Lower Columbia River Fish Population Indexing Analysis 2017

Discharge

Missing hourly discharge values for Hugh-Keenleyside Dam (HLK), Brilliant Dam (BRD) and Birchbank (BIR) were estimated by first leading the BIR values by 2 hours to account for the lag. Values missing at just one of the dams were then estimated assuming \(HLK + BRD = BIR\). Negative values were set to be zero. Next, missing values spanning \(\leq\) 28 days were estimated at HLK and BRD based on linear interpolation. Finally any remaining missing values at BIR were set to be \(HLK + BRD\).

The data were prepared for analysis using R version 3.5.0 (R Core Team 2018).

Condition

The expected weight of fish of a given length were estimated from the data using a mass-length model (He et al. 2008). Key assumptions of the condition model include:

• The expected weight is allowed to vary with length and date.
• The expected weight is allowed to vary randomly with year.
• The relationship between weight and length is allowed to vary with date.
• The relationship between weight and length is allowed to vary randomly with year.
• The residual variation in weight is log-normally distributed.

Only previously untagged fish were included in models to avoid potential effects of tagging on body condition. Preliminary analyses indicated that the annual variation in weight was not correlated with the annual variation in the relationship between weight and length.

Growth

Annual growth of fish were estimated from the inter-annual recaptures using the Fabens method (Fabens 1965) for estimating the von Bertalanffy growth curve (von Bertalanffy 1938). This curve is based on the premise that:

\[ \frac{\text{d}L}{\text{d}t} = k (L_{\infty} - L)\]

where \(L\) is the length of the individual, \(k\) is the growth coefficient and \(L_{\infty}\) is the maximum length.

Integrating the above equation gives:

\[ L_t = L_{\infty} (1 - e^{-k(t - t_0)})\]

where \(L_t\) is the length at time \(t\) and \(t_0\) is the time at which the individual would have had zero length.

The Fabens form allows

\[ L_r = L_c + (L_{\infty} - L_c) (1 - e^{-kT})\]

where \(L_r\) is the length at recapture, \(L_c\) is the length at capture and \(T\) is the time between capture and recapture.

Key assumptions of the growth model include:

• The mean maximum length \(L_{\infty}\) is constant.
• The growth coefficient \(k\) is allowed to vary randomly with year.
• The residual variation in growth is normally distributed.

Movement

The extent to which sites are closed, i.e., fish remain at the same site between sessions, was evaluated with a logistic ANCOVA (Kery 2010). The model estimates the probability that intra-annual recaptures were caught at the same site versus a different one. Key assumptions of the site fidelity model include:

• The expected site fidelity is allowed to vary with fish length.
• Observed site fidelity is Bernoulli distributed.

Length as a second-order polynomial was not found to be a significant predictor for site fidelity.

Length-At-Age

The expected length-at-age of Mountain Whitefish and Rainbow Trout were estimated from annual length-frequency distributions using a finite mixture distribution model (Macdonald and Pitcher 1979)

There were assumed to be three distinguishable normally-distributed age-classes for Mountain Whitefish (Age-0, Age-1, Age-2 and Age-3+) two for Rainbow Trout (Age-0, Age-1, Age-2+). Initially the model was fitted to the data from all years combined. The model was then fitted to the data for each year separately with the initial values set to be the estimates from the combined values. The only constraints were that the standard deviations of the MW age-classes were identical in the combined analysis and fixed at the initial values in the individual years.

Rainbow Trout and Mountain Whitefish were categorized as Fry (Age-0), Juvenile (Age-1) and Adult (Age-2+) based on their length-based ages. All Walleye were considered to be Adults.

Survival

The annual adult survival rate was estimated by fitting a Cormack-Jolly-Seber model (Kery and Schaub 2011, 220–31) to inter-annual recaptures of adults.

Key assumptions of the survival model include:

• Survival varies randomly with year.
• The encounter probability for adults is allowed to vary with the total bank length sampled.

Observer Length Correction

The annual bias (inaccuracy) and error (imprecision) in observer’s fish length estimates were quantified from the divergence of the length distribution of their observed fish from the length distribution of the measured fish. More specifically, the percent 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.

Capture Efficiency

The probability of capture was estimated using a recapture-based binomial model (Kery and Schaub 2011, 134–36, 384–88).

Key assumptions of the capture efficiency model include:

• The capture probability varies randomly by session within year.
• The probability of a marked fish remaining at a site is the estimated site fidelity.
• The number of recaptures is described by a binomial distribution.

Abundance

The abundance was estimated from the catch and bias-corrected observer count data using an overdispersed Poisson model (Kery and Schaub 2011, 55–56).

Key assumptions of the abundance model include:

• The capture efficiency is the point estimate from the capture efficiency model.
• The efficiency varies by visit type (catch or count).
• The lineal fish density varies randomly with site, year and site within year.
• The overdispersion varies by visit type (count or catch).
• The catches and counts are described by a gamma-Poisson distribution.

Stock-Recruitment

The relationship between the adults and the resultant number of age-1 subadults was estimated using a Beverton-Holt stock-recruitment model (Walters and Martell 2004):

\[ R = \frac{\alpha \cdot S}{1 + \beta \cdot S} \quad,\]

where \(S\) is the adults (stock), \(R\) is the subadults (recruits), \(\alpha\) is the recruits per spawner at low density and \(\beta\) determines the density-dependence.

Key assumptions of the stock-recruitment model include:

• The prior for \(\log(\alpha)\) is a truncated normal distribution from \(\log(1)\) to \(\log(5)\).
• \(\beta\) varies with the proportional egg loss.
• The residual variation in the number of recruits is log-normally distributed.

