From late December onwards we start to see female brown trout ‘cutting redds’ in the Kennet. If you are walking along the river you might be lucky enough to see it for yourself. The female (hen) fish lies on her side, vigorously flapping her tail, to scoop out a hollow in the gravelly river bed. Once she has created an oval shaped depression she lays hundreds of small, coral coloured eggs. The male (cock) trout is standing by to cover the eggs with sperm (milt) to fertilize them, before the hen covers them up with gravel.
If you look closely you can see clean gravel mounds on the river bed. A pair of polarized glasses can help you see through the water reflection more clearly. The size of the gravel mound varies depending on the size of the fish, anything from just 50cm to well over 1m is possible. Redds will remain quite visible for a few weeks after they are created.
The site that the hen trout chooses is very important. The gravel needs to be loose enough for the hen to be able to cut into it and the river bed needs to be clear of mud and silt. This ensures that once laid the eggs have a constant flow of oxygenated water passing over them. Too much silt and the eggs will suffocate before they get a chance to hatch.
The time it takes for eggs to hatch varies according to the river’s temperature, if it’s colder they take longer, and if it’s warmer they hatch faster. The average time to hatch is around two months, so our wild Kennet trout should hatch in the spring when there is more food around to support them. However, the timing will vary from river to river.
When trout hatch, they emerge as ‘alevins’. These tadpole-like creatures carry a ‘sac’ on their stomachs which provides all the food they need for the first few weeks of their lives. Equipped with this food source the alevins can stay safely hidden in the redd for a few more weeks before they have to venture out to find food. After three weeks the sac is absorbed into the baby trout’s body and they start to look more like small fish. These fry need to be able to swim to shallow margins to find a safe place to hide from predators. Reedy edges or scrubby willow margins are ideal.
The success of the wild brown trout depends on the river providing clean flowing water as well as a good habitat, which includes clean gravel, not choked with sediment and established marginal vegetation like reeds and flag iris.
On stretches of the Kennet, run as trout fisheries, extra fish are added to make sure there are enough trout for fishermen to catch. These ‘stocked fish’ are an important part of the fishing economy. Since 2015 only “triploid” fish have been allowed to be added to the wild stock. Making a fish triploid is an application of the same principle as that which creates a seedless water melon, and a triploid trout will be sterile. Changing an egg from diploid to triploid is done by shocking the trout eggs as they are developing.
Stocked trout are made sterile to avoid inter-breeding between farmed and wild brown trout, which might result in offspring less well adapted to survival in the wild, which would have serious implications for the long term future of wild brown trout.
ARK volunteer ‘redd spotters’ have been monitoring wild trout nests for several years to build up a picture of the health of the wild trout population. If you’d like to be part of this team please let us know and we can arrange training and a reach of river for you to adopt.
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