Copepod egg production estimates are biased by female mortality

Egg production rate (EPR, eggs female(-1)d(-1)) is a key measure of the nutritional state of metazoans such as copepods. Chlorophyll concentration is most often used to index responses of copepod vital rates to ambient food, but the fits to EPR are often poor. A known but unexplored bias in EPR estimates is the interaction between lifetime reproductive schedules of adult females and their mortality. We examined this interaction for Pseudodiaptomus forbesi, an abundant sac-spawning calanoid copepod, in the San Francisco Estuary (SFE), USA. The reproductive schedule of P. forbesi comprises a latent period followed by a generally steady rate of clutch production. We used a Bayesian approach to analyze data from two studies which included 141 individual estimates of proportion ovigerous, eggs per clutch, and mortality from 28 sampling events. Proportion ovigerous contributed 95% of the variation in EPR. Mortality was 0.01-0.73 d(-1) by one method and similar to 20% higher by another. When mortality was high, most females were young, few were past the latent stage, and similar to 25% were ovigerous. Proportion ovigerous up to 75% occurred only with mortality > similar to 0.1 d(-1). Adjusting for mortality increased estimates of proportion ovigerous by 3%-88% (median 36%) and allowed for a relationship with chlorophyll, which could not be fitted to the raw data. However, the adjustments increased uncertainty. Given the large effort required, we recommend that others investigate this phenomenon for other species and environments before this adjustment should be recommended for estimating copepod egg production.