TEMPORAL VARIABILITY OF EUPHAUSIID CONCENTRATIONS IN A NOVA-SCOTIA SHELF BASIN USING A BOTTOM-MOUNTED ACOUSTIC DOPPLER CURRENT PROFILER

The usefulness of bottom-mounted, upward-looking sonars for long-term quantitative zooplankton monitoring was demonstrated utilizing a 49 d record of acoustic volume backscattering strengths collected by a 150 kHz RDI Acoustic Doppler Current Profiler in LaHave Basin off Nova Scotia, Canada. Variations in the character and intensity of 2 distinct acoustic scattering horizons distinguished by differing diurnal migration patterns were noted. One horizon confined to the top 50 m and only present during the early parts of the recording was tentatively ascribed to juvenile fish. Another strongly migrating horizon identified with the euphausiid Meganyctiphanes norvegica was studied in detail. M. norvegica in daytime were broadly distributed at about 150 m depth. At night they were uniformly distributed in the upper 50 m. A 30 % decrease in nighttime integrated backscattering levels was consistent with euphausiid accumulation in the acoustically unobservable near-surface region. Average target strengths declined less-than-or-equal-to 1 dB during active vertical migration. This observation, interpreted by acoustic scattering theory, constrained average euphausiid inclinations to little more than 30-degrees during upward swimming. Column integrated population densities were roughly 290 m-2. Use of an inclined beam sonar geometry should result in backscattering levels less sensitive to organism orientation than use of a conventional vertically oriented sonar beam.