A bit about fish

With around 28,000 different species, fish make up about half of the World’s known species of vertebrate (animals with backbones). There are fish adapted to virtually every type of water environment, from tiny ponds, to lagoons, to oceans.

Most people are familiar with the idea of the pea-brained fish with a three-second memory. But scientists have been finding that fish are much more intelligent than they have ever been given credit for. Biologists and psychologists who study them claim that fish are capable of learning and remembering, and that they possess a range of other cognitive skills. Australian crimson spotted rainbowfish learned to escape from a net in their tank, and remembered how they had done it months later. And fish who survived an encounter with a net in early life will learned to swim away from approaching trawlers.

Writing in the journal Fish and Fisheries in 2003, biologists Calum Brown, Keven Laland and Jens Krause, cited more than 500 research papers that prove fish are now seen as highly intelligent creatures with very complex social relations. Fish co-operate to inspect predators and catch food and, sometimes, they will even pursue strategies of manipulation, punishment, and reconciliation.

Research in recent years has shown that fish recognize individual fish and social prestige, and even track relationships. Scientists have also observed that some fish have stable cultural traditions like tool use and complex nest building. There are South African fish, for example, who lay their eggs on leaves then carry them to a safe place, using the leaf as a transport basket. Other fish have been known to tend gardens, encouraging the growth of tasty algae by weeding out the types of plants they don't like.

Fish feelings

There are still many people who believe fish are dim-witted creatures unaware of their surroundings. This allows people to believe that fishing is okay and that the fish they catch don’t suffer. Nothing could be further from the truth. Fish actually have complex nervous systems that understand and respond to pain. As Dr. Donald Broom, scientific advisor to the British government, explains it: “The scientific literature is quite clear. Anatomically, physiologically and biologically, the pain system in fish is virtually the same as in birds and animals.” Scientists at Edinburgh University and the Roslin Institute in the United Kingdom report that in response to pain, fish also feel emotional stress and engage in a “rocking” motion that is quite similar to the motion seen in stressed mammals. The bottom line is that fish definitely suffer when hooked or caught in a net, just as any mammal would.

The threat of human mouths

Fish are eaten by other species of fish, by birds, snakes, turtles, and crustaceans, but their biggest threat comes from humans, who are quickly destroying them through intensive fishing. A study written in 2003 by Dr. Ransom Myers and Dr. Boris Worm of Dalhousie University in Halifax, estimated that 90% of large predator fish like swordfish, marlin and sharks, have been cleared from the seas in the past 50 years. The study also speculated that these fish, along with commercially important fish like tuna, cod, and halibut, are now much smaller in average size than their ancestors were.

According to the Food and Agricultural Organization (FAO), 52% of the world’s fish “stocks” are fully exploited, which means that they are being fished at their maximum biological capacity. 24% are over-exploited, depleted, or recovering from depletion, 21% are moderately exploited, and only 3% of the world's fish stocks are underexploited. Most Canadians are familiar with the example of cod. In 1992, thousands of people were suddenly without work when the once plentiful stocks of cod off the coast of Newfoundland completely collapsed due to over-fishing.

Did you know?

• In some species, female and male fish are differently shaped or coloured, but in many other species there is no male/female difference visible to humans.
• There are a few very unique fish species that have lungs and breathe air, but most fish breathe through gills. Gills are made up of thin sheets of tissue covered in blood vessels. As water passes over the gills, dissolved oxygen is absorbed into the blood and waste products such as carbon dioxide pass out into the water. The gills are protected by a bony plate called an operculum.
• Most fish have a lateral line along their sides, which is a row of scales stretching from their head to tail. Under these scales is a system of fluid-filled canals and specialized cells which transmit vibrations to the fish’s brain. The lateral line helps fish to detect other objects in the water, including predators and prey.
• If you were ever to touch a fish you might find it feels slimy. This “slime” is actually mucus the fish secretes from its skin. The mucus coating protects the fish against parasites and diseases, covers wounds to prevent infection, and helps it move through water faster. Some fish species also release toxins in their mucus that ward off attacking creatures.
• Marine biologist Ben Wilson of Canada's Bamfield Marine Science Centre lead a study in 2003 that found that "farting" may be very important in helping fish communicate with each other in the dark. They discovered that herring let out little fart-like noises, by gulping air at the surface and letting it out from a hole in their rear ends. These noises seem to help them locate each other at night without alerting predators.
• The climbing perch, native to India, can walk on land in search of water when its water hole dries up!
• Fish talk to each other with squeaks, squeals, and other low-frequency sounds that humans can hear only with special instruments. Some fish woo potential partners by singing to them.
• Fish like to be touched and often gently rub against one another—like a cat weaving in and out of your legs.
• Fish flesh is frequently contaminated with mercury (which can cause brain damage) and toxic chemicals like DDT, PCBs, and dioxin (which have been linked to cancer, nervous system disorders, and fetal damage), as well as disease-causing bacteria.