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All Aboard The Adur! – the link between rivers and migratory species

17 March 2025

A Sussex-Born Blueprint for Nationwide Recovery

What could lead to the belief that swallows house the souls of the dead? or that eels are born from mud?

The answer is migration!

Every year hundreds of thousands of species weave unique patterns of migration across the globe. Up to 4,000 different species of birds are regular migrants, with nearly 17 million birds passing through the UK during their migration every year. Not to mention the plethora of fish and invertebrates which face trans-continental travels every year too. Each individual creature, from birds to butterflies to bass, navigate treacherous journeys in pursuit of abundant seasonal resources and habitat.

Mythical Migration

Migration patterns have baffled humans for hundreds of years, leading to diverse regional folklore and beliefs about magical disappearing birds, barnacle-born geese and spirit-housing butterflies. Only recently have we begun to unravel the triggers, pathways, and capabilities of these trans-continental voyagers.

Forget about a satnav – these travellers rely on all manner of orienteering, including genetic disposition, environmental cues, and importantly, physical landmarks like rivers. Birds especially rely on rivers as superhighways to guide their passage whilst providing abundant food, refuge, and habitat. Whilst it goes without saying that rivers act as watery motorways for our fishy friends. So it is no surprise the River Adur acts as a launch-pad, path, and destination for a multitude of migratory species.

Left:White Storks introduced to the Knepp Estate make use of the Adur’s restored wetlands to feed, before migrating as far as North Africa.

Fishy Freeways

Travelling Trout

Migratory fish really pull out all the stops to get to their destination. In many cases, the driver for fish migration is the pursuit of suitable habitat and resources at different life stages. Trout are a classic example of a migratory species which frequent the River Adur – but did you know that only a proportion of a brown trout population will migrate to sea? Trout are an indecisive bunch. Some will migrate within the confines of a river, whereas some are far happier sticking to the familiar, and won’t migrate at all. Interestingly the percentage of the population that migrates is thought to be influenced by a host of environmental factors including food availability and stress.

Brown Trout – by Jack Perks.

Brown trout hatch in tributary streams and may migrate to sea in the spring, in pursuit of plentiful food. It makes sense then that most sea trout are female, since piling on the pounds is of great importance in producing lots of eggs. Once plump and primed, sea trout will head back to their original tributary and find a nice patch of slow-moving water to suit all their spawning needs.

Mystery in Motion

No species encapsulates the wonder of migration quite like the elusive European eels. If trout are your average holiday-goers in search of a seaside resort, eels are the mysterious backpackers – off in their adolescence for covert adventures through mysterious destinations. Eels hatch in the Sargasso Sea and will miraculously float thousands of miles to Europe on ocean currents. In fact, the complete absence of eel spawn in UK rivers led to the old belief that eels are spontaneously born from mud! As they find rivers like the Adur, they swim upstream, snacking on insects, crustaceans, and fish as they go. Often, they’ll try their luck as an expat, and become residents in lakes, streams, or brooks for anywhere between 6 to 30 years!

European Eel – by Jack Perks

The call of the deep returns eventually. In their twilight years, the now-aptly named ‘silver eels’ will begin the journey home. Travelling through the channel, eventually they’ll pursue the darkness beyond the great Atlantic Shelf. Where silver eels go is unknown, but each year a cloud of eel spawn will surface once more in the Sargasso Sea. The journey begins again. Unsurprisingly, these critically endangered species are prevalent characters in folklore. Once abundant in our Rivers, some Sussex fishermen believed eels harboured the souls of drowned sailors, returning home to their native rivers.

River Routes for Feathered Friends

Last Summer, the River Adur was met with much excitement when not one, but two, ospreys were spotted on the river. These magnificent raptors are thought to have earned their name from a derision of the Latin word – ossifragus, which means ‘bone-breaker’. It is no surprise then, that Ospreys are expert hunters, diving up to one metre below the waters surface to catch fish. What made the osprey sightings even more special is that just one hundred years ago, these birds were deemed extinct in England.

Left: An Osprey above the Adur. Jo Whiting.

Ospreys return to the UK from late March to build their nests and breed. Usually, they return to the same breeding and fishing locations each year. However, when migration calls, they may make some pit-stops along the way. Clearly the Adur was a suitable stop-over for these birds, providing much needed fuel for their journey to West Africa. Satellite tracking shows ospreys flying up to 430km in one day, which makes stocking up on tasty fish essential to fuel their migration. Young or non-breeding ospreys are much more likely to be tempted by mid-journey snacks, exploring different rivers to build up energy reserves on their way.

Ospreys may have caused a stir, but every year migratory passerines, gulls, terns, waterfowl, and waders, will all flock to the Adur in search of refuge, food, and breeding grounds. Little ringed plovers and common terns arrive in the spring returning from the African winter sun. Whereas redshank, dunlin, grey plover, and lapwing arrive in the winter, migrating south from chilly Scandinavia, Iceland, and Northern Europe.

Tiny Travellers

Often overlooked are the invertebrate migrants, which play a vital role in pollinating wildflowers along riverbanks and providing food for birds and bats. Butterflies like the Painted Lady and the Red Admiral, dragonflies, damselflies, moths, and even hoverflies escape harsh weather and brave thousands of miles in search of seasonal resources and reproductive success along the Adur.

Right: Red Admiral – Dawne Davis.

Seal Stopovers

Common seals are frequently spotted swimming, or basking on the banks of the River Adur. Whilst they aren’t technically a migratory species, their frequent visits earn them a special mention. Some populations of common seals will move up to a few hundred kilometres between feeding and breeding grounds, whilst young seals may disperse long distances away from their population in search of pastures new. Those spotted in the Adur are thought to be residents of the south-coast, but occasionally stragglers from other populations will pop-up, distracted by fish populations up the river. One female seal was traced all the way back to Belgium!

A Common Seal pops up on the Adur to say hello – John Pointing.

A Perilous Journey….

Searching for essential resources comes at a high price. The cost-benefit balance of migration teeters on a ‘knife-edge’, with even the tiniest changes in conditions capable of tipping the scales for, or against, migration.

Migration is already high-risk, and each year the journey gets harder. Climate change is pushing migratory destinations for butterflies and dragonflies further north. Whilst rising temperatures disrupt the genetic cues that trigger migration, causing birds to begin their journeys prematurely. Biodiversity loss and human disturbances to nesting and breeding grounds add even more pressure. These threats reduce reproductive success and put entire populations at risk.

Enhancing connectivity between rivers, ponds, wetlands and ditches, via restoring floodplains and removing manufactured barriers like weirs, is essential to support the movement of fish between ecosystems. Whilst protecting seasonal breeding grounds, reducing insecticide use, and minimising human disturbance can help make the River a safe refuge for our beloved bird visitors.

Keep on Moving…

Think of migration like a pulse of energy rippling across continents, shaping the world around us and connecting ecosystems in ways we rarely notice. It fuels habitats, drives food chains, and spreads genetic diversity, nutrients, and populations, across space and time. Migratory species are more than just travelers. They are living indicators of environmental change, revealing shifts in temperature, food availability, and habitat quality. Along the River Adur, these seasonal arrivals depend on the river’s water quality, mudflats, and riverbanks for food, shelter, and safe passage. In return, they help sustain the Adur’s delicate ecosystem, pollinating wildflowers, transferring nutrients, and enriching the web of life that makes this river thrive.