Chemical Messages Between Fish
Fish not only use their sense of smell to find food and stay around their home range, but also to pick up the chemical messages passed from fish to fish. These chemical messages -- more correctly called pheromones (from the Greek "pherein" meaning "to carry") -- allow fish to gather information about other members of their own species and to find mates. Pheromones also provide a host of other information, allowing fish to eavesdrop on their predators and the other species that share their habitat.
How important is chemical communication for fish breeding?
The single most important task in any animal's life is to breed and pass on its genes to its offspring. So when the breeding season comes round, the stakes are high. Fish use pheromones to detect the presence of mates, their readiness to breed and whether they have mated before. Fish are especially sensitive to the presence of sex pheromones, and are able to use them not only to home in on a potential mate, but also to detect whether their target is ready to breed. This means they are able to concentrate their efforts more effectively.
Male goldfish have an unusual trick that they use as part of their courtship display: they swim up to a female and then roll over and urinate over her nostrils. As strange as this seems, it allows the female to assess many things, including the male's genetic make-up, thus preventing her from breeding with a relative.
What happens if these chemical messages are intercepted?
Among fish species there are different strategies when it comes to parenthood. Some species make considerable investments of time and energy to defend their young, others invest in producing huge numbers of eggs and leave these to fend for themselves. In an ideal world, fish would produce masses of eggs and then defend them, but the energetic costs mean that they have to choose between one method or the other.
Some species, such as the catfish Synodontis multipunctatus, cheat the system by using others to raise their young for them.To do this, they respond to the presence of the sex pheromones of nesting species in their habitat and synchronize their breeding cycles with that species. Then they home in on the source of the sex pheromones and lay their eggs in the nest, leaving their young to be brought up by someone else and neatly side-stepping some of the costs of parenthood.
Can fish smell fear?
Each species of fish produces its own individual chemical signature, and other fish in the locale can detect these chemical cues and respond accordingly. This not only allows predators to find their prey, but also allows prey to recognize that predators are around and to take evasive action -- or so you might assume. Instead, prey fish often respond to the presence of fish predators by approaching them. They can then detect subtle chemical cues that tell them when the predator last ate (and therefore whether it is hungry) and what type of fish the predator has recently been preying on, all of which tells the prey how much of a threat the predator presents.
If and when the predator does attack and injures or captures a fish, this releases an alarm pheromone into the water. Prey species are acutely sensitive to this smell and will rapidly seek cover in response to detecting even tiny concentrations of it. The alarm pheromone is long-lasting, too, and even if the predator has managed to eat the prey, the pheromone continues to act as a chemical marker on the predator, taking away its ability to launch further surprise attacks.