While he could not see under water, his adversaries could. They had taken a hint from nature. The fish-hawk has no difficulty in spying his submarine prey. Flying high above the water, he can see his victims at a considerable depth, and wait his chance to pounce upon an unwary fish that comes too near the surface. It is said that the British trained sea-gulls to hunt submarines. Sea-gulls will follow a ship far out to sea for the sake of feeding on refuse that is thrown overboard. British submarines encouraged the birds to follow them, by throwing out bait whenever they came to the surface. Of course the birds could see the submarine even when it was submerged, and if they pursued it, they were always rewarded with plenty of food. The gulls drew no fine distinction between Hun and Briton, and so it came that Herr Kommandant often groped his way along in the dark sea, totally oblivious of the fact that he was attended by an escort of feathered folk who kept the British chasers informed of his presence.
In this connection it is interesting to note that the British trained sea lions to hunt submarines. The animals were taught at first to swim to a friendly submarine, locating it by the sound of its propellers. They were always rewarded with fish. These sea lions were muzzled so that they could not go fishing on their own account. Then they learned to locate enemy submarines and pointed them out by swimming directly toward them and diving down to them.
But there were human eyes, as well, that spied upon the U-boat. Fast seaplanes patrolled the waters, searching constantly for any trace of submarine. Its form could be vaguely outlined to a depth of from fifty to seventy-five feet, unless the sea were choppy, and once it was discovered, chasers or trawlers were signaled to destroy it with bombs or to entangle it in nets. Often a submarine would be discovered by a leak in its oil-tank which would leave a tell-tale trail. Sometimes when the U-boat itself could not be discerned, there would be slight shimmer, such as may be seen above a hot stove, caused by refraction of light in its wake. This was easily recognized by trained observers.
(C) Press Illustrating Service
A Depth-bomb Mortar in Action and a Depth-bomb snapped as it is being hurled through the air
Even better aërial patrols were the small dirigibles known as Blimps. They are a cross between a balloon and an airplane, for they have the body and the power-plant of an airplane, but the planes are replaced by a gas-bag. Blimps could cruise leisurely and search the sea thoroughly. They could stop and hover directly over a submarine and drop explosives upon it with great accuracy. And so Herr Kommandant could take no comfort in hiding under a blanket of waves unless the blanket were so thick as to conceal his form completely from the eyes overhead. This made it imperative to leave the shallower waters near shore and push out into the deep sea, where the small chasers could not pursue him. But he could not shake off his pursuers. Stream-trawlers are built to ride the heaviest gales and they took up the chase out into the ocean.
Courtesy of "Scientific American"
Airplane Stunning a U-boat with a Depth-bomb