Diel movement patterns and habitat preferences of the common thresher shark (Alopias vulpinus) in the Southern California Bight

The common thresher shark, Alopias vulpinus, is the basis of the largest commercial shark fishery in California waters. We used acoustic telemetry to determine the diel movement patterns and habitat preferences of this species inthe Southern California Bight (SCB),where commercial fishing for the common thresher sharkis concentrated. Eightcommonthreshers (forklength: 122-203cm) weretaggedwith temperature anddepth-sensingacoustic transmitters and tracked for periods ranging from 22 to 49h. Tracked sharks preferentially utilized deep offshore waters, and avoided shallower waters over the continental shelf. Mean rate of movement (ROMs.d.) was 2.15 � 0.46kmh � 1 . ROM and angularconcentration(r,ameasureofrelativelinearity)bothshowedastrongdaytimepattern,withhighestvaluesatdawn that decreased throughout the day, whereas nocturnal ROM and r were less variable. Daytime vertical movements consisted of either vertical excursions below the thermocline or relatively level swimming within the upper portion of the thermocline. Nocturnally, all sharks remained within the mixed layer. These findings suggest that the common thresher shark is primarily a daytime predator, and have relevance for estimating how the alteration of the set depth of fishing-gear could affect catch rates of this species in the SCB.