The lure of sharks - A Fascination for Fish: Adventures of an Underwater Pioneer - page 109

Back at Sea World, the shark box is lifted out of the boat for transfer
to the experimental shark pool. (Photo by author)
and appeared to do fine—but only for a month or so. Then we no-
ticed a change in their swimming posture: no longer perfectly hori-
zontal in the water, they began to swim in a slightly tail-down posi-
tion. Too, they seemed to be working harder at swimming. It became
clear that they were losing weight, and autopsies later showed that much
of the weight loss was from their liver.
Most species of pelagic, or free-swimming, sharks have large, oil-rich
livers that, in addition to their metabolic functions, act as a buoyancy
organ. As the oil in their liver was used up to provide energy they be-
came heavier in the water and had to work harder to stay up. It be-
came a vicious downward spiral: the harder they worked to stay up,
the more they consumed of their liver, which caused them to grow heav-
ier in the water, and so they had to work harder to stay up. In spite of
being given all the food they could eat, they were using up their stored
energy faster than they could replace it.
Our conclusion was that the pelagic blue shark is designed for long-