Bias Correction for Estimating Daily Ration with the Eggers Model Revisited

The Eggers (1977) model is considered a special case of the Elliott and Persson (1978) model (EPM) for estimating daily ration of fish when the quantity of food in the stomach or whole digestive tract is the same at the beginning and the end of a 24-hour period. When this conditions is not met (i.e. “lack-of-closure”), Eggers (1979) proposed a simple bias correction to his original method. Here we show that this bias correction is mathematically incorrect and produces biased estimates of daily ration itself. We then derive a simple new bias correction that produces identical results to the EPM when the sampling interval is constant throughout a day. When sampling is performed at irregular intervals throughout the day, daily ration should be estimated with the EPM as neither the Eggers (1977, 1979) models nor the bias correction proposed here are adequate in this situation.