On acoustic estimates of zooplankton biomass

Acoustic echo sounders have been used relatively successfully for several decades in the detection and possible classification of simple population (i.e. single size, single species) of fish. One assumption used in these studies is that the energy of the acoustic echo from a school of fish is related to the biomass of the animals via a simple linear regression curve. However, as a result of the natural species diversity within zooplankton aggregations, use of acoustic methods to quantify the populations of zooplankton represents a challenge because the acoustic scattering properties of each gross anatomical class of zooplankton are quite unique. As a result, large errors can occur if one relies on a simple regression curve to describe the echo energy/biomass relationship. Because of the great importance of understanding variability of echo levels due to changes in anatomical features of these animals, we embarked on an experimental investigation in which ship-board and land-based laboratory measurements of echo levels were made on freshly caught zooplankton. Our results indicate variation by factor of about 19 000 to 1 (43 dB) in the relative average echo energy per unit biomass due to animals ranging from fluid-like (salps) to elastic-shelled (gastropods).