Scattering of Underwater Sound in Saanich inlet, British Columbia

Limnology & Oceanography

Jaffe

Ohman

2000

Zooplankton commonly exhibit diel vertical migration (DVM), descending from food‐rich surface waters during the day. If DVM is a tradeoff between avoiding size‐selective visually hunting predators and maximizing energy gain, smaller bodied prey should enter surface waters earlier and leave later than larger, more visually conspicuous organisms. Conventional sampling technologies lack the temporal resolution to test this prediction. Here, we report on the first test of this prediction using a new submersible optical‐acoustic imaging system capable of resolving the timing of migration of the euphausiid crustacean Euphausia pacifica Hansen. Smaller bodied animals consistently ascended as much as 30 min earlier and descended up to 45 min later than adults. The timing of vertical migration reflects how the size‐dependent risk of attack by visual predators alters the tradeoff between feeding and predator avoidance, supporting the predator‐avoidance hypothesis for DVM.

Section: Size-dependent Visual Predation Risk and The Timing Of Vertisupporting

confidence: 87%

Section: Size-dependent Visual Predation Risk and The Timing Of Vertimentioning

confidence: 99%

Size‐dependent visual predation risk and the timing of vertical migration in zooplankton

Limnology & Oceanography

Jaffe

Ohman

2000

“…If excretion and defecation also occur higher in the aerobic portion of the water column, this would provide an example of carbon and nutrient transport that is counter to the more typical net downward transport of material by zooplankton through gravitational settling or diel vertical migration (Longhurst & Harrison 1989). In Saanich Inlet, O. obtusus are consumed by fishes in the oxic parts of the water column (Bary et al 1962), thus directly linking material consumed in the anoxic benthos to the pelagic food web. The benthic substrate is unlikely to be accessible to other planktonic organisms in the system during periods of water column anoxia.…”

Section: Discussionmentioning

confidence: 99%

“…The oxygen deficiency in this fjord exerts a strong influence on the community structure and spatial distribution of both benthic (Tunnicliffe 1981) and pelagic (Mackie & Mills 1983) organisms. In Saanich Inlet, the lysianassid amphipod Orchomene obtusus (Sars) (see Barnard 1964 for synonymies) is abundant in the water column in regions of low oxygen concentration (Bary et al 1962, Mackie & Mills 1983. The distribution of O. obtusus overlaps with a mid-water layer of flocculent suspended particles in the oxycline (Mackie & Mills 1983).…”

Section: Introductionmentioning

confidence: 99%

Eat and run: anoxic feeding and subsequent aerobic recovery by Orchomene obtusus in Saanich Inlet, British Columbia, Canada

Eiane²,

Rau³

2001

Mar. Ecol. Prog. Ser.

The lysianassid amphipod Orchomene obtusus (Sars) is known as an epibenthic scavenger, but it is abundant in the water column of Saanich Inlet, British Columbia, a temperate fjord with poorly oxygenated deep water. Sampling with vertically-stratified nets and baited traps indicates that O. obtusus is abundant within the oxycline at 100 to 140 m, is present in anoxic deep waters at low abundances, and is also abundant in the anoxic benthic environment. Shipboard incubations revealed that O. obtusus can survive between 10 and 33 h of anoxia at in situ conditions. Stable isotope and gut fluorescence measurements suggest that these amphipods feed on material produced in surface waters, and not on the isotopically dissimilar suspended detrital material present in the vicinity of the oxycline. The amphipods do not enter surface waters, and it appears that O. obtusus has adapted to oxygen gradients in this environment by feeding on surface-water-derived material in the food-rich and predator-poor anoxic benthos and subsequently migrating into the water column to conduct aerobic respiration. These vertical excursions are probably a modification of the swimming behavior exhibited by hyperbenthic amphipods in well-oxygenated environments, and may constitute an important mechanism for benthic-pelagic coupling.

“…OASIS was deployed during two midsummer cruises in 1996 and 1997 in Saanich Inlet, BC, an environment in which the macrozooplankton is known to be dominated by dense populations of the euphausiid Euphausia pacifica and the lysianassid amphipod Orchomene obtusus (Bary et al, 1962). This site was selected because the low diversity of large-bodied animals greatly simplifies the ecological interpretation of acoustic records.…”

Section: Study Sitementioning

confidence: 99%

Validation of acoustic echo counting for studies of zooplankton behavior

ICES Journal of Marine Science

2001

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