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

Abstract

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 in the Southern California Bight (SCB), where commercial fishing for the common thresher shark is concentrated. Eight common threshers (fork length: 122–203 cm) were tagged with temperature and depth-sensing acoustic transmitters and tracked for periods ranging from 22 to 49 h. Tracked sharks preferentially utilized deep offshore waters, and avoided shallower waters over the continental shelf. Mean rate of movement (ROM ± s.d.) was 2.15 ± 0.46 km h−1. ROM and angular concentration (r, a measure of relative linearity) both showed a strong daytime pattern, with highest values at dawn 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.