Diel movements of bat rays, Myliobatis californica, in Tomales Bay, California:: Evidence for behavioral thermoregulation?

We used ultrasonic telemetry to examine movement patterns of 11 bat rays, Myliobatis californica, in Tomales Bay, California. Tomales Bay is long (20 km) and narrow (1.4 km), and is hydrographically separated into outer and inner bay regions. The outer bay (the outermost 8 km) is characterized by oceanic conditions while the shallow inner bay (the innermost 12 km) features wide seasonal temperature shifts. Five rays were tracked monthly from October 1990 to November 1991 and six rays (four of which carried temperature-sensing transmitters) were tracked daily from 30 June to 16 July 1992. Mean bat ray movement rate was 8.84 m min(-1) (range 4.49 to 13.40 m min(-1)) and was not significantly affected by size (p = 0.592), tidal stage (p = 0.610), or time of day (p = 0.327). Movement direction was unrelated to tidal stage (p = 0.472) but showed a highly significant diel pattern (p < 0.001). From 2:50-14:50 h, rays moved toward the warmer and shallower inner bay, while from 14:50-2:50 h they moved toward the cooler and deeper outer bay. These telemetry data, along with known bat ray foraging patterns and respiratory temperature-sensitivity, argue for behavioral thermoregulation as the primary influence on this movement pattern.