Sharks have seventh sense

Sharks and rays belong to the taxonomic class Chondrichthyes, or cartilaginous fishes. Sharks do not have true bones, but instead their internal skeleton is made of flexible cartilage. Sharks inhabit every ocean in the world and there are approximately 500 species of shark known to exist. A variety of sharks are native to the Caribbean. The most common sharks in our region include nurse, blacktip, hammerhead, tiger, and reef. Sharks are generally slow-growing, late-maturing organisms that give birth to very few young, and are therefore well-recognized as being vulnerable to fishing pressure.

Hammerhead Shark

Hammerhead Shark

Sharks have evolved over 400 million years and play a critical role in ocean ecosystems. In common with land predators such as lions and wolves, sharks keep other marine populations in check and help maintain the balance of life in the sea. Sharks are one of the world’s misunderstood predators, as they rarely attack humans unless provoked. Humans kill approximately 26 to 73 million sharks every year, compared to five human deaths from shark attacks each year. Despite popular perceptions of sharks as invincible, shark populations around the world are declining because of overfishing, habitat destruction and other human activities. Sharks and rays are among the first marine groups to be systematically assessed by the International Union for the Conservation of Nature and Natural Resources (IUCN) Red List which states that of the 547 species listed, 20 per cent are threatened with extinction. This confirms suspicions that these mainly slow-growing species are disappearing at an unprecedented rate across the globe.

While over 100 nations fish for sharks, only a handful have created regulations to protect them. The once abundant bala shark in Sumatra and Kalimantan is a classic example of how important it is to protect sharks, regardless of the reason for hunting them. Despite its simple coloration, it became very popular all over the world as an aquarium fish. This popularity resulted in severe overfishing which effectively exterminated the wild populations. According to the National University of Singapore, it may well be extinct in these areas. The bala shark apparently migrates to specific breeding grounds. The collections for aquariums therefore took a serious toll not only of juveniles but breeding adults as well. The species has a short life span which means that regular large collections of specimens for the trade is necessary, and that capture of breeding individuals has very serious consequences for the population. These factors, together with collections even when it was reproducing at its breeding grounds, not surprisingly, resulted in a sharp drop in its numbers after a few years.

Sharks have remarkably well developed senses, many of which are equal or even more advanced of those in birds and mammals. Smell and taste are used to detect potential prey at long distance, but may also be important for discovering mates. Sharks have a sense of touch similar to other animals, and an additional sense for detecting pressure changes occurring in the waters immediately outside of the animal, allowing them to detect nearby prey, predators, or other sharks even if the other senses cannot be used. Sharks even have a seventh sense, allowing detection of the minute electric fields given off by living animals, inanimate objects, and water passing through the earth’s magnetic field. This sense is used to detect prey at very short distances, even when completely buried under sand. Perhaps partly to decipher the signals provided by these many senses, sharks and stingrays have relatively large brains. Some are even comparable in size and complexity to the brains of birds and mammals. According to Compagno (2005) in Sharks of the World, the hammerhead has the largest and most complex brain.

For many decades, the economy of the Cayman Islands relied very heavily on the turtling industry. By the 1930’s, however, it was evident that stocks were too low for this to continue, and turtlers began to look around for alternatives. Sailing the waters off the coasts of Costa Rica, Nicaragua and Honduras, many had noticed that numerous Nurse Sharks basked there. At this time, the leather industry was keen to use sharkskin, which was durable and scuff-resistant, so Caymanian fisherman began to catch sharks. Shark stocks were soon too depleted to be profitable for anyone. What had been plentiful in 1931 was almost exhausted by 1940 – the result of taking large numbers of breeding females. The Cayman Islands Government and merchants tried negotiating fishing rights in Cuban and Bahamian waters, with little success. Sharking declined and eventually died out. For more information visit the Sharking Information Sheet on the Trust website.

As an island community should be dedicated to ensuring that these valuable yet vulnerable animals survive for the benefit of ocean ecosystems and the people that depend on them. Any attempts to remove the sharks from local waters should be discouraged. Sharks are top predators in the marine environment and therefore are critical for maintaining the stability of the entire marine system. Photograph generously provided by Nick Buckley and Stephen Frink through Ocean Frontiers.

Protect Cayman’s Marine Wildlife! For more information, to share your knowledge or if you would like to get involved with the many activities in the National Trust’s Know Your Islands Program, please visit or call 949-0121.

Support local journalism. Subscribe to the all-access pass for the Cayman Compass.

Subscribe now