Elk Survival and Mortality Factors in the Blue Mountains of Washington, 2003-2006

Due to uncertainty about cumulative human-caused elk mortality in the Washington Blue Mountains, in particular the magnitude of illegal kills during 2000-2002, we implemented a study to estimate survival rates of elk  > 1 year-old and document sources of human-caused elk mortality, 2003-06. Our study area was focused on Game Management Units (GMUs) 162, 166, 169, and 175.

We radiomarked 190 elk (82 bulls and 39 cows > 1 year-old and 65, 11-month-old bulls). We marked most elk with both rumen implant radios and standard neck radiocollars, but we marked 60 elk in February 2003 with rumen radios only. We estimated annual survival rates using known fate models and explored various hypotheses about factors affecting elk survival using 3 alternate datasets and 9 a priori survival models.

Our 9 survival models invoked possible survival differences between subadult (2-3 yr-olds) and adult bulls (>4 yr-olds), differences in bull survival west and east of the Tucannon River, and differences in branch-antlered bull survival during 2003-05 vs. 2006. We used multi-model inference to select a subset of models that best explained our observed data.

We found that 90% of the available model weight was accounted for in a subset of 5 models for each alternate dataset. Four of the best models were common to all 3 datasets. Simpler models (3-4 parameters) generally had more support in the data than more complex models. We found little support for differences in survival between younger and older branch-antlered bulls or for differences in survival west and east of the Tucannon River for yearling bulls. We found modest support for survival differences east and west of the Tucannon River for branch-antlered bulls, and we found some evidence that branch-antlered bull survival was lower in 2006 relative to 2003-05.

Using estimates from the best-supported models, we estimated annual survival for yearling bulls was 0.41; for adult cows, survival estimates ranged 0.80-0.84 and for branch-antlered bulls, estimates ranged 0.80-0.85, depending on dataset. In models allowing different survival estimates for branch-antlered bulls east and west of the Tucannon River, we estimated annual survival was 0.79-0.83 and 0.80-0.86, respectively, across the 3 datasets. In models allowing survival to differ for branch-antlered bulls in 2003-05 vs. 2006, we estimated survival was 0.83-0.86 during 2003-05 and was 0.73-0.76 in 2006.

We recorded 78 deaths of our radiomarked elk during our study. Human-caused deaths predominated among general causes (n = 55). Most human-caused deaths were of yearling bulls killed by state-licensed hunters (n = 28). Most subadult bull deaths were from tribal hunting (n = 5), and most mature bulls died from natural causes (n = 6). We detected few illegal kills (n = 4). We detected deaths in every month of the year, but most mortalities occurred during Sept-Nov. Overall, tribal hunting was a nontrivial source of mortality, but its magnitude was only about 29% of that of state-licensed hunters. Mortalities of branch-antlered bulls were highest in 2006 (n = 13), with deaths almost equally split between natural causes (n = 6) and human-caused mortalities (n = 7).

We explored a variety of landscape factors as predictors of hunting mortality risks for yearling bulls using logistic regression models. We detected a relationship between road variables (distance-to-road and road density) and risks of hunting mortality, but other landscape factors (e.g., habitat class, ownership, topography) were not related to hunting mortality risks for our data. Risks of hunting mortality increased modestly as yearling bulls used areas closer to roads and as local road densities increased. Our models were not good absolute predictors of the fate of yearling bulls, suggesting other factors affected mortality risks or that vulnerability was variable across our study area. Our modest sample size of kill sites likely limited our ability to detect significant landscape effects (e.g., cover and topography) on elk harvest risks.

We concluded that aggressive enforcement efforts during 2000-03 greatly reduced illegal hunting activity in our study area. We also concluded that current spike-only general season regulations allow sufficient annual recruitment of young branch-antlered bulls to meet management objectives. Tribal harvest was a nontrivial source of mortality, suggesting that the reporting of off-reservation tribal harvest would be useful for state-tribal collaborative management of the Washington Blue Mountains elk herd.

Our results suggested that managing the density of open roads could be an effective strategy to reduce elk harvest vulnerability where that is a management objective.