Consequences of violating the recapture duration assumption of mark–recapture models: a test using simulated and empirical data from an endangered tortoise population

Summary

• 1
  Mark–recapture methods are frequently used to estimate survival rates, which inform wildlife management policies. Mark–recapture models assume zero mortality of marked animals during the period of recapture and therefore that this period is of negligible duration relative to the interval between recapture events. This ‘recapture duration assumption’ is frequently violated yet is rarely tested for its potential to bias survival estimates. We investigated whether violating the recapture duration assumption increased precision at the cost of increased bias in estimates of survival.
• 2
  Using both simulation and empirical approaches, annual survival and recapture rates were estimated under recapture period durations that either conformed to, or violated, the recapture duration assumption. Empirical data were derived from a population of endangered Madagascar ploughshare tortoises Geochelone yniphora.
• 3
  Estimates of ploughshare tortoise survival were higher and had smaller standard errors when the recapture duration assumption was violated. As the ‘true’ ploughshare tortoise survivorship was unknown, a simulation approach was used to assess whether short or long recapture periods gave the most reliable estimates.
• 4
  The simulation study showed that violating the recapture duration assumption increased precision but did not increase bias in parameter estimates, even if up to 50% of the annual mortality occurred during the recapture period. The most precise and least biased survival estimates were obtained when sample size (number of marked individuals) was greatest, i.e. when both survival and recapture rates were high. Bias in survival estimates was negligible unless the recapture rate during the recapture period was < 0·2. We conclude that the most precise ploughshare tortoise parameter estimates were generated by using lengthy recapture occasions that violated the recapture duration assumption.
• 5
  Synthesis and applications. To achieve the most precise and least biased survival estimates we recommend violating the recapture duration assumption and using a recapture period that maximizes sample size and achieves a recapture rate of > 0·2 during the recapture period. If both survival and recapture rates are relatively constant during the recapture period, survival estimates will then have negligible bias. If a recapture rate of > 0·2 during the recapture period cannot be achieved, methods other than mark–recapture should be used to quantify demographics. These recommendations apply to any species with an annual survival rate > 0·05.