Estimating mortality: Clarifying assumptions and sources of uncertainty in vertical methods

Abstract Accurate estimates of stage-dependent mortality rates are critical for understanding zooplankton dynamics. Existing methods for estimating mortality from survey data include assumptions that are not always evident, meaning practitioners may be unaware of errors or whether a particular method is appropriate for their data. Here, a quantitative analysis of three vertical (single time point) methods is conducted to help improve confidence in mortality estimates. The underlying theory is reviewed to clarify assumptions and sources of error. The Basic method propagates errors, the Ratio method smoothes stage-to-stage variation, and the Alternative method is unrealistic when rates are constant. Contrary to perception, all three methods neglect the influence of advection. Application to field data for Calanus finmarchicus in the Northwest Atlantic illustrates dependence of estimates on input variables and choice of method. Variation among methods was generally smaller than that caused by uncertainty in physiological rates, and frequent violations of assumptions common to all methods seriously limited their utility. Simulations with an individual-based model reveal unacceptable sensitivity for the Basic method, whereas Ratio and Alternative methods are relatively robust except for dynamic or advective environments. Recommendations are given regarding ways to detect whether these methods can be applied to survey data, as well as to assess uncertainty.