New discoveries on sea trout behaviour helps inform international conservation efforts

Key points

New tracking technologies have given researchers clearer insight into sea trout smolt survival during their outward migration and adult behaviour whilst at sea.
Using acoustic tags and data storage tags, scientists tracked hundreds of sea trout in three rivers and their estuaries that feed into the English Channel as well as collecting data on sea trout behaviour at sea.
Sea trout smolts survived best when they swam faster, travelled shorter distances, and avoided complex estuaries.
These findings emphasise the importance of minimising barriers and delays in estuaries, to allow fast migration which optimises sea trout survival.
Adult sea trout were shown to spread widely within the English Channel with trout from each river generally using distinct marine areas.
There was a pattern in behaviour at sea from west to east, with adult sea trout from the most westerly river reaching the sea earlier, staying at sea longer, and diving deeper relative to the most easterly river.
These findings highlight how coordinated, nearshore protection across borders is crucial, as sea trout move between international waters. Management must be seasonal and locally tailored, as migration patterns differed by river of origin and region.
Existing marine fishing rules could be strengthen by new insights, since sea trout often use deep waters and vary their diving behaviour by time of day.

Background

Sea trout are a form of brown trout that migrate from freshwater to the sea to grow and feed before returning to rivers to spawn. This migration helps them grow larger and produce more offspring, but it also puts them at risk from natural predators and human activities like fishing, pollution, and barriers in rivers and estuaries.

Sea trout go through several life stages, starting in rivers as juveniles, then transforming into smolts when they are ready to migrate to the sea. This transition is a vulnerable time, as the young fish must adapt to saltwater, avoid predators, and find their way through complex river mouths and estuaries. Research has shown that the survival of these smolts can vary widely depending on local conditions, such as how long the estuary is, how easy it is to navigate, and how fast the fish move through it.

After spawning, adult sea trout, called kelts, return to the sea. Some may make this migration multiple times during their lives, and while most kelts stay near the coast in shallow water, some travel far into open sea. Their behaviour in the marine environment is not well understood, especially in the southern parts of their range, such as the English Channel. This is partly because it’s difficult to track fish over such long and varied journeys.

Understanding aspects of sea trout behaviour and their life cycles is critical for their conservation and understanding the risks the face around bycatch, predation and exposure to environmental pressures. Until recently very little was known about key life stages such as early marine phases, such as when sea trout smolts move through estuaries and migrate through marine waters as well as how sea trout kelts (adults) use the water column whilst at sea.

New technologies like acoustic and data storage tags have made it possible to track both the movement and diving behaviour of sea trout. This research is important because sea trout populations have declined in recent decades. Understanding how, where and when they are most at risk can help scientists and policymakers design better protections, such as improved fisheries rules or habitat restoration projects.

What they did

Recently published studies from the GWCT have provided new information across three aspects of sea trout life cycle and behaviour. Researchers used advanced tracking technologies to follow sea trout during two key stages of their life:

As smolts (juvenile trout) leaving rivers and moving through estuaries to the sea, in three different rivers - two in England (Tamar and Frome) and one in France (Bresle) (Artero 2023).
As adult kelts (post-spawning trout) returning to sea, from the same three rivers as above, which discharge into the English Channel (Artero et al. 2025a & b).

The studies used acoustic tags to track the movements of hundreds of individual smolts as well as data storage tags surgically implanted within over 60 kelts to record data during their marine migration. This allowed scientists to estimate not only where sea trout travelled, but also how they moved through different environments, including how deep they swam, how fast they travelled, and when they died.

The researchers also tested for effects of environmental conditions in estuaries, such as water temperature, salinity, and complexity of the estuary shape, to see how these might influence smolt survival.

What they found

The studies come together to provide a better picture of the behaviour and habitat of the trout at different life stages.

Survival rates for sea trout smolts moving through estuaries varied widely. Sea trout smolt survival ranged between 73% and 97%. More complex estuaries or longer migration routes were linked to lower survival. Faster-moving fish tended to survive better, and this might be because they spent less time at risk from predators or challenging conditions. River-type had a slight impact on total survival, with higher levels in rain-fed rivers compared to aquifer-fed rivers.

