The piscatory arms race

The battle between predatory fish and their prey is a never-ending war of attrition across generations with continuous evolution

FISH have evolved many strategies to avoid being eaten. Some, such as shoaling, are more obvious than others, but over millions of years each behaviour and trait has given them a slightly better chance of survival, leading to its gradual spread throughout the population. Those fish with a handy trait will have a slightly better chance of survival. Multiplied over many generation this will see their progeny becoming more successful. This is pure Darwinian evolution – small changes, magnified by time, giving improved survival. Of course, it is not just the prey fish that are evolving. The predators will be adapting just the same, giving rise to an evolutionary arms race as predator and prey evolve in unison. One of the most common strategies that is thought to have evolved in response to predation pressure is shoaling. Just as in herding animals on land, many individuals have a much better chance of spotting a potential danger sooner than an individual. This means that the risk of being eaten for each fish in a shoal is less than if they were on their own. Shoal mates can also spend more time feeding, which helps to offset the extra competition they face by living in such close proximity to others. Shoaling is only effective when fish can quickly communicate the danger to others. This is something that they can do in literally a blink of an eye as their body language transmits the alert signal to their shoal mates. A stiffening of the fins, jerky movements and a bending of the back are all signals of approaching danger. Of course, predatory fish such as pike, have learned to combat the many eyes of a shoal of fish. The broken camouflage pattern produced by the vertical stripes on a pike help to disguise it. The slow, tiny movements of a pike also do not draw attention to it, enabling it to creep up close to prey fish. Perhaps the biggest adaptation that a pike uses is to hunt mainly at dawn and dusk when light levels are low. With their large eyes pike are able to operate well in the dull gloom of early

morning, while the less acute eyes of roach are not able to operate as effectively, giving the pike a big advantage. Good eyesight, especially in low light, is a useful trait for a predator. Big cats and owls share the same ability as many predatory fish. Zander share the same adaptation as lions, having a layer of reflective cells in the back of the eye, called the tapetum, which reflect light back on to the cone and rod cells in the eyeball. This gives them greatly improved eyesight in low light, enabling them to hunt effectively even under the restricted light of a new moon. Spines are another useful weapon against predation, and perch use these to good effect. The success of pike hunting perch is significantly worse than hunting roach of a similar size. This can be put down to the spines, which increase the time it takes for a pike to swallow a perch and make its initial catch less successful. Predators can evolve to deal with a wide range of prey and, generally speaking, the most common prey fish in a body of water will make up the bulk of a pike’s diet. This has been seen most clearly in lakes where new, highly successful, species have been introduced. Here, the pike switch to feeding on the most abundant prey and adapt their behaviour accordingly. Not all predators are able to consume all prey, and many are limited to feeding on a narrow range of prey sizes. Pike, for example, can consume prey at least as large as their mouth gape, but are much more effective when handling prey that are no more than 60% as long as their mouth gape. For a prey fish it is important to grow fast and minimise the time that they are a handy snack size for a predator. The same applies for prey fish of all sizes, from being tiny fry, where most of their predators will be invertebrates, such as water scorpions and dragonfly larvae, to young adults which may be subjected to cormorant predation. Prey fish populations can also, over a few generations, alter their body shape to reduce their risk of being predated. In lakes with pike, crucian carp tend to have a rounder body and deeper profile than in lakes with no pike. The deep body makes the crucians more difficult to eat, so their survival is slightly better. When pike are introduced to lakes where they were previously absent the crucian population switches to deeper bodied fish in just a few years, hinting at the benefits of changing shape. To ensure that they are bigger than their prey, predatory fish will normally lay their eggs a few weeks before their prey. Pike, for example, spawn in late March, whereas their main prey in many waters – roach – spawn in mid-May. By the time the roach eggs hatch the pike are already big enough to start hunting and eating them, ensuring that there is no easy escape for the poor roach. Many fish use camouflage to help reduce their risk of being eaten. If you look at most coarse fish they have a dark back and a lighter coloured belly. When viewed from above the dark back blends in with the bottom,

“Pike are more effective when handling prey that are no more than 60% as long as their mouth gape”

making them more difficult for predatory birds to spot. From a pike’s viewpoint looking upwards, the reflective belly blends in with the bright surface, again affording them a degree of camouflage. Predation is certainly one of the strongest pressures driving evolution. Although it normally happens at a glacial speed, often taking hundreds of generations and thousands of years to give even small changes, evolution has given us the interesting predator-prey dynamics that we see today.