More often, less each: Averaging can hide opposing trends in prey occurrence and consumer stomach content.

Averages of prey mass in fish stomachs are widely used to track changes in feeding. Yet, a mean over all stomachs is the exact product of two quantities-the frequency of occurrence of a prey and its mean content in the stomachs that contained it-which can move in opposite directions. In Atlantic cod (Gadus morhua L.) in the Baltic Sea, a stable mean benthos content concealed a falling content of consumers and a rising frequency of occurrence, opposing trends that can signal density dependence or responses to hypoxia.