An acoustic tracking of a megamouth shark, Megachasma pelagios: a crepuscular vertical migrator
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A 4.9 m TL megamouth shark, only the sixth specimen known to science, was tracked continuously for 50.5 h, during which it exhibited distinct vertical migrations at the dawn and dusk transitions. The male shark was captured on 21 October 1990 in a drift gill net off Dana Point, California, restrained overnight in a harbor, and released at sea the next afternoon. Horizontally, the shark moved slowly southward, covering 62 km on a relatively straight path with no significant diel changes. For the major part of the tracking, its rate of movement was 1.15 km h−1, as determined from positions at 15 min intervals. Considering a probable head current of 10–25 cm sec−1, its estimated through-the-water swimming speed was more likely 1.5–2.1 km h−1 (X¯ = 1.8, representing 0.1 body lengths sec−1). Vertically, the shark stayed shallow at night (12–25 m depth range, X¯ = 17) and deep during the days (120–166 m, X¯ = 149) but still well above the bottom at 700–850 m. The four twilight depth-change events were very distinct and always spanned the times of sunset or sunrise. The ascent and descent profiles are a reasonble match to isolumes on the order of 0.4 lux for an extinction coefficient (0.07) calculated from water transparency measurements. Furthermore, the steepest parts of the shark's profiles correspond closely to the times of maximum rate-of-change of illumination. These findings suggest that, except during nights, the shark's chosen depth was to a large degree determined by light level.
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- An acoustic tracking of a megamouth shark, Megachasma pelagios: a crepuscular vertical migrator
Environmental Biology of Fishes
Volume 49, Issue 4 , pp 389-399
- Cover Date
- Print ISSN
- Online ISSN
- Kluwer Academic Publishers
- Additional Links
- ultrasonic telemetry
- diel behavior
- Author Affiliations
- 1. Department of Biological Sciences, California State University, Long Beach, CA, 90840, U.S.A
- 2. Department of Biological Sciences, University of California, Santa Barbara, CA, 93106, U.S.A
- 3. Section of Ichthyology, Natural History Museum of Los Angeles County, Los Angeles, CA, 90007, U.S.A