Improved agreement of net and acoustical methods for surveying euphausiids by mitigating avoidance using a net-based LED strobe light system

Abstract

Abstract Wiebe, P. H., Lawson, G. L., Lavery, A. C., Copley, N. J., Horgan, E., and Bradley, A. 2013. Improved agreement of net and acoustical methods for surveying euphausiids by mitigating avoidance using a net-based LED strobe light system. – ICES Journal of Marine Science, 70: 650–664. Euphausiids are known for their ability to avoid capture by standard plankton nets. Repeat sampling was conducted in the Gulf of Maine with a 1 m2 MOCNESS equipped with a light-emitting diode (LED)-based strobe light concurrent to multifrequency acoustic data collection. On cruise one, the strobe light's efficacy in reducing euphausiid net avoidance was evaluated on two horizontal tows where four of the eight nets were sampled with the strobe flashing and four with the strobe off. With the strobe light on, there was a significant increase in euphausiid abundance by a factor of 4.5 at night and by a factor of 11.0 during the day. There was also a significant increase in biovolume catch of zooplankton when the strobe light was on by factors of 2.2 at night and 5.5 during the day due to a higher abundance of 10 and 35 mm euphausiids. Euphausiids caught with the strobe light on accounted for most of the observed backscattering, and acoustic and net estimates of their abundance agreed. Similar results were obtained on cruise two. Agreement of acoustic and net estimates when using the strobe light suggests that reduced avoidance enhanced the efficiency of catching euphausiids.