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...

No abstract

Time varying geomagnetic fields observed in southern British Columbia and Alberta are analyzed using optimum "transfer function" techniques for periods from 4 to 240 min. Estimates of the frequency and spatial dependence of the "coast effect" are obtained. Inland anomalies are delineated and attributed to variations in conductivity in the upper-to-middle crust coupled with an abrupt east–west transition in deeper conductivity. Regional attenuation of the uncorrelated portion of the vertical magnetic component is indicative of high conductivity near the crust mantle transition zone under the western part of the Cordillera. The conclusion is in agreement with magnetotelluric observations and other geophysical data.

No abstract