ICES Journal of Marine Science: Journal du Conseil Advance Access originally published online on April 2, 2009
ICES Journal of Marine Science: Journal du Conseil 2009 66(6):1318-1325; doi:10.1093/icesjms/fsp055
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This article appears in the following ICES Journal of Marine Science issue: The Ecosystem Approach with Fisheries Acoustics and Complementary Technologies [View the issue table of contents]
Diel changes in acoustic and catch estimates of krill biomass
1 Marine Sciences Institute, University of Québec at Rimouski, 310 Allée des Ursulines, Rimouski, Québec, Canada G5L-3A1, and Maurice Lamontagne Institute, Fisheries and Oceans Canada, Mont-Joli, Québec, Canada G5H-3Z4
2 IFREMER, DYNECO, Centre de Brest, BP 70, 29280 Plouzané Cedex, France
Correspondence to Y. Simard: e-mail: yvan.simard{at}dfo-mpo.gc.ca or yvan_simard{at}uqar.qc.ca.
Simard, Y., and Sourisseau, M. 2009. Diel changes in acoustic and catch estimates of krill biomass. – ICES Journal of Marine Science, 66: 1318–1325.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 (
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 
= 11°; s.d. = 4°). The Sv120–38 was smaller for lower densities and during night-time. It appears that the 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 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.
Keywords: avoidance, diel vertical migration, feeding, in situ orientation, krill, Meganyctiphanes norvegica, St Lawrence Estuary, strobe light, target strength, Thysanoessa raschi
Received 8 August 2008; accepted 31 December 2008; advance access publication 2 April 2009.