The carrying capacity \(K\) is given by the relationship:

\[ K = \frac{\alpha}{\beta} \quad.\]

Scale Age

The age of individual Mountain Whitefish was estimated from their circuli distances using a hand-coded algorithm. The algorithm involved a series of steps. First, the circuli distances were differenced to get the circuli increments. Next, a linear mixed model on the circuli number with the encounter as a random effect was fitted to the increments to account for differences in scale size and/or shape. A simple moving average (of window sma1) was then fitted to the residuals from the linear mixed model (circuli increment deviations) to remove noise. Small increment deviations, which are associated with annuli, were then identified by testing whether they are smaller than their neighbours based on a second simple moving average (of window sma2) and a multiplier. Finally blocks of small increment deviatons greater than a threshold size (block) were considered to be annuli. The initial circuli increment deviations below a second threshold were considered to have been laid down during the first year of life and were not included in the age (block count).

The algorithm was tuned by varying each of sma1, sma2, multiplier, block threshold and age-0 threshold within a narrow range of values and selecting the combination that minimized the age-averaged chi-squared statistic for encounters of known age. The age of an encounter was considered to be known if the length-at-age model was able to assign it to a single age-class with a probability \(\geq 0.98\) or if the recaptures age was known at first capture.

Age-Ratios

The proportion of Age-1 Mountain Whitefish \(r^1_t\) from a given spawn year \(t\) is calculated from the relative abundance of Age-1 & Age-2 fish \(N^1_t\) & \(N^2_t\) respectively, which were lead or lagged so that all values were with respect to the spawn year:

\[r^1_t = \frac{N^1_{t+2}}{N^1_{t+2} + N^2_{t+2}}\]

The relative abundances of Age-1 and Age-2 fish were taken from the proportions of each age-class in the length-at-age analysis.

As the number of Age-2 fish might be expected to be influenced by the percentage egg loss \(Q_t\) three years prior, the predictor variable \(\Pi_t\) used is:

\[\Pi_t = \textrm{log}(Q_t/Q_{t-1})\]

The ratio was logged to ensure it was symmetrical about zero (Tornqvist, Vartia, and Vartia 1985).

The relationship between \(r^1_t\) and \(\Pi_t\) was estimated using a Bayesian regression (Kery 2010) loss model.

Key assumptions of the final model include:

• The log odds of the proportion of Age-1 fish varies with the log of the ratio of the percent egg losses.
• The residual variation is normally distributed.

The relationship between percent dewatering and subsequent recruitment is expected to depend on stock abundance (Subbey et al. 2014) which might be changing over the course of the study. Consequently, preliminary analyses allowed the slope of the regression line to change by year. However, year was not a significant predictor and was therefore removed from the final model. The effect of dewatering on Mountain Whitefish abundance was expressed in terms of the predicted percent change in Age-1 Mountain Whitefish abundance by egg loss in the spawn year relative to 10% egg loss in the spawn year. The egg loss in the previous year was fixed at 10%. The percent change could not be calculated relative to 0% in the spawn or previous year as \(\Pi_t\) is undefined in either case.

Table 1. Parameter descriptions.

Mountain Whitefish

Table 2. Model coefficients.

Table 3. Model summary.

Rainbow Trout

Table 4. Model coefficients.

Table 5. Model summary.

Walleye

Table 6. Model coefficients.

Table 7. Model summary.

Table 8. Parameter descriptions.

Mountain Whitefish

Table 9. Model coefficients.

Table 10. Model summary.

Rainbow Trout

Table 11. Model coefficients.

Table 12. Model summary.

Walleye

Table 13. Model coefficients.

Table 14. Model summary.

Table 15. Parameter descriptions.

Mountain Whitefish

Table 16. Model coefficients.

Table 17. Model summary.

Rainbow Trout

Table 18. Model coefficients.

Table 19. Model summary.

Walleye

Table 20. Model coefficients.

Table 21. Model summary.

Mountain Whitefish

Table 22. The estimated upper length cutoffs (mm) by age and year.

Rainbow Trout

Table 23. The estimated upper length cutoffs (mm) by age and year.

Table 24. Parameter descriptions.

Mountain Whitefish

Table 25. Model coefficients.

Table 26. Model summary.

Rainbow Trout

Table 27. Model coefficients.

Table 28. Model summary.

Walleye

Table 29. Model coefficients.

Table 30. Model summary.

Table 31. Parameter descriptions.

Mountain Whitefish

Rainbow Trout

Walleye

Table 42. Parameter descriptions.

Mountain Whitefish

Rainbow Trout

Walleye

Table 53. Parameter descriptions.

Mountain Whitefish

Table 54. Model coefficients.

Table 55. Model summary.

Rainbow Trout

Table 56. Model coefficients.

Table 57. Model summary.

Table 58. The optimal parameter values for aging fish from their circuli distances.

Table 59. Parameter descriptions.

Table 60. Model coefficients.

Table 61. Model summary.

Subadult

Mountain Whitefish

Rainbow Trout

Adult

Mountain Whitefish

Rainbow Trout

Walleye

Mountain Whitefish

Rainbow Trout

Walleye

Mountain Whitefish

Rainbow Trout

Adult

Mountain Whitefish

Rainbow Trout

Walleye

Mountain Whitefish

Subadult

Adult

Rainbow Trout

Subadult

Adult

Walleye

Adult

Mountain Whitefish

Subadult

Adult

Rainbow Trout

Subadult

Adult

Walleye

Adult

Mountain Whitefish

Rainbow Trout