Na. Cochrane et al., TEMPORAL VARIABILITY OF EUPHAUSIID CONCENTRATIONS IN A NOVA-SCOTIA SHELF BASIN USING A BOTTOM-MOUNTED ACOUSTIC DOPPLER CURRENT PROFILER, Marine ecology. Progress series, 107(1-2), 1994, pp. 55-66
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.