Nick Upton / European eel (Anguilla anguilla) elver migrating upstream, using an eel pass on the side of a weir, Somerset Levels, England, UK, July.

Dave Bevan / Male Blackbird (Turdus merula) catching European eel (Anguilla anguilla) elvers as they migrate up a waterfall, Ceredigion, Wales, UK, July.

Dave Bevan / Male Blackbird (Turdus merula) catching European eel (Anguilla anguilla) elvers as they migrate up a waterfall, Ceredigion, Wales, UK, July.

Dave Bevan / European eel (Anguilla anguilla) elvers migrating up a wet rock next to a waterfall, Ceredigion, Wales, UK, July.

SCOTLAND: The Big Picture / European eel (Anguilla anguilla) migrating upstream bypassing a waterfall by leaving the water completely. Cairngorms National Park, Scotland, UK, July.

Dave Bevan / European eel (Anguilla anguilla) elvers migrating up a rock face next to a stream, Ceredigion, Wales, UK, July.

Dave Bevan / Male Blackbird (Turdus merula) catching European eel (Anguilla anguilla) elvers as they migrate up a waterfall, Ceredigion, Wales, UK, July.

Dave Bevan / European eel (Anguilla anguilla) elver trying to migrate up a rock face, before slipping back, Ceredigion, Wales, UK, July.

Dave Bevan / European eel (Anguilla anguilla) elvers migrating up a wet rock next to a waterfall, Ceredigion, Wales, UK, July.

Remi Masson / European eel (Anguilla anguilla) in the River Rhone, Alps, France. Critically endangered species.

Jean E. Roche / Fishermen carrying net of live Eels (Anguilla anguilla) Camargue, France, November.

Jean E. Roche / Fishermen carrying net of live Eels (Anguilla anguilla) Camargue, France, November.

Jean E. Roche / European eel (Anguilla anguilla) portrait, Camargue, France.

Jean E. Roche / Trapping European eels (Anguila anguilla) in the Camargue, for scientific research by the biological research station of Tour du Vala.

Jean E. Roche / European eels (Anguilla anguilla) released after scientific research, La Gacholle, Camargue, France. April.

Jean E. Roche / Scientists releasing European eel (Anguilla anguilla) after they were caught during research, La Gacholle, Camargue, France. April.

Jean E. Roche / Scientists measuring European eels (Anguilla anguilla) during research, La Gacholle. Camargue, France, April.

Loic Poidevin / Grey heron (Ardea cinerea) feeding on European eel (Anguilla anguilla), Marais Breton, Vendée, France, January.

Loic Poidevin / Grey heron (Ardea cinerea) feeding on European eel (Anguilla anguilla), Marais Breton, Vendée, France, January.

Dave Bevan / Little grebe (Tachybaptus ruficollis) diving and catching a European eel (Anguilla anguilla) elver, Pembrokeshire, Wales, UK. October.

Dave Bevan / Grey heron (Ardea cinerea) trying to swallow a European eel (Anguilla anguilla), Pembrokeshire, Wales, UK. September.

Dave Bevan / Grey heron (Ardea cinerea) trying to swallow a European eel (Anguilla anguilla), Pembrokeshire, Wales, UK. July.

Nick Upton / European eel (Anguilla anguilla) elvers swimming in a glass holding tank, caught during their annual migration upstream for a reintroduction project, Gloucester, England, UK, March. Captive.

Nick Upton / European eel (Anguilla anguilla) elvers swimming in a glass holding tank, caught during their annual migration upstream for a reintroduction project, Gloucester, England, UK, March. Captive.

Nick Upton / European eel (Anguilla anguilla) elvers swimming in a glass holding tank, caught during their annual migration upstream for a reintroduction project, Gloucester, England, UK, March. Captive.

Roger Powell / Great cormorant (Phalacrocorax carbo) catching and attempting to eat a European eel (Anguilla anguilla) on the side of a tidal pool. Druidge Pools Nature Reserve, Northumberland, England, UK. August....

Nick Upton / European eel (Anguilla anguilla) elvers being released from an inflatable boat by Elliot Gough during a reintroduction project run by Severn and Wye Smokery and UK Glass Eels, Lake Llangorse, Breacon...

Nick Upton / European eel (Anguilla anguilla) elvers being released from an inflatable boat by David Blennerhassett during a reintroduction project run by Severn and Wye Smokery and UK Glass Eels, Lake Llangorse,...

