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.
Despite the growing number of recreational anglers targeting white seabass Atractoscion nobilis in California, no information is available on the mortality rates of juveniles following catch and release. We captured juvenile white seabass (430-577 mm total length) with 4/0 offset circle hooks (n ϭ 113) or 4/0 J-type hooks (n ϭ 108) or with a net (control, n ϭ 30) to investigate the effect of hook type on anatomical hook location. We also examined how hook location affected growth and survival for 90 d following release. Offset circle hooks penetrated the lip region significantly more frequently (73%) than did J-type hooks (41%). Hook location was directly correlated with mortality; all mortalities involved hook damage to the visceral region. A 10% postrelease mortality rate was observed for fish caught on both offset circle and J-type hooks. All mortalities occurred within 5 d postrelease. Survival was enhanced when deeply embedded hooks were left in place rather than removed from the visceral tissue. Hook-caught fish showed no reduction in growth when compared with control fish, but deeply hooked fish grew at a significantly lower rate than did fish hooked in the mouth. Angler success rates did not differ significantly between hook types. We recommend that catch-and-release mortality be incorporated into the management plan for white seabass.
White seabass Atractoscion nobilis (family Sciaenidae) form transient spawning aggregations in the spring and summer throughout southern California and northern Baja California, Mexico. Although spawning activity has been linked with sound production in other sciaenid fishes, the function and acoustic characteristics of white seabass sounds have not previously been identified. We described the spawning behavior and characterized the sounds generated by 62 white seabass maintained within the seminatural conditions and free‐field acoustic environment of a net‐pen moored in Catalina Harbor, California. In addition to visual observations, video and audio recordings were made during periods of peak spawning activity in 2001‐2003. The physical characteristics of white seabass sounds were described and illustrated with sonogram, waveform, and power spectrum displays. Gravid females were identifiable during courtship and spawning by shifts in behavior and the development of dark bars across the dorsal region. During spawning, 1‐9 males tightly surrounded a gravid female and the resultant pack shuddered in unison as gametes were simultaneously broadcast into the water column. Five distinct types of sound were produced by white seabass: Single and multiple pulse trains during courtship, drumrolls and thuds during spawning, and booms during yawning and burst swimming. During the actual release of gametes, a rapid succession of overlapping drumroll and thud sounds resulted in identifiable spawning chants lasting 7‐55 s. Consistent physical, behavioral, and acoustical patterns during courtship and spawning indicated that white seabass utilize visual, tactile, and sonic cues to communicate their reproductive state.
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