ON ACOUSTIC ESTIMATES OF ZOOPLANKTON BIOMASS

Acoustic echo sounders have been used relatively successfully for seve ral decades in the detection and possible classification of simple pop ulations (i.e. single size, single species) of fish. One assumption us ed in these studies is that the energy of the acoustic echo from a sch ool of fish is related to the biomass of the animals via a simple line ar regression curve. However, as a result of the natural species diver sity within zooplankton aggregations, use of acoustic methods to quant ify the populations of zooplankton represents a challenge because the acoustic scattering properties of each gross anatomical class of zoopl ankton are quite unique. As a result, large errors can occur if one re lies on a simple regression curve to describe the echo energy/biomass relationship. Because of the great importance of understanding variabi lity of echo levels due to changes in anatomical features of these ani mals, we embarked on an experimental investigation in which ship-board and land-based laboratory measurements of echo levels were made on fr eshly caught zooplankton. Our results indicate variation by a factor o f about 19 000 to 1 (43 dB) in the relative average echo energy per un it biomass due to animals ranging from fluid-like (salps) to elastic-s helled (gastropods).