Duncan Lardeau Juvenile Rainbow Trout Abundance 2019b

Length Correction

The annual bias (inaccuracy) and error (imprecision) in observer’s fish length estimates when spotlighting (standing) and snorkeling were quantified from the divergence of their length distribution from the length distribution for all observers (including measured fish) in that year. More specifically, the length correction that minimised the Jensen-Shannon divergence (Lin 1991) between the two distributions provided a measure of the inaccuracy while the minimum divergence (the Jensen-Shannon divergence was calculated with log to base 2 which means it lies between 0 and 1) provided a measure of the imprecision.

Abundance

The abundance was estimated from the count data using an overdispersed Poisson model (Kery and Schaub 2011, 55–56). The annual abundance estimates represent the total number of fish in the study area.

Key assumptions of the abundance model include:

• The lineal fish density varies with year, river kilometer as a polynomial, and randomly with site.
• The observer efficiency at marking sites varies by study design (GPS versus Index).
• The observer efficiency also varies by visit type (marking versus count) within study design and randomly by snorkeller.
• The expected count at a site is the expected lineal density multiplied by the site length, the observer efficiency and the proportion of the site surveyed.
• The residual variation in the actual count is gamma-Poisson distributed.

Condition

The condition of fish with a fork length \(\geq\) 500 mm was estimated via an analysis of mass-length relations (He et al. 2008).

More specifically the model was based on the allometric relationship

\[ W = \alpha_c L^{\beta_c}\]

where \(W\) is the weight (mass), \(\alpha_c\) is the coefficent, \(\beta_c\) is the exponent and \(L\) is the length.

To improve chain mixing the relation was log-transformed, i.e.,

\[ \log(W) = \log(\alpha_c) + \beta_c \log(L).\]

Key assumptions of the condition model include:

• \(\alpha_c\) can vary randomly by year.
• The residual variation in weight is log-normally distributed.

Fecundity

The fecundity of females with a fork length \(\geq\) 500 mm was estimated via an analysis of fecundity-mass relations.

More specifically the model was based on the allometric relationship

\[ F = \alpha_f W^{\beta_f}\]

where \(F\) is the fecundity, \(\alpha_f\) is the coefficent, \(\beta_f\) is the exponent and \(W\) is the weight.

To improve chain mixing the relation was log-transformed.

Key assumptions of the fecundity model include:

• The residual variation in fecundity is log-normally distributed.

Spawner Size

The average length of the spawners in each year (for years for which it was unavailable) was estimated from the mean weight of Rainbow Trout in the Kootenay Lake Rainbow Trout Mailout Survey (KLRT) using a linear regression. This approach was suggested by Rob Bison.

Egg Deposition

The egg deposition in each year was estimated by

1. converting the average length of spawners to the average weight using the condition relationship for a typical year
2. adjusting the average weight by the annual condition effect (interpolating where unavailable)
3. converting the average weight to the average fecundity using the fecundity relationship
4. multiplying the average fecundity by the AUC based estimate of the number of females (assuming a sex ratio of 1:1)

Stock-Recruitment

The relationship between the number of eggs (\(E\)) and the abundance of age-1 individuals the following spring (\(R\)) was estimated using a Beverton-Holt stock-recruitment model (Walters and Martell 2004):

\[ R = \frac{\alpha_s \cdot E}{1 + \beta_s \cdot E} \quad,\]

where \(\alpha_s\) is the maximum number of recruits per egg (egg survival), and \(\beta_s\) is the density dependence.

Key assumptions of the stock-recruitment model include:

• The residual variation in the number of recruits is log-normally distributed with the standard deviation scaling with the uncertainty in the number of recruits.

The age-1 carrying capacity (\(K\)) is given by:

\[ K = \frac{\alpha_s}{\beta_s} \quad.\] and the \(E_{K/2}\) Limit Reference Point (Mace 1994, E_{0.5 R_{max}}), which corresponds to the stock (number of eggs) that produce 50% of the maximum recruitment (\(K\)), by \[E_{K/2} = \frac{1}{\beta_s}\]

The LRP was also converted into a number of spawners in a typical year (assuming 6,000 eggs per spawner and a sex ratio of 1:1).

