Validation of acoustic echo counting for studies of zooplankton behavior

Abstract

Acoustic methods can be employed to census zooplankton in large volumes of water rapidly, but ecological interpretations of the results are often complicated by ambiguities concerning the identity of acoustic backscatters. Here, a multi-step approach is developed to evaluate the potential of acoustic echo counting for behavioral studies of zooplankton using OASIS (Optical-Acoustic Submersible Imaging System), an instrument designed for concurrent optical and acoustic imaging of zooplankton. A combination of field and laboratory analyses (1) demonstrate general agreement between echo counts and depth-stratified net sampling, (2) show that fish and zooplankton can be distinguished on the basis of target strength (TS), (3) indicate that zooplankton taxa (euphausiids and gammarid amphipods) could not be differentiated on the basis of TS, and (4) demonstrate a positive relationship between body size and TS in euphausiid crustaceans. Simulation of the system's performance suggests that at high densities of reflective targets, echo counting increasingly underestimates the true number of targets present. These results confirm that echo counting is a valuable method for behavioral studies and underscore the importance of independent verification of the identities of acoustic backscatters. International Council for the Exploration of the Sea

Second-order seasonal variability in diel vertical migration timing of euphausiids in a coastal inlet

“…The gammarid amphipod Orchomene obtusus is also abundant in Saanich Inlet (Bary et al 1962, De Robertis 2001, but resides at 100 to 125 m depth and does not migrate (De Robertis et al 2000, De Robertis 2001. Since the present study focuses on diel vertical migration behavior in the upper 50 m of the water column, O. obtusus is unlikely to contribute to the major acoustic scatterers.…”

“…Low biomass of copepods (Bary et al 1962) and their low target strength (TS) values at 200 kHz due to their small body size (Trevorrow 2005) suggest that their contribution to volume backscattering strength (S v ) is also minimal. Deca pods, mysids, shrimps, physonectid siphonophores, gastropods, hydromedusae, cteno phores, chae tognaths and cephalopods have been observed in previous studies, but their rare oc currence and low density in scattering layers suggest insignificance as acoustic scatterers (Bary et al 1962, Pieper 1971, Mackie & Mills 1983, De Robertis 2001. Although the thecosome pteropod Limacina helicina and the gas-filled pneumatophores of siphonophores are strong acoustic targets (Stanton et al 1994), scattering model calculations of L. helicina, which are < 2 mm in diameter, indicated that their contribution to echoes is not significant at 445 kHz (De Robertis 2001).…”

“…m −3 (Bary et al 1962, Bary 1966, Pieper 1971, Mackie & Mills 1983, accounting for ~70% of the daytime and ~76% of the night-time scattering layers based on the forward problem predictions of acoustic scatterers (Holliday & Pieper 1995, De Robertis 2002. Euphausia pacifica is the most abundant euphausiid throughout the year, constituting 77 to 100% of all euphausiids followed by Thysanoessa spinifera and T. raschii (Bary et al 1962, Pieper 1971, De Robertis 2001. E. pacifica live 1 to 2 yr (Tanasichuk 1998) with their main spawning period from early May to mid-July, coinciding closely with the periods of higher phytoplankton abundance (Parsons et al 1967, Heath 1977.…”

Trophic relationships and oceanography on and around a small offshore bank

“…2, and next paragraph). Our net samples did not reveal other planktonic organisms that are strong sound scatterers and sometimes abundant in the Gulf of Maine, such as siphonophores (which can leave a trace when nets are pulled slowly, as in vertical tows) and pteropods (which would be retained by the nets) (De Robertis 2001, Warren et al 2001.…”

Turbulence and internal waves in Patricia Bay, Saanich Inlet, British Columbia

“…Yearround dominance of euphausiids in Saanich Inlet is documented in previous studies through optical images (Jaffe et al, 1998), visual observations (Mackie and Mills, 1983), and net samplings (De Robertis, 2001;Sato et al, 2013). Euphausia pacifica is the most abundant euphausiid species throughout the year, constituting 77-100% of all euphausiids (Bary et al, 1962;Pieper, 1971;De Robertis, 2001).…”