Acoustic backscatter measurements with a 153kHz ADCP in the northeastern Gulf of Mexico: determination of dominant zooplankton and micronekton scatterers

Determining dominant scatterers of sound in mixed zooplankton populations

“…Though the correlation between observed and predicted scattering was not as good as some other studies ͑Flagg and Smith, 1989; Wiebe et al, 1997;Ressler, 2002;Fielding et al, 2004͒, the key difference is that this study includes data from multiple locations, collected at different times of the day and night, at different times of the year, at multiple frequencies, and in heterogeneous zooplankton populations in which one particular type of scatterer does not often dominate at all frequencies. It is also of interest to note that the daily vertical migration of zooplankton, in which some zooplankton taxa migrate but others do not, changes the relative day/night abundances of different zooplankton at any given depth, potentially affecting the balance of dominant scatterers.…”

“…An occasional deep scattering layer, most often observed during the daytime, had an almost flat frequency response. There is evidence in the literature that scattering from myctophids, small fish, some with and some without swimbladders, can give rise to a relatively flat, or even decreasing, backscattering spectrum between 38 and 200 kHz ͑Kloser et al., 2002;Mair et al, 2005͒. Myctophids, and fish more generally, are known to effectively avoid net systems such as the 1-m 2 MOCNESS, and very few individuals were captured in the nets.…”

Diel variation in the vertical distribution of deep-water scattering layers in the Gulf of Mexico

“…The depths at which SSLs occur are dynamic and often dependent on the depth of the water column and time of day, which gives rise to well-recognized and remarkable diel vertical migration (DVM) patterns (Balls, 1948;Kampa and Boden, 1954;Hersey and Backus, 1962). The main organisms that comprise SSLs are euphausiids, crustaceans, cephalopods, siphonophores, myctophids, and other mesopelagic fish species (Torgensen et al, 1997;Ressler, 2002;Båmstedt, 2003;Benoit-Bird and Au, 2006;Collins et al, 2008;Gødo et al, 2009) that serve as primary prey resources for larger nektonic predators (Markaida et al, 2008;Benoit-Bird, 2004;Kaltenberg et al, 2007). In the Gulf of Mexico, SSLs are primarily characterized by mesopelagic fishes with gonostomatids (bristlemouths) being numerically dominant, followed by myctophids (lanternfishes) (Hopkins and Lancraft, 1984).…”

“…The distinct DVM of scattering layers is believed to be due to changing ambient light conditions (Ressler, 2002;Gødo et al, 2009), with descent away from the surface during daytime and the ascent to shallower waters in the night (Neilson and Perry, 1990;Ohman, 1990). The diel dynamics are believed to be a behavioral response to feeding opportunity and predation risk (Cohen and Forward, 2009) or metabolic advantages (Lampert, 1989).…”

“…Studies of the vertical migration of macrozooplankton and mesopelagic fishes in the slope waters of the Gulf of Mexico have primarily focused on net sampling (e.g., Hopkins andBaird, 1985a, 1985b;Gartner et al, 1987;Ross et al, 2010), with occasional complementary use of single frequency acoustic Doppler current profilers (Ressler, 2002;Kaltenberg et al, 2007). However, advances in multifrequency approaches have facilitated improved interpretation of acoustic backscatter permitting scattering attributes to be assigned to relevant taxonomic groups (Korneliussen and Ona, 2003;Jech and Michaels, 2007;Lavery et al, 2007;Warren and Wiebe, 2008;Benoit-Bird, 2009;Fernandes, 2009;De Robertis et al, 2010;Gauthier et al, 2014).…”