We analyzed size-dependent variations in spatial patchiness of the eight numerically dominant euphausiid species in the California Current System (Euphausia pacifica, Nematoscelis difficilis, Nyctiphanes simplex, Thysanoessa gregaria, Euphausia recurva, Euphausia gibboides, Thysanoessa spinifera, and Euphausia eximia). Patchiness was measured by using a count-based statistic using euphausiid densities and applied to 11 yr of detailed size-specific enumerations from the California Cooperative Oceanic Fisheries Investigations program. We rejected the hypothesis of size-independent patchiness for seven of the eight species. The most common pattern observed was a "U-shaped" curve, showing elevated patchiness in the smallest size classes, a rapid decrease in patchiness of intermediate-sized euphausiids, and a later increase in patchiness of adults following the onset of reproductive maturity. These size-dependent changes parallel ontogenetic changes in spatial dispersion observed for some marine fishes. The initial descending limb of the patchiness curve appears to be caused by turbulent diffusion, while the later ascending limb of the curve is consistent with the onset of predator-induced aggregation behavior. The patterns were surprisingly consistent across years and different reproductive characteristics (egg-brooding vs. broadcast spawning euphausiids). © 2010, by the American Society of Limnology and Oceanography, Inc.

A three-dimensional multibeam tracking sonar system (FishTV) was deployed in the stratified waters of Saanich Inlet in July-August of 1996 and 1997 to assess the swimming behavior of euphausiids in situ. Here, a new algorithm is used to estimate swimming velocities of animals from the uncorrelated displacements of acoustic targets in pairs of sonar frames with increasing time delays between frames. Assuming isotropic motions of euphausiids, the superior spatial resolution of the sonar in one dimension (range) is used to infer the three-dimensional motions. We applied the algorithm to sonar tracks of groups of 10 000 - 25 000 euphausiids recorded in the daytime at depths just above the oxycline on six different occasions. The animal movements were described with a probability density function for velocity which suggests that about 60-70% of the animals were moving less than or equal to 0.3-1.2 cm·s-1. These distributions imply remarkably low swimming speeds and quiescent behavior. Limited motility in the daytime would confer two major advantages on the euphausiids: reduction of encounter rates with ambush predators and decreased metabolic costs.

A new instrument has recently been developed: OASIS (optical-acoustical submersible imaging system), for recording in-situ acoustic target strengths (TS) of crustacean zooplankton with concurrent optical imaging of the animals. The system consists of a three-dimensional acoustic imaging system (Fish TV) and a sensitive optical CCD camera with a red-filtered strobe. The sonar triggers the acquisition of an optical image when it detects the presence of a significant target in the precise location where the camera and strobe are co-registered. Acoustic TS can then be related to the optical image which permits the identification of the animal and its 3-D orientation. The system was deployed in Saanich Inlet, BC, Canada in the summers of ’86 and ’87. Target strengths of many acoustic reflectors were recorded in parallel with the optical images triggered by the presence of an animal in the correct location of the sonar system. Inspection of the optical images revealed that the primary zooplankton taxa at the site were euphausiids [esp. Euphausia pacifica] and [gammarid] amphipods [esp. Cyphocaris challengeri] and permitted an exact correlation of TS and taxa. Extrapolation of this information to many other acoustic targets, imaged by the Fish TV system will allow more precise interpretation of both abundance and behavior.

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.