All animals must feed in order to maintain an energy supply to fuel their metabolic processes. Each individual faces competition for scarce food resources, and only those that are consistently successful are able to grow and reproduce.
Finding sufficient food is not just about being able to fight for a share when rich pickings are on offer; it is also about innovating and diversifying -- trying new kinds of food and exploiting untapped resources. Over tens of thousands of years, fish have evolved to be expert foragers, feeding on a huge range of different food sources, both animal and vegetable. Some specialize on a particular kind of food, others are opportunists, feeding on whatever they can. One thing all species have in common is that they are experts in spotting an opportunity and hunting out their next meal.
Where do different fish species find their food?
Fish use a variety of techniques when foraging. They can either seek out their prey or they can lie in ambush and wait for it to come to them. If they look for their own food, they often visit rich feeding grounds where their chances of success will be much greater. Fish in rivers often rely on the current to provide them with food. Food particles, including fly larvae that have been dislodged by the current or the fry of other fishes, are unable to negotiate the flow and so drift downstream to be met by eager hunters.
In lakes, shallow waters are particularly rich in food, perhaps because they contain weed beds that harbor small prey animals. For herbivores, the substrate in these areas is often bathed in sunshine and the algae grows rapidly here. Another means of finding food is to sniff it out -- fish have an excellent sense of smell and use it to home in on their food. If they pick up the faint odor of a prey animal, they are often able to follow the trail back to its source and feast on whatever awaits them there.
How do fish forage effectively?
Quite naturally, many of the animals that are the preferred prey of fishes hide. Some, such as water fleas (Daphnia sp.) even respond to exposure to fish by developing different shapes, including growing spiky projections that make it more difficult for fish to handle them. But as fast as their prey work out ways to keep themselves out of harm's way, fish find a means to overcome the problem. Often when fish try a novel and difficult type of prey, they are not especially efficient at dealing with it. But practice makes perfect and fish have shown themselves to be experts at learning new tricks. Researchers looking at how quickly fish can learn have shown that most species can go from novice to expert in their dealings with an unusual food in as few as five attempts. Some species hunt more actively, fanning the bottom substrate with their pectoral fins to expose hidden prey, others dig down for the same reason. Some cichlids actually turn over small rocks and pebbles to find the animals hidden beneath.