Age-1 to Age-2 Survival

The relationship between the number of age-1 individuals and the number of age-2 individuals the following year was estimated using a linear regression through the origin where the slope was constrained to lie between 0 and 1 by a logistic transformation.

Key assumptions of the survival rate model include:

• The residual variation in the number of age-2 individuals is log-normally distributed.

Reproductive Rate

The maximum reproductive rate (the number of spawners per spawner at low density) not accounting for fishing mortality was calculated by multiplying \(\beta_s\) (number of recruits per egg at low density) from the stock-recruitment relationship by the inlake survival by the average number of eggs per spawner in a typical year (assumed to be 3,000 based on 6,000 eggs per spawner and a sex ratio of 1:1). The inlake survival from age-1 to spawning was calculated by dividing the subsequent number of spawners by the number of recruits assuming that equal numbers of fish spawn at age 5, 6 and 7.

Expected Spawners

The expected spawners without in river and/or in lake variation was calculated from the stock-recruitment relationship and assuming in age-1 to spawner survival of 0.66%.

Abundance

  data {

    int<lower=0> nMarked;
    int<lower=0> Marked[nMarked];
    int<lower=0> Resighted[nMarked];
    int<lower=0> IndexMarked[nMarked];

    int<lower=0> nObs;
    int Marking[nObs];
    int Index[nObs];
    int<lower=0> nSwimmer;
    int<lower=0> Swimmer[nObs];
    int<lower=0> nYear;
    int<lower=0> Year[nObs];
    real Rkm[nObs];
    int<lower=0> nSite;
    int<lower=0> Site[nObs];

    real SiteLength[nObs];
    real SurveyProportion[nObs];

    int Count[nObs];
  }
  parameters {
    real bEfficiency;
    real bEfficiencyIndex;

    real bDensity;
    real<lower=0> sDensityYear;
    vector[nYear] bDensityYear;
    
    vector[4] bDensityRkm;
    real<lower=0> sDensitySite;
    vector[nSite] bDensitySite;

    real bEfficiencyMarking;
    real bEfficiencyMarkingIndex;

    real<lower=0,upper=5> sEfficiencySwimmer;
    vector[nSwimmer] bEfficiencySwimmer;

    real<lower=0> sDispersion;
  }
  model {

    vector[nObs] eDensity;
    vector[nObs] eEfficiency;
    vector[nObs] eAbundance;
    vector[nObs] eCount;

    sDispersion ~ gamma(0.01, 0.01);

    bDensity ~ normal(0, 2);
    bDensityRkm ~ normal(0, 2);
    sDensitySite ~ uniform(0, 5);
    bDensitySite ~ normal(0, sDensitySite);
    sDensityYear ~ uniform(0, 5);
    bDensityYear ~ normal(0, sDensityYear);

    bEfficiency ~ normal(0, 5);
    bEfficiencyIndex ~ normal(0, 5);
    bEfficiencyMarking ~ normal(0, 5);
    bEfficiencyMarkingIndex ~ normal(0, 5);
    sEfficiencySwimmer ~ uniform(0, 5);
    bEfficiencySwimmer ~ normal(0, sEfficiencySwimmer);

    for (i in 1:nMarked) {
      target += binomial_lpmf(Resighted[i] | Marked[i],
        inv_logit(
          bEfficiency +
          bEfficiencyIndex * IndexMarked[i] +
          bEfficiencyMarking +
          bEfficiencyMarkingIndex * IndexMarked[i]
        ));
    }

    for (i in 1:nObs) {
      eDensity[i] = exp(bDensity +
                        bDensityRkm[1] * Rkm[i] +
                        bDensityRkm[2] * pow(Rkm[i], 2.0) +
                        bDensityRkm[3] * pow(Rkm[i], 3.0) +
                        bDensityRkm[4] * pow(Rkm[i], 4.0) +
                        bDensitySite[Site[i]] +
                        bDensityYear[Year[i]]);

      eEfficiency[i] = inv_logit(
        bEfficiency +
        bEfficiencyIndex * Index[i] +
        bEfficiencyMarking * Marking[i] +
        bEfficiencyMarkingIndex * Index[i] * Marking[i] +
        bEfficiencySwimmer[Swimmer[i]]);

      eAbundance[i] = eDensity[i] * SiteLength[i];

      eCount[i] = eAbundance[i] * eEfficiency[i] * SurveyProportion[i];
    }

    target += neg_binomial_2_lpmf(Count | eCount, sDispersion);
  }

Block 1. Abundance model description.

