Foods and Feeding 151
speeds. However, White Sharks are primarily daytime feeders, at least
where their major prey consists of day-active marine mammals.
In some species, the difference in activity and location is quite predict-
able. Gray Reef Sharks at Rangiroa Atoll in the Pacific were inactive in
a small lagoon area by day but swam outside the lagoon via a pass in the
reef at about the same times each evening, returning to the lagoon via the
same route in the morning. One tagged individual exited the lagoon be-
tween 6:40 and 6:50 on three consecutive evenings. Sunset and decreasing
light are relatively constant at tropical latitudes, and the shark was likely
responding to these predictable cues.
How often must sharks feed?
How often and how much a shark must eat depends on species, age, and
activity levels. Overall, juvenile sharks need to feed more often than adults.
Relatively inactive sharks, such as adult Nurse Sharks, consume about 0.2%
to 0.3% of their body weight each day and take about six days to digest an
average meal. More active species such as Sandbar and Blue sharks consume
0.2% to 0.6% of their body weight each day and take only three to four
days to digest a meal. Very active sharks such as makos, which are warm-
blooded (see “Are sharks cold-blooded or warm-blooded?” in chapter 2),
consume 3% of their body weight per day, digesting a meal in 1.5 to 2.0
days. A 100-kg (220-lb) mako would therefore have to eat 3 kg (6.6 lb) of
fish per day. It appears that sharks digest most of one meal before they re-
sume feeding. And as high as these numbers may seem, they are only about
half what a comparable-sized bony fish would need to maintain itself.
Low energy demands appear to characterize even the superpredatory
White Shark. A 4.3-m-long (14-ft) White Shark was outfitted (harpooned
actually) with a transmitting thermometer. Researchers were able to calcu-
late its oxygen and energy demands by looking at how quickly its internal
temperature changed as it swam through water masses of differing tem-
peratures. The shark used up energy at an hourly rate of 0.2 calories for
each kilogram of body weight (0.2 Kcal/kg/hr). By comparison, bony fishes
use about twice that amount, and humans use up about eight times as much
energy per unit body weight.
We can translate these numbers into how much a White Shark would
need to eat to maintain itself. White sharks often feed on dead whales.
Whale blubber has a very high caloric value: a half pound of whale blubber
(the size of a tuna fish can) provides 1,600 calories. Researchers captured
a shark that was feeding on a dead whale and found 30 kg (66 lb) of whale
blubber in the shark’s stomach. At an energy consumption rate of 0.2 Kcal/
kg/hr, a belly full of whale blubber could maintain a White Shark for about