Estimation of evacuation rates in the field

Two methods are presented to calculate evacuation rates based on observed diel changes in occurrence and mean mass of prey in predator stomachs. The methods do not require predators to exhibit prolonged non‐feeding periods, but the ingestion of each particular prey type must be restricted to certain diel periods. Data from >7500 whiting Merlangius merlangus collected at five locations in the North Sea were used to demonstrate the methods. The evacuation rates estimated from field data were similar to laboratory results, though a tendency for estimates to exceed literature values slightly was noted. Bias was introduced if a large proportion of the prey was evacuated completely in the interval between subsequent samples and if significant amounts of other food were present in the stomach together with the prey in question. The methods can be used to supplement laboratory estimates of evacuation rates or provide first estimates for species that are not easily maintained in the laboratory.