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

Jack Dykinga / Moss-covered branches of Big leaf maple (Acer macrophyllum) trees with the Elwah River in the background, Olympic National Park, Washington, USA. With the removal of dams, salmon can now migrate up ri...

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...

Alex Mustard / Split level photo of group of Sockeye salmon (Oncorhynchus nerka) fighting their way upstream as they migrate back to the river of their birth to spawn, trees showing autumnal colours. Adams River, Br...

Nick Upton / Gorse spider mites (Tetranychus lintearius) emerging in large numbers from silk tent shrouding a Gorse bush (Ulex europaeus) ready to migrate to other bushes. Tiny black ladybird (Stethorus punctillum...

Nick Upton / Gorse spider mites (Tetranychus lintearius) massing outside silk tent covering Gorse bush (Ulex europaeus) ready to migrate to other bushes. These mites are spreading in the UK and are used for biolog...

Nick Upton / Gorse spider mites (Tetranychus lintearius) massing outside their silken tent shrouding a Gorse bush (Ulex europaeus) ready to migrate to other bushes. These mites are spreading in the UK and are used...

Nick Upton / Gorse spider mites (Tetranychus lintearius) emerging in large numbers from silk tent shrouding Gorse bush (Ulex europaeus) ready to migrate to other bushes. These mites are spreading in the UK and are...

Lisa Hoffner / Underpass built for African elephants (Loxodonta africana) to migrate under the busy Nanyuki-Meru road, northern Kenya.

Lisa Hoffner / Underpass built for African elephants (Loxodonta africana) to migrate under the busy Nanyuki-Meru road, northern Kenya

Donald M. Jones / Osprey (Pandion haliaetus) nest with reserved sign on it to keep geese from nesting there before the osprey migrate back, Libby, Montana

Konrad Wothe / Army Ants (Eciton burchelli) workers carrying pupae during migration phase with large soldier ant, in rainforest at Tambopata river, Tambopata National Reserve, Peru, South America.