Hard Choices in Assessing Survival Past Dams Using Telemetry

Mark recapture models are widely used to assess survival of migrating salmon smolts past dams. Reach survival is has two components; natural risk and the added risk associated with the structure.  A single release above the dam generates an unresolved survival estimate.  Paired releases of tagged fish below (and above) the dams have been effectively used to produce an unbiased estimate of dam related mortality. This benefit is well characterized but it comes at a cost of releasing tagged fish below the dam.  All things being equal, the SE of survival past a dam increases by having fewer tags available to estimate survival.  There is also a mathematical inflation of SE.   Thus there is a tradeoff between bias and error in survival estimate approaches.  We modeled an idealized system with a single dam to assess this tradeoff using “mean squared error (MSE)” as criteria.  Simulations were run under various defined conditions (dam mortality, number of tags, background mortality and detection probability) to characterize the conditions under which use of the paired release produced a theoretical advantage in minimizing MSE.