Disentangling residency and migration in a partial migratory system where detection is much less than one

Background/Question/Methods

The largest population of the endangered humpback chub (Gila cypha) is located downstream of Glen Canyon Dam throughout Grand Canyon National Park. Most individuals are found either in the Little Colorado River (LCR), a tributary of the Colorado River, or in portions of the mainstem Colorado River near the LCR confluence. Spawning occurs principally in the LCR, however many young chub pass into the main Colorado River as juveniles or sub-adults. Fish that survive to adulthood in the Colorado River then adopt a migratory strategy, spawning in the LCR during April and May of most years (i.e., skip-spawning).  Fish that survive to adulthood in the LCR, on the other hand, often maintain a resident life style for their adult life. Recent efforts have focused on sampling in both the mainstem and the LCR, and multistate analyses have shown remarkable life history differences between resident and migrants. Here we focus on a longer-time series of mark-recapture data where sampling was focused only in the LCR during both the spawning and non-spawning seasons, and ask whether we can determine whether rates of residency have changed over time.

Results/Conclusions

We develop models with separate transition matrices for residents, spring migrants, and fall migrants (believed to be a small portion of the population) and determine the relative frequency of these strategies. We compare these models to ones in which individual strategies are not constant through time. Humpback chub are believed to have declined dramatically in the late 1990s and recovered in the 2000s, however the causes of this decline and subsequent recovery are unknown and it is unclear if this decline affected both life history strategies equally. We show that residents declined, however there is uncertainty regarding the extent of the decline of migrants. Failure to account for different life strategies leads to underestimates of adult population size throughout the past decade due to heterogeneity in capture probabilities. We discuss our results in the context of a small, but growing, literature that examines how anthropogenic changes are affecting partial migratory systems.