Low water flow also affects fish populations in the Aura River

The low water level at Halistenkoski has this summer provided an interesting opportunity to observe the fish in the river. On a sunny day, plenty of small fish can be seen in the shallow water, and large schools of fish can even be spotted at the fish ladder.

According to Johanna Piipanoja, a fisheries adviser at Länsi-Suomen Kalatalouskeskus ry, there currently appears to be plenty of bleak at Halistenkoski. The bleak come to the rapids in search of food. The flowing water contains plenty of insects and plankton, which the bleak gather to feed on. The still-warm water of the Aura River also keeps the bleak active.

However, low water flow does not affect all fish species in the same way.

Low water flow changes fish habitats

The effects depend particularly on how low the flow becomes, how long the period of low water lasts, and what the structure of the river is like. In small rivers and streams, the effects can be significant even during a single dry summer.

In a regulated river such as the Aura River, the effects of low water flow are somewhat different. Dams alter the natural flow and variation of the water. Above a dam, the flow slows down, the water stays in the area for longer, and the water warms more easily during the summer. In deeper and more nutrient-rich areas, oxygen levels may also decrease.

These changes affect fish species differently. Migratory fish are more sensitive to high water temperatures and low oxygen levels than, for example, perch, pike and cyprinids. During periods of low water flow, fish therefore seek out places where there is enough water, food and oxygen.

River flow is created across the entire catchment area

For fish, it is not only what happens in the river channel itself that matters. The entire catchment area affects how much water reaches the river and how quickly it does so.

A natural catchment area retains water and releases it more slowly, helping to keep the river’s flow more even. In a heavily ditched catchment area, water runs off more quickly. During rainfall, this can result in rapidly increasing flows, while during longer dry periods there is less water available to the river.

Drainage can therefore intensify variations in river flow: large amounts of water arrive quickly when there is plenty of it, but during dry periods the catchment releases less water.

This also affects fish populations.

In spring and during heavy rainfall

• water flows rapidly into the river channel
• the flow rises quickly
• suspended sediment loads can increase
• fine sediment can be carried onto spawning gravel
• strong flows can wash away eggs and young fish or alter their habitats.

In summer

• there is less water stored in the catchment area
• groundwater influence may decrease
• flows in streams and small rivers decline
• the water becomes warmer
• the habitat available to fish becomes smaller.

In autumn

• low water flow can make it more difficult for migratory fish to move upstream
• the timing of spawning migrations can be disrupted
• small tributaries can become impassable.

It is worth retaining water in the catchment area

Retaining water in the catchment area can help improve both water quality and fish habitats. When water is stored in the landscape and released more slowly, the river’s flow can remain more stable even during dry periods.

All measures that retain water in the catchment area are therefore beneficial both for water quality and fish populations. Migratory fish in particular benefit from these measures.

When we keep water in the landscape for longer, we also keep our rivers alive.

Johanna Piipanoja, fisheries adviser at Länsi-Suomen Kalatalouskeskus ry.