Sonar estimates of daytime activity levels of Euphausia pacifica in Saanich Inlet

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