A stage-based estimator of daily egg production

<p>Snapper (<em>Chrysophrys auratus</em>) in South Australia spawn over a wide range of daily times and hatch in ∼1.5 d. Traditional estimates of daily egg production (i) divide sampled eggs into daily age cohorts, e.g. days 1 and 2, (ii) compute cohort ages by assuming that all spawning occurs at a fixed daily peak hour, and (iii) link the estimation of spawning egg density with egg mortality rate by regressing cohort egg densities against cohort age. We propose a method for estimating daily egg production, the number of eggs spawned per unit area per day (<em>P</em>₀), without these assumptions. We use a range of estimates of snapper egg mortality rate obtained from prior studies to backcorrect for egg mortality to the age 0 spawning egg density for each stage of eggs individually rather than aggregating into daily cohorts. <em>P</em>₀ is estimated as a mean of sample tow densities rather than as a regression intercept. This stage-based <em>P</em>₀ estimator avoids errors associated with assuming a fixed daily spawning hour and classifying eggs into day 1 or 2. It requires no regressions of sampled daily cohort egg densities against age, which often provide imprecise estimates of mortality rates. Simulation testing showed 1–2% accuracy for this estimator of <em>P</em>₀. The uncertainty of assuming egg mortality rates is moderated by the insensitivity of estimates of <em>P</em>₀ to the input value of egg mortality rate.</p>