A Population Assessment for the North Cascades Elk Herd: 2006-2011

The North Cascades (Nooksack) elk herd declined during the 1980s, prompting a closure to recreational and subsistence harvest by state and tribal hunters in 1997. The Washington Department of Fish and Wildlife (WDFW) and the Pt. Elliot Treaty Tribes initiated collaboration in the late 1990s to promote herd recovery. The principal strategies used were the temporary recreational / subsistence harvest moratorium and 2 translocations of elk from the Mt. St. Helens (MSH) elk herd (fall 2003 and fall 2005).

In 2005, WDFW and the Pt. Elliot Treaty Tribes initiated a joint study to evaluate the size and demographics of the current Nooksack elk herd, judge the effectiveness of the recovery strategy, and develop a rigorous monitoring strategy. Two principal monitoring approaches were concurrently evaluated: sightability-correction modeling and mark-resight modeling. We report the results from work conducted during the fall of 2005 through the spring of 2011 (winters 2005-2006 through 2010-2011).

We collected data during intensive late winter helicopter surveys (2 total area surveys per year). We used data from Feb-Apr flights, 2006-2007 to fit a logistic regression model to predict the sightability of elk groups based on group and environmental covariates. Several covariates influenced sightability in univariate logistic regression models. We then used multi-model inference and an information-theoretic criterion (AIC_c) to compare several alternative multivariate models of varying complexity; our results indicated the best multivariate model predicted sightability of elk groups based on: 1) group size, 2) forest canopy cover (%), and 3) a categorical activity covariate (active vs. bedded). Predicted sightability increased with increasing group size, with decreasing cover, and when elk were active. The final logistic regression model was effective at correctly discriminating sighted and unsighted groups from the model building dataset, and we applied it to the full aerial survey datatset (2006-2011). The sightability model indicated relatively steady and modest herd growth during 2006-2011, but model estimates for years we had good minimum-known-alive estimates were negatively biased. The sightability model estimated the Nooksack elk population was about 350 elk in the spring of 2006, rising to about 550 elk by the spring of 2011.

Among available mark-resight models, we principally used the recently developed logit-normal mixed effects (LNME) model to generate estimates of total elk population size and the sizes of the cow and branch-antlered bull subpopulations. We explored 15 a priori LNME models to predict total population size and 12 models to predict subpopulation sizes. We again used multi-model inference and AIC_c to evaluate the evidence in our data for the various models in the a priori model sets. Our results supported evidence for individual heterogeneity in resighting probabilities and variation in resighting probabilities across some years. The LNME model estimates suggested growth in the total elk population and the gender-based subpopulations during 2006-2011. Estimates of total population size increased from 644 (95% CI = 570-706) in spring 2006 to 1,248 (95% CI = 1,094-1,401) in 2011. The cow subpopulation was estimated to have increased from 381 (95% CI = 338-424) in spring 2006 to 573 (95% CI = 507-639) in 2011. The branch-antlered bull subpopulation estimates increased from 87 (95% CI = 54-119) to 180 (95% CI = 118-241) from spring 2006 to spring 2011.

The LNME model estimates were consistently and substantially higher than the sightability model estimates across years. The LNME model estimates were higher than minimum-known-alive estimates, but seemed reasonable in comparison. The trends among total population size (sightability model and LNME model derived) and cow elk subpopulation size (LNME model) were very similar and suggested a finite growth rate of λ ≈ 1.07-1.12 or an exponential growth rate of r â‰ˆ 0.07-0.11. The mean estimates of £f and r from an age-structured stochastic population model using empirical estimates of vital rates were 1.10 and 0.10, very similar to the trends estimated from both sightability modeling and mark-resight modeling. Collectively, these results provided considerable evidence that the core Nooksack elk population (GMU 418) grew modestly during 2006-2011.

We also used radiomarked elk to estimate survival rates and explore possible sources of variation in survival. We explored 16 a priori survival models with known-fate models implemented in Program MARK, using AIC_c and model weights to draw conclusions about Nooksack elk survival during 2000-2008. Based on early results from the population assessment work, limited bull harvest had been reinitiated in the fall of 2007. Our results suggested bull elk survival was high during the harvest moratorium (0.92; 95% CI = 0.76-0.99), but declined after the resumption of limited-entry bull harvest (0.68; 95% CI = 0.50-0.82). Cow survival was high under the best supported model (0.93; 95% CI = 0.90-0.95), except adult cows translocated from MSH in fall 2003 had lower survival for the first year post-translocation (0.68; 95% CI = 0.51-0.82). There was little evidence of any early post-translocation effect on adult cow survival for cows moved from MSH in the fall of 2005.

Our results suggested that the strategy jointly pursued by WDFW and the Pt. Elliot Treaty Tribes was effective in promoting recovery of the Nooksack elk herd and that the recent trajectory for the population has been consistently positive. Recent levels of limited bull harvesting have reduced bull survival, as expected, but survival seems high enough to protect the bull subpopulation from over-exploitation and meet management objectives. Despite the conservation closure since 1997, adult cow elk were regularly harvested under damage permits and seasons in the agricultural valleys of the Nooksack herd area during 2005-2011. We documented at least 270 elk deaths and 91 cow elk deaths during 2005-2011. Losses of adult cows were principally from damage hunts and roadkills along local highways.

We suggest mark-resight is the best approach for monitoring the population, but the use of this approach will require periodic remarking of elk. Both the scales of the landscape and of the elk population make mark-resight a viable management option. We offer further suggestions regarding monitoring strategies and overall herd management in light of our results.