Sameoto, B., N. Cochrane, and A. Herman. 1993. Convergence of acoustic, optical, and net-catch estimates sf euphausiid abundance: use of artificial light to reduce net avoidance. Can. 1. Euphausiid concentrations in the Scotian Shelf basins were sampled with BIONESS, a multinet sampling system, and quantitatively assessed simultaneously with an in sitle optical zooplankton counter (BPCb mounted on BlONESS and with an acoustic backscattering system operating simultaneously at 50, 122, and 200 kHz. Supplementary observations were made with vertically dropped video cameras and video and 35-mrn frame cameras mounted on a remotely operated vehicle (ROV). The use of a light source on BlOlVESS during sampling increased the catch of euphalesiids by 10-20 times by reducing active avoidance reactions to the net. Consequently, conventional net sampling has greatly underestimated euphausiid concentrations, a conclusion suggested by previous acoustic measurements. Experimental acoustic target strengths for 28-mm euphausiids averaged -77.5, -73.4, and -68.4 dB at 50, 122, and 200 kHz, respectively, using measured S, levels and assuming 180% net sampling efficiency. Theoretical scattering models based on randomly oriented cylinders require the elephausiids to be oriented within about 5" of the horizontal to approximate both the experimentally observed target strength amplitudes and their frequency dependence. Acoustic interpretation has been enhanced by incorporation of transducer sensitivity versus temperature corrections and modeling techniques that allow for finite transducer beam widths.La concentration des euphausiaces dans les bassins du plateau Scotian a et6 6valu6.e par kchantillonnage au moyen du BION ESS, un systeme de filets mleltiples, et simu Itan6ment par des d~nombrements sur place al e moyen d'un numerateur optique du zooplancton install6 sur te BlONESS ainsi qu'au moyen d'un systeme de retrodiffusion acoustique fonctionnnant A la fois 2 50, 122 et 200 kHz. B'autres observations ont kt6 faites au rmoyen de cameras video plongees verticalement dans l'eau, ainsi que gr2ce 2 des cam6ras video et des appareils photo 35 mm installes sur un vehicule t~l6rnanipuIt5. h'emploi d'une source lurnineuse sur BIONESS au cours de Of~chantillonnage a fait augmenter la capture d'euphausiaces par un facteur de 70 2 20 du fait que les reactions de fuite active devant les filets se sont trouvees reduites. Par consequent, 116chantillonnage classique au filet a conduit sous-estimer consid6rablement la concentration dfeuphausiac6s, une conclusion d'ailleurs suggkree par de precedentes mesures acoustiques. Dans le cas d'euphausiaces de 28 mm, la force des 6chos acoustiques experimentaux ktait d'en moyenne -77,5, -73,4 et -68,4 dB 2 50, 122 et 280 kHz, respectivement, aux niveaux S,, mesures et en supposant une efficacitk d'echantillonnage au filet de 100 %. hes msdeles theoriques de diffusion, fond& sur I'orientatisn aleatsire de cylindres, pr6supposent une orientation des euphausiaces $interieur d'un angle d'environ 5" par rapport 2...
The deep basins on the Nova Scotia shelf contain high concentrations of Calanus finmarchicus, C. glacialis, and C. hyperboreus at depths below 200 m. From May to late fall these were as high as 20 000 m-3. The dominant specles by numbers was C. finmarchicus; however, its biomass was equalled or exceeded by C. hyperboreus. The life cycle of C. finmarchicus in the region of 2 of the largest basins showed that breeding started late in winter with a peak in April. High concentrations of Calanus suggested that the basins had higher levels of C. finmarchicus production than the adjacent shelf with mean depths less than 100 m. All 3 species of Calanus started to accumulate below 200 m in May as Stages CIV and CV C. glacialisand C. hyperboreusdid not reproduce in significant numbers on the NW half of the shelf but did accumulate in the basins as they were advected from the NE shelf region. Resting stages of Calanus resided at depths below 200 m in water at between 8.5 and 11 "C for an estimated 7 to 8 mo. The fate of these copepods is uncertain, but evidence suggested that a large proportion were preyed on by euphausiids in the deep regions of the basin. The deep basins on the shelf make it possible for C. finmarchicus populations to dominate the Nova Scotia shelf zooplankton community for most of the year; without the basins the shelf zooplankton would probably resemble that of the Grand Banks which has no deep basins, and is dominated by microzooplankton.
Acoustic backscatter observations at 12, 50, and 200 kHz in the inner Scotian Shelf basins reveal large populations of silver hake (Merluccius bilinearis) and its principal prey, the euphausiid Meganyctiphanes norvegica. Multichannel sonar colour imagery facilitates separation of fish from euphausiids and delineation of their separate diurnal migration patterns. Silver hake acoustic abundances are consistent with midwater trawl sampling. Acoustic column densities for mature M. norvegica of approximately 1000/m2 exceed net sampling column densities by over 1 order of magnitude indicating strong net avoidance. Numerical simulation of euphausiid backscattering using a fluid cylinder model and natural distribution of scatterer orientations yields an approximate 20 log L target strength length dependence and little frequency domain dependence in the "geometric" scattering regime. Quantitative interpretation of euphausiid scattering in situ must account for randomness in their orientations.
An optical plankton counter (OPC) mounted on a towed Batfish vehicle was used to detect and estimate the abundance of euphausiids in the Emerald and La Have Basins of the Scotian Shelf, northwest Atlantic Ocean. Euphausiids (Meganyctiphanes norvegica and Thysanoessa inermis) were identified from their characteristic diameter distributions measured by the OPC. The diameter distributions containing euphausiid peaks were compared to several depth strata known to contain these animals. The depth strata containing euphausiids were established from acoust~cal tows made on the same cruise and the identification of animals was accomplished by sampling these acoustical layers with the BIONESS net sampler. The die1 migration patterns of the euphausiids were also identified from these diameter distributions. Euphausiid reaction to artificial lights mounted on Batfish and BIONESS showed increases in abundance estimates by a factor of 2 x and 50x respectively. Experiments with artificial lights mounted on a remotely operated vehicle and camera platforms are also described. An artificial light mounted on a towed Batfish resulted in a distinct increase in euphausiid abundance estimates as measured by the OPC. Abundance estimates from the OPC are also compared to results from acoustical tows and showed agreement to within a factor of 2 and better.
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