Sussex Sea Trout: Damned by Beaver Dams?

Did you know that the sea trout is actually the same species as our beloved brown? In this blog, our Conservation Officer Andy Thomas ponders how trout migrating up and down small Sussex streams could be impacted by the return of beavers.

If you flicked through your average fly-fishing comic, you could be forgiven for thinking that the enigmatic sea trout is only found where there are mountains and lively spate river systems, like those typically found in the north and far west of England, Wales and Scotland. Not so: they lurk in many surprising locations, Sussex being one! In fact, many folks who are interested in sea trout are amazed to hear that they can be found in virtually every river that drains the heart of the Weald in South East England.

It’s not widely known, but fully migratory trout – having a lifecycle not too dissimilar to Atlantic salmon – populate almost all rivers that have at least some potential to hold resident wild brown trout. For centuries, we’ve been doing our level best to lock migratory sea trout out of many lowland rivers, via the construction of numerous dams and weirs. You might imagine that this would pretty much exclude them, but even on rivers where fully migratory trout are not thought to exist, they almost always try to gain access. This is even more true in the headwaters of a few key tributaries, where access to suitable spawning and nursery habitat is essential to maintaining viable trout communities – both resident browns and those that like to travel.

The Thames is a great example: when I worked for the Thames Conservancy more than 45 years ago, on the ill-fated Thames Salmon Scheme, we would regularly find wonderful sea trout of all sizes in the salmon trap located on Molesey Weir in Surrey. These fish were as wild as the wind, and we had no real sense of where they were trying to go. But they kept coming back, with no realistic hope of reaching a river that offered even the remotest chance of successful spawning.

We also used to regularly find them at the bottom of the Wey system, when electrofishing in search of salmon broodstock during the autumn. This was despite the fact that the Wey Navigation – constructed nearly 400 years ago – has done a spectacular job of keeping them out, via a string of impassable weirs and locks that run from Godalming to Weybridge. The headwaters of the Wey are thick with resident wild brown trout, and some are obviously genetically predisposed to an epic downstream migration that, while difficult, is still occasionally possible. The return journey, however, presents a greater challenge to a big adult sea trout, fattened up from rich feeding in our coastal waters.

In the tributaries and headwaters that drain the Weald, returning sea trout have better prospects. River systems like the Sussex Arun, Rother, Adur, and Ouse all support spectacular sea trout, despite having their fair share of hideous water-level control structures. What I find truly amazing about these systems is the way that trout populations appear to be sustained by a network of tiny tributary streams, all of which join the main river in the lower half of their various catchments. Many of these have also been dredged and impounded, but some still support high-quality spawning habitat for trout, albeit sometimes in very small areas.

I often muse about the fact that these little Sussex streams have been sustaining giant trout since the last ice age – the largest sea trout, on average, to be found in any UK river. I therefore feel a deep responsibility for clearing my throat when a new potential pressure comes onto the scene. The Wild Trout Trust, for example, has been keen to engage with other conservation bodies who are advocating beavers as the panacea to the problems that impact many of our rivers, particularly regarding dam building as a form of natural flood management.

I understand where they’re coming from, and there is no doubt that in some locations – and certainly some river systems – the eco-services that beavers provide could be very exciting. But, and it’s a big but, I also know that a string of dams (whether man- or beaver-made) will have a cumulative impact that could pose significant problems for sea trout; fish that live in a world where freedom of movement means survival. If they get trapped in a pool above or below a dam, they make easy pickings for predators. This is why the Wild Trout Trust has actively engaged with the conservation community on beaver reintroduction: to ensure that the benefits and issues are being thought through.

When it comes to the dam building activities of beaver on small streams, such as those draining the Sussex Weald, we’re a tad more concerned about sea trout smolts leaving in springtime than we are about adult sea trout returning in autumn. The reasons are a little complicated, but centre around flow conditions and the timing of movement. Returning adult sea trout – particularly Sussex sea trout – are big, powerful fish. They will often hang around estuaries from September to early January, waiting for the all-important lift in flows that typically follows autumn storms. These are the conditions they need to power their way back to their birthplaces, in readiness for spawning.

I have long since given up predicting whether an adult sea trout can make upstream progress past a woody dam, either natural or manmade, because they usually find a way. This sometimes corresponds with a full flood, when they can take advantage of the river ​“breaking out” around the sides of a high impoundment, rather than attempting to pass over the top of it. I won’t pretend that this is ideal, and any delay in upstream migration risks the loss of individual fish and their spawning potential.

But our main concern, especially on these lowland systems, is how juvenile sea trout will migrate downstream past a network of dams and structures. Manmade weirs and dams with at least a little flow over their tops often pose few problems for juvenile smolts, which will passively drop back with the flow. But having looked at standard beaver dams, we can’t quite work out how an average sea trout smolt is going to make it through the diffuse flow that percolates through such structures.

In a bygone era, when beavers were here and doing well, we suspect that springtime flow characteristics would have been very different to today’s average. These days, I’m not sure that a sea trout can count on snowmelt to raise river levels enough to lift them over a well-built beaver dam, or to offer them a route around its sides via floodwater. The spring of 2025 was the fourth driest on record since 1875, and the 10 years preceding it weren’t much better. The reality is that the springtime window for smolt emigration is often comparatively short, and I’m convinced that a mighty tough challenge awaits the sea trout that has found itself on the top end of a tiny Sussex tributary, with one or more beaver dams located below.

So what’s the solution? Well, as always, the devil will be in the detail. I do believe we can have beavers and sea trout on the same systems, as the existing science seems to indicate. In some systems, where the pools created by beaver dams will provide good feeding opportunities, for adult brown trout, we’ll likely see growth rates improve and populations thrive. The larger a hen trout is, the more eggs she can hold, which obviously benefits the population as a whole.

But for me, it would be an environmental crime if we lost the fully migratory component of the trout community. We must seek simple, pragmatic solutions to ensure that sea trout smolts can still move freely; even in a dry April, when a wall of mud and woven sticks stands in the way of any obvious downstream escape route. The Wild Trout Trust has been working with the Beaver Trust, and other environmental charities, to create the Beaver Dam Assessment Methodology for Salmonids (BDAMS) – a tool that might help people to assess how passable these dams are at key times of the year. It’s crucial that we look out for our sea trout, as they go on the adventure of a lifetime.