Diel changes in acoustic and catch estimates of krill biomass

Type Article
Date 2009-07
Language English
Author(s) Simard Yvan, Sourisseau Marc
Affiliation(s) 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.
Source ICES Journal of Marine Science (1054-3139) (Oxford university press), 2009-07 , Vol. 66 , N. 6 , P. 1318-1325
DOI 10.1093/icesjms/fsp055
WOS© Times Cited 22
Keyword(s) Thysanoessa raschi, Target strength, Strobe light, St Lawrence Estuary, Meganyctiphanes norvegica, Krill, In situ orientation, Feeding, Diel vertical migration, Avoidance
Abstract 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.
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