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

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.

Estimation of biological parameters of marine organisms using linear and nonlinear acoustic scattering model-based inversion methods

“…Although the backscattering cross section is proportional to the square of the aspect ratio of euphausiids, this parameter does not vary too much between individuals for adult euphausiids (Stanton et al, 1998) and, for simplicity, is set to a constant value of L/a ¼ 0.5 L/W ¼ 18. The value is derived from measurements of euphausiids sampled in the Gulf of Maine reported in Wiebe et al (2013), and is also very similar to what we have used before in other modeling studies of elongated scatterers Stanton et al, 1993;Stanton et al, 1998). For euphausiids of uni-modal distribution, the parameter r L is about 10% or less (Foote et al, 1990;Chu and Wiebe, 2005), but could be larger for bimodal length distribution (Lawson et al, 2006(Lawson et al, , 2008a.…”

“…Biological sampling methods such as nets, trawls, and pumps typically have been used to provide direct data (Wiebe and Benfield, 2003). However, these techniques can only provide sparse discrete information and are time consuming, notwithstanding the inherent biases due to sampler avoidance (Kelley, 1976;Wiebe et al, 2013). Acoustic remote sampling techniques can continuously sample a much larger portion of the water column during a given time period than the direct methods.…”

Spatial and temporal variations in the abundance, distribution, and aggregation of krill (Thysanoessa raschii and Meganyctiphanes norvegica) in the lower estuary and Gulf of St. Lawrence

“…significantly (Sameoto et al, 1993;Wiebe et al, 2004) and produce density estimates more comparable to acoustic estimates (Wiebe et al, 2013). Further, the highly patchy distribution of some krill species confounds the point-sampling and fixed-station design of typical plankton-net surveys, which lack the high sampling coverage and resolution required to adequately describe krill horizontal and vertical distribution.…”

Physical controls on copepod aggregations in the Gulf of Maine

“…The ecosystem continues to change; it is imperative that we understand the system so that we may understand these changes and adapt accordingly. New technology, including optical plankton counters, video plankton recorders, and broadband acoustic systems allow for previously unattainable high spatial-and temporal-resolution pictures of phytoplankton, copepods, and fish (Lawson et al (2007), Wiebe et al (2013)). Autonomous gliders and other AUVs are able to map three-dimensional physical and biological fields rapidly in time and space (Rudnick et al (2004), Baumgartner and Fratantoni (2008)).…”