Using incidental mark-encounter data to improve survival estimation

1. Obtaining robust survival estimates is critical, but sample size
limitations often result in imprecise estimates or the failure to obtain
estimates for population subgroups.  Concurrently, data are often
recorded on incidental re-encounters of marked individuals, but these
incidental data are often unused in survival analyses. 2. We evaluated the
utility of supplementing a traditional survival dataset with incidental
data on marked individuals that were collected ad-hoc.  We used a
continuous time-to-event exponential survival model to leverage the
matching information contained in both datasets and assessed differences
in survival among adult and juvenile and resident and translocated Mojave
desert tortoises (Gopherus agassizii). 3. Incorporation of the incidental
mark-encounter data improved precision of all annual survival point
estimates, with a 3.4–37.5% reduction in the spread of the 95% Bayesian
credible intervals.  We were able to estimate annual survival for
three subgroup combinations that were previously inestimable. 
Point estimates between the radio-telemetry and combined datasets were
within |0.029| percentage points of each other, suggesting minimal to no
bias induced by the incidental data. 4. Annual survival rates were high
(> 0.89) for resident adult and juvenile tortoises in both study
sites and for translocated adults in the southern site.  Annual
survival rates for translocated juveniles at both sites and translocated
adults in the northern site were between 0.73 and 0.76.  At both
sites translocated adults and juveniles had significantly lower survival
than resident adults.  High mortality in the northern site was
driven primarily by a single pulse in mortalities. 5. Using exponential
survival models to leverage matching information across traditional
survival studies and incidental data on marked individuals may serve as a
useful tool to improve the precision and estimability of survival
rates.  This can improve the efficacy of understanding basic
population ecology and population monitoring for imperiled species.