Shark Ecology 89
the vertical migrations of some sharks are impressive. Vertical migrators
experience greater changes in temperature and pressure over a few hours
than do sharks moving the length of a coastline or across an ocean over
many days. The diving profiles of these sharks, as revealed by ultrasonic
telemetry and archival tags, show a yo-yo pattern: animals move up and
down several hundred meters repeatedly during the day and night or over
the course of several days. Tiger Sharks have been described as “bounce
divers,” moving almost continuously between the surface and 90 m (300
ft) down. White Sharks may move up and down between the surface and
200-m (660-ft) depths as often as 96 times in 24 hours; others may spend
days at the surface and then more days at depth. Female Gray Reef Sharks
observed in Palau exhibited a distinctive, complex up-and-down move-
ment pattern that extended over only 15 m (50 ft) each day. They tended to
move to shallower water (30 m, or 100 ft, deep) at dawn and dusk but spent
daylight hours at 45-m (150-ft) depths. The depths at which they stayed
at night were usually shallower than daytime depths, except on full moon
nights, when they descended to 60 m (200 ft).
Bigeye Threshers, Megamouths, Shortfin Makos, and School Sharks
spend daytime at depths of several hundred meters and then move to within
10 to 100 m (33–330 ft) of the surface for the night, probably tracking food.
Whale Sharks engage in both irregular and day versus night vertical move-
ments, moving between the surface and 1,000 m (3,300 ft). They spend
most of their time near the surface regardless of light, especially when
feeding. Their attraction to the surface is probably because their food—
recently spawned fish and invertebrate eggs—is concentrated in shallow
water. Basking Sharks around the British Isles move up and down in the
classic pattern of going deeper (to 70–80 m, 225–265 ft) by day and staying
shallower (10–30 m, 33–100 ft) at night when they are in deep water that is
colder at depth. But in shallower areas where there is no temperature dif-
ference with depth, these sharks stay in shallower water by day and deeper
water at night. In both the classical daytime-deep / nighttime-shallow and
reversed patterns, it appears the Basking Sharks are following the move-
ment of their zooplankton prey.
A few exceptional sharks undertake a daily vertical migration clearly
linked to food availability. Many bony fishes (lanternfishes, lampfishes,
bristlemouths, lightfishes) live at mesopelagic (middle-ocean) depths be-
tween 200 and 1,000 m (660 and 3,300 ft) during daylight hours. They
move up near the surface in the evening to feed under the cover of dark-
ness on the zooplankton that is more abundant in the shallows. They are
met there by a number of predators, such as dolphins, billfishes, tunas, and
Megamouth Sharks. These predators find their prey in the dark because
mesopelagic fishes are often bioluminescent, giving off a green glow from