Stroll along a chalk stream in winter, and you might see a wildlife wonder in progress. At this time of year, the 'gin-clear' water is alive with amorous brown trout and salmon. As the females get busy building their nests, the males compete for the chance to be their spawning partners.

In the UK, both of these species typically spawn between October and January, although the exact timing varies by location. By breeding in the colder months, the fish benefit from some of our chalk streams' most unique qualities: their constant flow and water temperature.

Because our chalk streams are fed by an underground aquifer, they maintain a fairly steady flow and emerge at around 11 °C all year round. This makes them relatively stable habitats that are cold enough to hold lots of dissolved oxygen - ideal places for salmonid spawning.

Laying eggs at this time of year also means that their development - which can take several months depending on the water temperature - will be finished by the spring. This gives the juvenile fish plenty of time to feed and grow before the arrival of the following winter.

How redds are made

To create a nest, or ‘redd’, a female first selects a suitable location. This can involve using her fins to test the composition of the stream bed. If a spot seems promising, she turns on her side and flexes her body; this directs the flow downwards, forming a hollow and sweeping away the finer sediment.

Once this hollow has been formed, she lays her eggs and a nearby male swoops in to fertilise them. He will have waited by her side during the redd's creation, defending his position against competitors. Even if he emerges victorious, speed is vital to stop another male from sneaking in at the last second!

With fertilisation complete, the female moves upstream and repeats her flexing gesture; this covers the eggs with a layer of gravel. Though hidden from view and shielded from strong currents, the eggs will still receive a steady supply of oxygen from water flowing through the gaps in their gravelly home.

The redd-making process can take several days, but the result is a dip followed by a slight mound just downstream. A freshly made redd is easier to spot, since the disturbed patch is a lighter colour than the area around it. As sediment and algae reaccumulate, the redd gradually becomes less visible.

When things go wrong

Of course, not every breeding attempt will be successful - this is one reason why females often make multiple redds in a season. Moreover, a female may start forming a redd only to abandon it upon finding a better location. Beyond this natural uncertainty, however, there are factors that make the odds less favourable.

Watercourse barriers like dams, weirs, and sluices can be impassable for some fish, which divides them into smaller populations with limited space for spawning. You might see many redds crowded together - or even atop one another - in the vicinity of such obstacles.

Low water levels after a dry year can leave redds exposed, putting the eggs at risk of frost and dehydration. Flash floods, by contrast, can destroy redds and wash the eggs away. The march of climate change exacerbates these issues, and may leave the streams too warm for spawning.

Excess sediment can fill the gaps between larger pieces of gravel, thereby preventing water from flowing through them. This deprives the eggs of oxygen, which is crucial to their development. Dredging can even remove gravel from a stream bed entirely, leaving only fine sediment behind.

In some cases, this sediment can actually compact the stream bed, which makes it harder for females to carve out a hollow. If they reject the site, their spawning options become more limited; if they make a shallower redd, their eggs are more vulnerable to exposure or storms.

Pictured below (click to expand): At least ten trout redds formed in very close proximity and partially overlapping. This is due to the presence of a significant barrier to fish migration about 100 metres upstream. Some redds have displaced gravel mounds large enough to break the stream's surface.

Cherishing our chalk streams

So how do we aid our salmonids in coping with such issues? To put it simply: by keeping their chalk stream homes in a healthy condition. Tackling bank erosion, removing barriers, conserving water, and restoring habitat diversity will all help these amazing animals to thrive.

The Watercress and Winterbournes scheme is surveying redds and supporting wild fish on the Test and Itchen headwaters. If you’d like to help, or make habitat improvements on your land, we’d love to hear from you. If you're based elsewhere, guidance is available from our friends at Wild Trout Trust.