FN Archimer Export Format PT J TI Diel changes in acoustic and catch estimates of krill biomass BT AF SIMARD, Yvan SOURISSEAU, Marc AS 1:;2:; FF 1:;2:PDG-DOP-DCB-DYNECO-PELAGOS; C1 Univ Quebec, Inst Marine Sci, Rimouski, PQ G5L 3A1, Canada. Fisheries & Oceans Canada, Maurice Lamontagne Inst, Mont Joli, PQ G5H 3Z4, Canada. IFREMER, Ctr Brest, DYNECO, F-29280 Plouzane, France. C2 UNIV QUEBEC, CANADA MPO, CANADA IFREMER, FRANCE SI BREST SE PDG-DOP-DCB-DYNECO-PELAGOS IN WOS Ifremer jusqu'en 2018 copubli-int-hors-europe IF 1.92 TC 22 UR https://archimer.ifremer.fr/doc/2009/publication-6591.pdf LA English DT Article DE ;Thysanoessa raschi;Target strength;Strobe light;St Lawrence Estuary;Meganyctiphanes norvegica;Krill;In situ orientation;Feeding;Diel vertical migration;Avoidance AB Krill-biomass estimates can be compromised by diel variabilities in acoustic backscatter and the catch efficiencies of various nets. This paper describes an effort to quantify these variabilities at fine temporal and spatial scales during a three-day experiment at a fixed location, using high-resolution, stratified Bioness samples and echo-integration, and assuming a fixed distribution of krill orientations. Night-time catches in the krill scattering layer (SL) were 15 times the acoustic estimates. The situation was reversed during daytime, when the acoustic estimates in the SL were 5 times larger than the catches. This collectively resulted in a ±10-dB gradual diel cycle in the difference of vertically integrated biomass from both sampling methods. Use of a strobe light on the Bioness reduced avoidance of the net by krill and significantly increased (x10) daytime catches in the SL, but had no significant effect on night-time catches. The difference in volume-backscattering strength at 120 and 38 kHz ({Delta}Sv120–38) in the densest parts of the SL agreed with predictions using a target-strength (TS) model and an assumed normal distribution of tilt (mean {theta} = 11°; s.d. = 4°). The {Delta}Sv120–38 was smaller for lower densities and during night-time. It appears that the {theta} and, therefore, TS distributions of krill significantly change during their diel vertical migrations. At twilight and at night, when they are feeding and swimming vertically, they exhibit lower mean TS and {Delta}Sv120–38 and react less to strong strobe-light pulses, in contrast to daytime. Diel patterns in TS and net avoidance should be taken into account in krill-biomass assessments that use round the clock acoustic-survey data and multi-frequency TS models for target classification. PY 2009 PD JUN SO ICES Journal of Marine Science SN 1054-3139 PU Oxford university press VL 66 IS 6 UT 000267221600051 BP 1318 EP 1325 DI 10.1093/icesjms/fsp055 ID 6591 ER EF