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 Sco tia, Canada. Variations in the character and intensity of 2 distinct a coustic scattering horizons distinguished by differing diurnal migrati on patterns were noted. One horizon confined to the top 50 m and only present during the early parts of the recording was tentatively ascrib ed to juvenile fish. Another strongly migrating horizon identified wit h the euphausiid Meganyctiphanes norvegica was studied in detail. M. n orvegica in daytime were broadly distributed at about 150 m depth. At night they were uniformly distributed in the upper 50 m. A 30 % decrea se in nighttime integrated backscattering levels was consistent with e uphausiid accumulation in the acoustically unobservable near-surface r egion. Average target strengths declined less-than-or-equal-to 1 dB du ring active vertical migration. This observation, interpreted by acous tic 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 son ar geometry should result in backscattering levels less sensitive to o rganism orientation than use of a conventional vertically oriented son ar beam.