Distribution of Sound Scattering Layers Caused by Euphausiids and Their Relationship to Chlorophyll a Concentrations in the Gulf of St. Lawrence Estuary

A 3-yr study in the Gulf of St. Lawrence estuary using a 120-kHz sounder has shown the existence of a large population of euphausiids consisting of the species Thysanöessa raschii, T. inermis, and Meganyctiphanes norvegica. The sound scattering produced by these animals varied in intensity as the density and biomass m−3 of the animals. Chlorophyll a concentrations measured within 5 m of the surface at the same time as the sound scattering layers showed a significant correlation between the numbers of euphausiids m−3 and the concentration of chlorophyll a m−3.