Considering non-predatory death in the estimation of copepod early life stage mortality and survivorship

Abstract

Estimation of early stage mortality is essential for predicting copepod population dynamics and ecological linkages. Standard methods do not distinguish among types of mortality, nor do they consider that samples may include dead individuals, which can lead to misinterpretation and bias. Here, we develop theory to explain how non-predatory death, or "expiration", influences in situ abundances. We present an amended population dynamics model that accounts for the production of non-viable eggs, expiration of live individuals and losses of dead individuals. This model is used to derive generalizations of four vertical mortality estimation methods, including the widely used Vertical Life Table approach. These new formulae are applied to data for Calanus finmarchicus in the Labrador Sea to illustrate the potential effects of reduced viability on estimated early stage loss rates and survivorship. Results show that even slight reductions in viability can impart significant changes, with the nature of the effect varying among methods, consistent with previous studies. We explain the reasons for these differences and how the common practice of aggregating stages masks the ecological significance of egg viability. Our analysis reinforces previous recommendations for scientists to consider expiration in their estimates of mortality, and in designing their empirical studies.