You'll often hear about swims on barbel rivers that will produce fish during the day, yet the chances of catching them from the same swim after dark are virtually nil. But why do fish often move in such a predictable way?
Moving is inherently risky and uses energy, so there must be a strong survival advantage to undertaking such journeys. From daily feeding movement, to annual spawning migrations, fish populations are constantly on the move, and those movements are often essential to their survival.
Day and night areas
Many fish species will move on a daily basis, often heading upstream at dusk and back down before dawn. These ‘diurnal’ migrations normally correspond to feeding, with the fish moving upstream to find areas where invertebrate food is most abundant.
Often this will be directly below shallow riffles, where the stony riverbed harbours vast numbers of caddis larvae and other invertebrates. During the day the fish will take advantage of areas offering protection from predators, perhaps beneath overhead cover, or in side streams where bigger fish cannot pass.
How far the fish will move from day to day really depends on the distance between their safe resting areas used during the day and feeding areas used at night. On a big river like 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 food.
Incredibly, each fish in a shoal can find its way back to the exact spot that it prefers, often to within 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.
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Reward vs risk
In every population of fish, around a third will rarely stray away from their home range. 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 covering two or three times the home range. These fish may 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 miles on big rivers and rarely, if ever, return. These fish face by far the greatest risks, but the rewards may be greater, too. Fish that move further will be at lesser risk from local events, such as flooding or pollution incidents, that could otherwise wipe them out. While the local population could be catastrophically affected, the survival of the true wanderers will in turn ensure that future generations are protected.
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 ofwhich are likely to be living there every day. Others, though, may be just passing through. So a swim can be productive, but not necessarily because the same shoal of fish always lives there.
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Falling Temperatures
As a general rule,in cold weather fish tend to become more lethargic and reluctant to move. As cold-blooded creatures, their metabolism slows, so they’re less inclined to feed as they need less energy to move. At such times, they’ll often tuck themselves away in areas of slower water such as in slacks, behind cover that deflects the river’s flow, or on the edges of creases.
Many species will often also become more nocturnal during cold snaps in a bid to avoid warm-blooded
predators such as herons, kingfishers and otters. However, after being harder to catch for the first few days, once they’ve adapted to the change in temperatures, their feeding and movements start to return to normal.
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Boundaries to fish
Natural boundaries, such as shallow riffles, may reduce fish movement in times of low river flow, but in a natural lowland river there are rarely any natural boundaries that will halt fish movement. Yet, humans have installed weirs, sluices and dams on rivers, which stop all fish passage.
These barriers can often prevent spawning migrations, and 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 their population 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 movements may consume precious energy and carry inherent risks, they are an essential part of the behaviour of river fish, whether these are short hops to feed, or epic migrations.
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