Sharks a Hazard For Submarine Cables

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There is nothing more difficult to take in hand, more perilous to conduct, or more uncertain in its success, than to take the lead in the introduction of a new order of things.

— Niccolo Machiavelli

I have spent my share of time on a ship dealing with electronics being operated underwater -- it is never easy or simple. It is easy to forget that there are creatures down there that also may want to cause you trouble. Figure 1 is a video showing a shark chomping a fiber optic communication cable (source).

Problems like this have been reported since the first transatlantic cables were laid in the 1800s. Here is a quote from this article about a situation back in the 1980s.

We’ve long known squirrels are a major problem to anyone laying cable, but according to a report by the International Cable Protection Committee cable bites—by sharks and other fish—remain a surprisingly persistent problem. In the 1980s, a deep-ocean fiber-optic cable was cut four times. Researchers blame crocodile sharks for those attacks after finding teeth in the cable.

I have never heard more than mere speculation as to why the sharks go after the cables. Here is an excerpt from the New York Times. This excerpt mentions that sharks are very sensitive to electromagnetic (EM) fields. When I was a boy, the news articles used to mention a fish's lateral line as being sensitive to EM fields.

Also, some researchers believe there may be something unusual about the electrical current in the fiber-optic lines that attracts sharks and that may trigger an automatic feeding reflex.

The finding that sharks are supersensitive to electrical signals, able to detect electric fields as faint as a few millionths of a volt per centimeter in water, is a recent significant discovery in marine science, Dr. Nelson said.

The sharks may detect a faint field near the cable and attack. ''Not knowing any better, they try to eat it,'' Dr. Nelson said. ''It's programmed in their genes. Whether the field comes from a cable or from a tin can, sharks are prone to behave as if they were encountering a food item, and try to eat it up.''

Some folks have asked me why a fiber optic cable would have EM field around them. The cables that I have dealt with all had internal copper wire that carried high voltage (~8 kV) to power optical amplifiers within the cable that were mounted every 30 km or so. So it is conceivable that there are significant fields around these cables. Figure 2 (source) shows a submarine cable with amplifiers (aka repeaters) in it.

Figure 2: Illustration of a Fiber Optic Cable with Repeaters. (Source)

The Wikipedia also discusses sharks biting cables.

After AT&T installed the first deep sea fiberoptic cable between Gran Canaria and Tenerife in the Canary Islands in September 1985, the system suffered a series of shorts that necessitated costly repairs. It was discovered that attacks from the crocodile shark were responsible for most of the failures, possibly because they were attracted to the electric field around the cables. Since crocodile sharks are not benthic in nature, they were presumably biting the cables as they were being deployed. The problem was solved by protecting the cables with a layer of steel tape beneath a dense polyethylene coating.

Figure 3 shows what a crocodile shark looks like (source).

Figure 3: Wikipedia Photo of Crocodile Shark. (Source)

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