Meanwhile, adult sea trout from different rivers showed clear differences in how far they travelled, how long they stayed at sea and how deep they dived. Trout from the most westerly river, the Tamar, entered the sea earlier and stayed longer than the more easterly populations from the Frome and Bresle, which stayed more localised. Sea trout tagged in French rivers were found in UK waters and vice versa, showing that sea trout use wide water bodies and require international conservation efforts. Sea trout that had migrated before showed different behaviour compared to first-time migrants – for instance, they used more direct routes or swam in deeper marine waters.

Around 80% of the time at sea was spent within 45km of the coast, though some individuals ventured into deeper or more distant waters, particularly midway through their migration. Most dives happened during daylight, and sea trout generally stayed in shallow waters. However, some individuals made regular deep dives - incredibly, some sea trout kelts were recorded to dive beyond 40 metres. Dives became deeper and more frequent as spring progressed into summer. Larger fish and those completing full marine migrations tended to dive more deeply and more often than smaller or individuals that died whilst at sea.

What this means

The key lessons from these findings include:

Estuaries are not just passageways; they can be survival bottlenecks. Investments in restoring and protecting estuarine habitats could have a big impact on sea trout survival.
Quick migration means safer migration. Minimising barrier and disturbances that slow sea trout down could improve outcomes.
Marine protection must be nearshore and coordinated across borders. Since most sea trout stay close to shore and cross international boundaries, protection measures must cover inshore water and involve international cooperation.
Diving depth matters for fishing rules. Sea trout are more active at certain depths and times of day. Existing protections that assume fish stay in very shallow water may not go far enough.
We need targeted, seasonal management. Migration timing and behaviour varies by river, so a one-size-fits-all approach won’t work. Local knowledge and seasonal data should guide decision-making.

These studies provide the clearest picture yet of how sea trout use estuaries and coastal waters in the English Channel, however with new insights comes new questions.

For example, the Frome estuary had the lowest smolt survival (73% compared to 96-97% in the other two rivers); this may be due to its more complex structure compared to the other estuaries studied. The Frome estuary is wider with multiple directions of water flow, which could cause the smolts to swim around it further, making them more at risk of predation and, therefore, lowering their survival rate. More detailed monitoring is needed to assess this and understand the differences in smolt survival found between different types of rivers.

Highly managed estuaries and rivers, such as the Bresle, have floodgates that block the main migration route out to sea for fish. This highlights the critical impact that obstacles and dams can have on smolt survival when located along key migration routes and so should be carefully considered in the planning processes and conservation efforts looking to improve habitat for trout.

It is clear from these studies that conservation of sea trout requires cross-border collaboration. Conservation plans cannot stop at national coastlines because sea trout regularly move between different countries’ waters. At the same time, differences in when, how far, and where in the marine environment trout populations migrate, might mean some populations may face more risk than others. Subsequent conservation efforts may need to prioritise protection or recovery efforts of specific rivers or migration routes to combat this.

It is concerning to observe sea trout relying so heavily on nearshore zones in the English Channel, given that these areas are most heavily impacted by human activity including fishing, pollution and coastal development. Data from these recent studies also challenge the assumption that trout remain in shallow water, with findings suggesting that sea trout often used depths greater than 3m during parts of their marine migration. Future research to understand why sea trout use these areas is key to protecting the right habitats.

The result that diving patterns were influenced by individual fish size (measured at tagging, prior to migration) also raises questions about whether vertical behaviour could be linked to fitness, and whether size prior to migration could serve as a future indicator of individual health or survival. Similarly, differences in diving patterns of sea trout from different rivers could be due to environmental factors, inherited traits, or prior experience. This highlights the need for further work to understand how much of this variation is shaped by local conditions, and how different populations might respond to pressures like climate change or marine development.

Finally, the finding that sea trout often use deeper nearshore waters during migration challenges the knowledge informing existing fishery regulations, particularly the idea that sea trout remain at the surface. If sea trout routinely use depths greater than 3m, current protections may not fully safeguard sea trout and could leave them vulnerable to bycatch or illegal capture. These results highlight the importance of incorporating vertical space use into future policy design, and suggest that depth-specific protections may be necessary to effectively conserve sea trout during their time at sea.

Overall, these findings provide crucial information needed to support calls for more integrated and science-based management of sea trout – considering the full range of habitats they use, and the complex pressures they face throughout their lives.

Read the original papers

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