Nick Upton / European eel (Anguilla anguilla) elvers being packaged at UK Glass Eels for transport to Wales for a reintroduction project, Gloucester, UK, October 2016

Nick Upton / European eel (Anguilla anguilla) elvers being packaged at UK Glass Eels for transport to Wales for a reintroduction project, Gloucester, UK, October 2016

Nick Upton / European eel (Anguilla anguilla) elvers transported in insulated boxes on an inflatable boat during a reintroduction project run by Severn and Wye Smokery and UK Glass Eels, Lake Llangorse, Breacon Be...

Nick Upton / European eel (Anguilla anguilla) elvers transported in insulated boxes on an inflatable boat during a reintroduction project run by Severn and Wye Smokery and UK Glass Eels, Lake Llangorse, Breacon Be...

Nick Upton / European eel (Anguilla anguilla) elver, part of a shipment prepared by UK Glass Eels for transport to Wales for a reintroduction project, Gloucester, UK, October 2016

Nick Upton / European eel (Anguilla anguilla) elver, part of a shipment prepared by UK Glass Eels for transport to Wales for a reintroduction project, Gloucester, UK, October 2016

Nick Upton / Eel pass with clumps of nylon bristles to allow migration of young European eel (Anguilla anguilla) elvers, or glass eels, up the side of a weir on a drainage channel, Somerset Levels, UK, April.

Nick Upton / Anna Carey fishing under license with a legally sized dip net for young European eel (Anguilla anguilla) elvers, or glass eels, on a rising tide on the River Parrett at night, Somerset, UK, March 201...

Nick Upton / Anna Carey fishing under license, casting a legally sized dip net for young European eel (Anguilla anguilla) elvers, or glass eels, on a rising tide on the River Parrett at dusk, UK, March 2016

Nick Upton / Overflow weir with adjustable gates and an Eel pass to allow migration of young European eel (Anguilla anguilla) elvers, or glass eels up a drainage channel on the Somerset Levels, UK, April 2016

Nick Upton / Young European eel (Anguilla anguilla) elvers, or glass eels, caught during their annual migration up rivers from the Bristol channel, swimming in a large holding tank at UK Glass Eels, which supplies...

Nick Upton / Young European eel (Anguilla anguilla) elvers, or glass eels, caught during their annual migration up rivers from the Bristol channel, swimming in a large holding tank at UK Glass Eels, which supplies...

Nick Upton / Young European eel (Anguilla anguilla) elvers, or glass eels, caught during their annual migration up rivers from the Bristol channel, swimming in a large holding tank at UK Glass Eels, which supplies...

Nick Upton / Glass eels, young European eel (Anguilla anguilla) elvers being packed in insulated boxes with ice and water at UK Glass Eels for transport to Germany for reintroduction projects, Gloucester, UK, Marc...

Nick Upton / Wild caught Glass eels / young European eel (Anguilla anguilla) elvers, part of a large shipment prepared by UK Glass Eels for transport to Germany for reintroduction projects, Gloucester, UK, March 2...

Nick Upton / Wild caught Glass eels / young European eel (Anguilla anguilla) elvers, part of a large shipment prepared by UK Glass Eels for transport to Germany for reintroduction projects, Gloucester, UK, March 2...

Nick Upton / Glass eels / young European eel (Anguilla anguilla) elvers, packaged in insulated boxes at UK Glass Eels being loaded into a van for onward transport to Germany by air for reintroduction projects, Glo...

Nick Upton / Glass eels, young European eel (Anguilla anguilla) elvers, packaged in insulated boxes at UK Glass Eels, being loaded into a light aircraft for transport to Germany for reintroduction projects, Glouce...

Nick Upton / Young European eel (Anguilla anguilla) elvers, or glass eels retained in a collecting bucket with a fine mesh base during a fishing session on the River Parrett at night, caught under licence Somerset...

Nick Upton / Young European eel (Anguilla anguilla) elvers, or glass eels, caught under licence in a legally sized dip net on a rising tide on the River Parrett at night, Somerset, UK, March 2016

Nick Upton / Geoff Way of the Environment Agency counting European eel (Anguilla anguilla) elvers using an eel pass beside a weir on a drainage channel on the Somerset Levels by reviewing remote infra red video fo...

Nick Upton / Geoff Way of the Environment Agency counting European eel (Anguilla anguilla) elvers using an eel pass beside a weir on a drainage channel on the Somerset Levels by reviewing remote infra red video fo...

Nick Upton / European eel (Anguilla anguilla) elver wriggling up an Eel pass between clumps of nylon bristles on the side of a weir on a drainage channel as it migrates upstream at night, Somerset Levels, UK, Apri...

