Even if the predator does detect its prey, the game is far from up. Fish have a variety of behavioral strategies available to them to avoid ending up as lunch. Beyond this, fish populations that co-exist with large numbers of predators often show adaptations that help them cope with this pressure. Some show additional behavioral defenses, others have developed different body shapes in response to predator cues, making them more difficult as targets. Others cope by maturing and producing offspring earlier.
Can fish learn how to deal with predators?
If a fish can prevent its first encounter with a predator from being its last, it needs more than just the ability to learn quickly; it must be able to recognize the threat before it has even happened. But how on earth can this work? In fact, there are two ways. First of all, fish are born with the innate ability to recognize certain characteristics of predators, such as their body size and their large eyes and mouth. Secondly, they can learn from other members of their population.
Many fish, including glowlight tetras (Hemigrammus erythrozonus), release a chemical alarm substance if they are injured. If another tetra comes across this smell, it reacts by taking evasive action. Fish are also able to link the smell of the alarm substance to the appearance of the predator in question, so that information about the danger passes through the population. In the case of the glowlight tetras, research has shown that the smell of the alarm substance causes the fish to show a much more dramatic response to the image of a cichlid than they might otherwise do. Once they have associated the cichlid with danger, the lesson remains learned.
Can prey fish tell if a predator means business?
One way of learning about a predator is to take a closer look at it. Although this sounds potentially suicidal, it is precisely what prey fish of many different species do. As they approach they are careful to avoid what is known as the "attack cone" -- the area immediately in front of the predator, where it might lunge forward to attack. By carrying out this predator inspection, fish can gather an incredible amount of information, including whether the predator has eaten recently and, if so, whether it has eaten one of their own species (based on how it smells). From this they can gauge the level of the threat posed by the hunter. Often this inspection is carried out by only one or two individuals, the remainder of the local population remaining behind in safety. The risk to the inspectors is great but the pay-off for them can also be high. Studies have shown that males who carry out predator inspection become far more attractive to watching females.
How do fish populations adjust to the presence of predators?
Many fish species show what is known as developmental plasticity. This simply means that as they develop they adapt to their conditions. Populations of fish that live alongside their predators develop deeper bodies compared to members of their own species from other populations.This makes them harder for a predator to handle. As well as altered body shape, the hunted often show less vivid coloration. Male guppies from rivers that contain pike cichlids are far less bright than males that live apart from these piscivores. In addition to being less showy, the males from these populations also mature at an earlier age, giving them a better chance of breeding before they wind up as a meal.