Improving confidence in copepod mortality estimates: Choosing formulas and quantifying errors

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Estimates of copepod mortality rates from survey data are used to analyze trophic interactions, assess the importance of predation vs. advective losses, and specify model parameters. Accurate estimates are critical, and even small uncertainty in mortality can produce large uncertainty in predicted copepod population dynamics and production. The literature abounds with methods for calculating stage-dependent mortalities, each comprising a host of underlying assumptions that are not always evident. This makes it difficult to choose appropriate formulas or determine associated errors. Here, we provide researchers the means to improve confidence in their mortality estimates by conducting a quantitative analysis of different candidate formulae. We review the common vertical life table approach, and then explain its mathematical relationship to other steady-state equations. The effect of specific assumptions is quantified by comparison of mortality estimates for Calanus finmarchicus in the Northwest Atlantic using different estimation methods. An individual-based model of C. finmarchicus is then used to evaluate how these errors scale with those due to violation of other assumptions (e.g. steady-state). Finally, we make recommendations for types of field data that can be used to help reduce uncertainty in estimated mortality rates