Nick Upton / European eel (Anguilla anguilla) elver wriggling up an Eel pass between clumps of nylon bristles on the side of a weir on a drainage channel as it migrates upstream at night, Somerset Levels, UK, Jul...

Nick Upton / Tray of European eel (Anguilla anguilla) elvers at UK Glass Eels ready to be packed for transport to Wales for a reintroduction project, Gloucester, UK, October 2016

Nick Upton / European eel (Anguilla anguilla) elvers wriggling up an Eel pass between clumps of nylon bristles on the side of a weir on a drainage channel as they migrate upstream at night, Somerset Levels, UK, Ju...

Nick Upton / European eel (Anguilla anguilla) elvers swimming in a large holding tank at UK Glass Eels, ahead of being reintroduced to a lake in Wales, Gloucester, UK, October 2016

Nick Upton / European eel (Anguilla anguilla) elvers swimming in a large holding tank at UK Glass Eels, ahead of being reintroduced to a lake in Wales, Gloucester, UK, October 2016

Nick Upton / European eel (Anguilla anguilla) elvers being collected in a sieve for packaging and transport to Wales for a reintroduction project as a holding tank is drained down at UK Glass Eels, Gloucester, UK,...

Nick Upton / Split level view of European eel (Anguilla anguilla) elvers being released from an insulated transport box during a reintroduction project run by Severn and Wye Smokery and UK Glass Eels, Lake Llangor...

Nick Upton / Richard Cook releasing European eel (Anguilla anguilla) elvers during a reintroduction project run by Severn and Wye Smokery and UK Glass Eels, Lake Llangorse, Breacon Beacons, Wales, UK, October 2016

Nick Upton / Elliot Gough releasing European eel (Anguilla anguilla) elvers during a reintroduction project run by Severn and Wye Smokery and UK Glass Eels, Lake Llangorse Breacon Beacons, Wales, UK, October 2016

Nick Upton / Richard Cook releasing European eel (Anguilla anguilla) elvers during a reintroduction project run by Severn and Wye Smokery and UK Glass Eels, Lake Llangorse Breacon Beacons, Wales, UK, October 2016

Nick Upton / European eel (Anguilla anguilla) elvers being released from an insulated box into a Welsh lake during a reintroduction project run by Severn and Wye Smokery and UK Glass Eels, Lake Llangorse, Breacon...

Nick Upton / Split level view of European eel (Anguilla anguilla) elvers being released from an insulated transport box during a reintroduction project run by Severn and Wye Smokery and UK Glass Eels, Lake Llangor...

Nick Upton / Split level view of European eel (Anguilla anguilla) elvers being released from an insulated transport box during a reintroduction project run by Severn and Wye Smokery and UK Glass Eels, Lake Llangor...

Nick Upton / Split level view of European eel (Anguilla anguilla) elvers being released from an insulated transport box during a reintroduction project run by Severn and Wye Smokery and UK Glass Eels, Lake Llangor...

Nick Upton / European eel (Anguilla anguilla) elvers being released from an inflatable boat by Nigel Wateridge during a reintroduction project run by Severn and Wye Smokery and UK Glass Eels, Lake Llangorse, Breac...

Norbert Wu / European Eel (Anguilla anguilla) live very deep in the Sargasso Sea and return to rivers as adults

Sebastia Torrens/ BIA / Grey Heron (Ardea cinerea) predating on European Eel (Anguilla anguilla) prey, Mallorca, Spain

Sebastia Torrens/ BIA / Grey Heron (Ardea cinerea) with European Eel (Anguilla anguilla) prey, Majorca, Spain

Neil Aldridge / A European eel (Anguilla Anguilla) swims amongst reeds in a wetland in Gloucestershire, England. The European eel is critically endangered and one of the most trafficked species on the planet.

Nick Upton / European eel (Anguilla anguilla) migratory adult 'silver eel'trapped on a rock ledge on a sandy beach on a very low tide, Dunraven Bay, Glamorgan, Wales UK, September. Protected by their mucus...

Nick Upton / European eel (Anguilla anguilla) migratory adult 'silver eel'trapped in and partially emerged from a tide pool on a sandy beach on a very low tide, Dunraven Bay, Glamorgan, Wales UK, September...

Nick Upton / European eel (Anguilla anguilla) migratory adult 'silver eel'trapped in a tide pool on a sandy beach on a very low tide, Dunraven Bay, Glamorgan, Wales UK, September.

David Perpinan / European eel (Anguilla anguilla) swimming in front of the camera at night, Barcelona, Spain, May.