Trapping and Tracking Tweed Trout

Trout are one of the Tweed's most important fish species. Native to the river, they not only play a big role in the river's ecosystem, but the Tweed is increasingly becoming renowned as a top location for Trout fishing, attracting anglers from all over the world looking for a chance to catch large Tweed Trout.

Monitoring the health of the Tweed's Trout populations and understanding their population dynamics is therefore a big part of the Tweed Foundation's work.

There are two types of trout at home in the Tweed: Brown trout and Sea trout. Both belong to the same species (Salmo trutta) but they differ in their behaviour, with Brown trout generally staying in the river while Sea trout venture out into the North Sea.

Despite showing different behavioural traits, both Brown trout and Sea trout have very similar life cycles.

The Trout lifecycle starts off in late autumn when adult Trout make their way upstream to spawn. The female trout lays her eggs in a nest called a redd before the male trout fertilizes the eggs by releasing milt. The eggs are then covered over by gravel and left to develop, protected in the redd.

Trout start off life as tiny eggs. At only about 3mm in diameter, the eggs develop, becoming known as eyed eggs before hatching out in early Spring. The speed at which trout eggs develop is dependent on several factors including light and oxygen levels, but largely on water temperatures. This is why the number of days it takes for an egg to hatch is referred to as "degree days".

The eggs hatch into tiny alevin. Alevin emerge attached to a small yolk sac from which they feed. As they feed, their yolk sacs gradually disappear and the trout grow in size, remaining in the safety of the redd until they are big enough to start swimming freely.

Once their yolk sacs have disappeared and they are large enough to start swimming freely in the water column, trout become known as fry. The fry leave the safety of the redd, swim up to the surface of the water to fill their swim bladders with air and then find a hiding spot in amongst the stones in the gravel bed. Here the fry will feed on tiny invertebrates.

After spending up to a year in the river, trout become known as parr. By this stage they have grown to approximately 10cm in length. This makes them much more appealing to predators, so trout parr will often hide under cover in slightly deeper water for protection. This often results in trout parr having to drop downstream into slightly larger channels where they can find suitable habitat with plenty of cover.

It is at this stage that trout decide whether they stay in the river or head out to sea. The trout that stay in the river (Brown trout) find a good spot in the river in which to feed and continue to grow, while the trout that are to head out to sea (Sea trout) first turn into smolts.

Having found a good spot in the river with lots of cover and rich feeding, Brown trout parr continue to grow until they mature into adults. Once sexually mature, Brown trout are able to reproduce, migrating back upstream in order to spawn come the autumn each year before dropping back down to their feeding grounds.

Once they are fully smolted, Sea trout appear silver in colour and develop dark fins. This is in preparation for them leaving the river and heading out to sea. Between April and June each year, sea trout smolts will gather before migrating on mass down the river and out into the North Sea where their feeding grounds are.

Once out at sea, the Sea trout smolts stay in the North Sea, swimming down the East coast and often being found off the coast of Denmark. Going out to sea allows Sea trout to grow and therefore mature faster. Some Sea trout will therefore return to the river having only spent one winter out at sea in order to spawn. However, many will spend longer out at sea before returning to the river. The Tweed is well known for its big Sea trout.

Having reached maturity, both Brown trout and Sea trout can spawn multiple times throughout their life.

With trout being such an important fish species in the Tweed (both for the river's ecological health and for angling), it is crucial to monitor their populations in order to ensure their numbers stay healthy.

The Tweed Foundation use a number of methods to monitor Trout throughout the catchment. One of these methods is running fish traps on trout spawning burns.

Trout traps work by blocking off both upstream and downstream passage for the fish in spawning burns. Instead, the majority of the flow is channeled through a meshed box trap, luring the fish into the trap where they sit in a sheltered, slack bit of water. Each trap is then checked daily throughout the duration of the spawning run, ensuring that any caught fish are not in the trap for too long before being processed and released.

The Tweed Foundation started monitoring trout through trapping in the 1970's. Since then, we have run traps on 11 trout spawning burns spread throughout the catchment.

Most of these traps have been temporary structures, with all bar one having been run for nine years or less before being removed. Our longest running trap is the Stanhope trout trap, located on the Stanhope Burn in Tweedsmuir. Having been constructed in 2000, this has provided us with over 20 years of valuable data on Tweed trout.

Trapping trout allows us to monitor how many fish are spawning in each burn, when their run times are, as well as what size the trout tend to be. It also gives us the opportunity to look at whether trout populations are predominantly made up of Brown trout or Sea trout and whether they are predominantly male or female. With this information, we have gained a much better understanding of how population dynamics of trout differ between burns in different parts of the catchment.

Migratory fish are often thought of as being those that journey between river and sea, as is the case with the Tweed's Salmon, Sea Trout and European Eels. However, despite spending their whole lives in the river, Brown trout are also migratory.

Adult Brown trout spend much of the year feeding in the larger channels of the Tweed system. However, throughout October and November, Brown trout will leave their feeding grounds and migrate upstream to spawn. Which spawning grounds trout come from and how far they travel within the Tweed catchment are important management questions for the trout fishery to consider. To help in answering these questions, the Tweed Foundation have been tagging and tracking adult Brown trout in order to monitor their movements throughout the Tweed catchment.

Since 2018, the spawning migrations of 47 Brown trout (ranging between 8 and 24 inches in length) have been monitored. The trout are tagged with small, acoustic tags that send out regular sonic ‘pings’ containing a unique identification code. Tracking receivers, which record the identification codes of any tagged trout that swims past, are then strategically located throughout the catchment, allowing us to record the movements of each fish during their spawning migrations.

There are two components to our Trout Acoustic Tracking project: tracking post spawning migrations in trout as they leave a burn in the Upper Tweed, and tracking spawning migrations of trout caught by anglers within particular sections of the River Tweed.

By establishing a link between the spawning grounds and feeding grounds of trout, we can work with angling associations and target our efforts in improving the nursery grounds for the trout caught in their waters.

It's not just trapping and tracking that the Tweed Foundation use to monitor trout. Catch log books and spot pattern recognition software are both important tools that we use to monitor the numbers of trout throughout the catchment.

The reputation of the River Tweed as a fishery for wild Brown trout is growing year on year. With help from anglers and volunteers, the Tweed Foundation gathers the relevant information required to advise angling clubs and associations on the best management of their association waters to keep the Tweed's Brown trout populations healthy.

This encompasses a variety of different monitoring methods. With each method providing valuable data, it can all then be brought together to give us a much better understanding of Tweed trout and their activities in the river.