when more than one target (e.g. a fish school) is located in an acoustic beam at the same depth, and it is not possible to resolve them separately.  The total acoustic energy backscattered by the school or aggregation is integrated together, and this total is divided by the (previously determined) backscattering coefficient of a single animal, giving an estimate of the total number. Echo-integration assumes that the total acoustic energy scattered by a group of targets is the sum of the energy scattered by each individual target.