Condition

 data {
  int nYear;
  int nObs;

  vector[nObs] Length;
  vector[nObs] Weight;
  int Year[nObs];

parameters {
  real bWeight;
  real bWeightLength;
  real sWeightYear;

  vector[nYear] bWeightYear;
  real sWeight;

model {

  vector[nObs] eWeight;

  bWeight ~ normal(-10, 5);
  bWeightLength ~ normal(3, 2);

  sWeightYear ~ normal(-2, 5);

  for (i in 1:nYear) {
    bWeightYear[i] ~ normal(0, exp(sWeightYear));
  }

  sWeight ~ normal(-2, 5);
  for(i in 1:nObs) {
    eWeight[i] = bWeight + bWeightLength * log(Length[i]) + bWeightYear[Year[i]];
    Weight[i] ~ lognormal(eWeight[i], exp(sWeight));
  }
..

Block 2.

Fecundity

 data {
  int nObs;

  vector[nObs] Weight;
  vector[nObs] Fecundity;

parameters {
  real bFecundity;
  real bFecundityWeight;
  real sFecundity;

model {

  vector[nObs] eFecundity;

  bFecundity ~ uniform(0, 5);
  bFecundityWeight ~ uniform(0, 2);

  sFecundity ~ uniform(0, 1);
  for(i in 1:nObs) {
    eFecundity[i] = log(bFecundity) + bFecundityWeight * log(Weight[i]);
    Fecundity[i] ~ lognormal(eFecundity[i], sFecundity);
  }
..

Block 3.

Stock-Recruitment

model {
  a ~ dunif(0, 1)
  b ~ dunif(0, 0.1)
  sScaling ~ dunif(0, 5)

  eRecruits <- a * Stock / (1 + Stock * b)

  for(i in 1:nObs) {
    esRecruits[i] <- SDLogRecruits[i] * sScaling
    Recruits[i] ~ dlnorm(log(eRecruits[i]), esRecruits[i]^-2)
  }
..

Block 4. Stock-Recruitment model description.

Age-1 to Age-2 Survival

model {
  bSurvival ~ dnorm(0, 2^-2)
  sRecruits ~ dnorm(0, 2^-2) T(0,)

  for(i in 1:nObs) {
    logit(eSurvival[i]) <- bSurvival
    eRecruits[i] <- Stock[i] * eSurvival[i]
    Recruits[i] ~ dlnorm(log(eRecruits[i]), sRecruits^-2)
  }
..

Block 5. In-river survival model description.

Abundance

Table 1. Parameter descriptions.

Condition

Table 6. Parameter descriptions.

Table 7. Model coefficients.

Table 8. Model summary.

Fecundity

Table 9. Parameter descriptions.

Table 10. Model coefficients.

Table 11. Model summary.

Stock-Recruitment

Table 12. Parameter descriptions.

Age-1 to Age-2 Survival

Table 17. Parameter descriptions.

Length Correction

Abundance

Age-1

Age-2

Condition

Fecundity

Spawner Size

Egg Deposition

Stock-Recruitment

Age-1

Age-1 to Age-2 Survival

Age-2

Inlake

Reproductive Rate (age-1)

Reproductive Rate (age-2)

Expected Spawners

Conclusions

• Modeling the density in each year as a random effect (as opposed to a fixed effect) substantially reduced the estimated density for 2006.
• The apparent survival from age-1 to age-2 was a density-independent 27%.
• The Limit Reference Point is 387,000 eggs or 129 AUC spawners (based on fecundity in a typical year).
• The egg deposition has fallen below the LRP since 2015.
• Although the 2005 spawn year resulted in double the expected recruitment, the following four years were all below average.
• Calculations suggest that the spike in the number of spawners was solely due to changes in the inlake survival.