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

A three-dimensional multibeam tracking sonar system (FishTV) was deployed i n 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 d isplacements of acoustic targets in pairs of sonar frames with increasing t ime delays between frames. Assuming isotropic motions of euphausiids, the s uperior spatial resolution of the sonar in one dimension (range) is used to infer the three-dimensional motions. We applied the algorithm to sonar tra cks of groups of 10 000 - 25 000 euphausiids recorded in the daytime at dep ths just above the oxycline on six different occasions. The animal movement s were described with a probability density function for velocity which sug gests 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 an d quiescent behavior. Limited motility in the daytime would confer two majo r advantages on the euphausiids: reduction of encounter rates with ambush p redators and decreased metabolic costs.