RIVER FISH ON THE MOVE

We explore exactly what it is that prompts fish to be so mobile

YOU’LL often hear about swims on various rivers that will produce barbel during the day, yet the chances of catching from the same swim after dark are virtually nil. But why do fish often move in such a predictable way? The answer is down to a wide range of reasons. Yet, because moving is inherently risky and uses energy, there must be a strong survival advantage to undertake such journeys. From daily feeding movement, to annual spawning migrations, fish populations are constantly on the move, and those movements are often critical for their survival.

Day and night areas

Many fish species will move on a daily basis, often heading upstream at dusk and then back downstream before dawn. These diurnal migrations normally correspond to feeding, with the fish moving upstream to find areas where invertebrate food is more abundant. Often this will be directly below shallow riffles, where the stony riverbed harbours vast quantities of caddis larvae and other invertebrates. The change in flow here also means that any drifting food will be carried straight to the waiting fish. During the day the fish take advantage of areas where they are offered protection from predators, perhaps in overhead cover, or in side streams where bigger fish cannot pass. How far the fish will move on a daily basis really depends on the distance between their safe resting areas used during the day and feeding areas used at night. On a big river, such as the Trent, bream may move several hundred metres each night. On a small stream fish may only need to move a few metres to find the right conditions. These feeding migrations can happen as regular as clockwork for several months, and may be repeated year after year. Incredibly, each individual fish in a shoal can find its way back to the exact spot that it prefers, often to within just a few centimetres. Experiments have shown that fish have a very detailed ‘neural map’ of their environment that enables them to navigate precisely over large areas. This explains why fish always know where every snag is when you hook one!

Reward versus risk

In every population of fish around a third will rarely stray away from their home range, which may be as small as the area covered by their daily feeding migration. As long as there is enough food to be found then this is a good strategy as it minimises risk. Another third will move over a wider area and may be found covering two or three times the home range. These fish may well have access to richer food larders, and will likely have less competition for food, but this is countered by the increased risk. The final third are true wanderers that can travel many kilometres

on big rivers and rarely, if ever, return. These fish face by far the greatest risks, but the rewards may also be greater too. Fish that move further may be at less risk of local events that could wipe them out. Floods and pollution, for example, that could see a local population catastrophically affected, will see their future generations protected if some of the fish have moved out of the area. In many rivers well-known productive swims will often hold plenty of fish, but these might not be the same fish all the time. An overhanging tree may be a great spot for chub, some of which are likely to be living there every day. Some others though, may be just passing through and use the spot temporarily. So a swim can be very productive, but not necessarily because the same shoal of fish lives there all the time.

Boundaries to fish movement

Natural boundaries, such as shallow riffles, may reduce fish movements, especially at times of low river flow, but in a natural lowland river there are rarely any natural boundaries that will completely halt fish movement. Yet, humans have installed a vast number of weirs, sluices and dams on rivers which stop all fish passage. These barriers can often prevent spawning migrations, which are often essential for many fish species. Radio-tracking studies have shown that even small weirs can block almost all upstream movements of powerful fish, such as barbel. Only during floods are such structures passable. Many fish will remain below the barrier before giving up, so the natural spread of the fish is disrupted. By migrating upstream to spawn, fish give their young several advantages. Larval fish are poor swimmers and will often be washed downstream after hatching, particularly during floods. By spawning upriver the youngsters have the best chance of spreading out naturally and finding suitable habitat. Once the adult fish have migrated back downstream the young will face less competition if they stay upstream, often in small shallow streams. Predation may also be less severe here as well. So, while it may consume precious energy, movements are an essential part of the behaviour of river fish, be they short hops to feed, or epic migrations.

“Incredibly, each fish in a shoal can find its way back to the exact spot it prefers”