggernaut.