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

Michel Roggo / Atlantic salmon (Salmo salar) migrating upstream to spawn, Quebec, Canada Photographed for the Freshwater Project.

SCOTLAND: The Big Picture / Atlantic salmon (Salmo salar) leaping up a small waterfall in order to reach breeding grounds further upstream, Cairngorms National Park, Scotland, UK, October.

Ann & Steve Toon / Atlantic salmon (Salmo salar) leaping on upstream migration, River Tyne, Hexham, Northumberland, UK, November

Yva Momatiuk & John Eastcott / Sockeye Salmon (Oncorhynchus nerka) group migrating upstream, Adams River, Roderick Haig-Brown Provincial Park, British Columbia, Canada

Ingo Arndt / Sockeye Salmon (Oncorhynchus nerka) group swimming upstream during spawning run, Adams River, Roderick Haig-Brown Provincial Park, British Columbia, Canada

Will Watson / Atlantic salmon (Salmo salar) leaping a weir, Shrewsbury, River Severn, Shropshire, England, UK. November.

Wim van den Heever / Bottlenosed dolphin (Tursiops truncatus) porpoising during annual sardine run, Port St Johns, South Africa. June.

Yva Momatiuk & John Eastcott / Sockeye Salmon (Oncorhynchus nerka) migrating upstream, Adams River, Roderick Haig-Brown Provincial Park, British Columbia, Canada

Alex Mustard / A 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,...

Michel Roggo / Atlantic Salmon (Salmo salar) in pool on upstream spawning migration. Quebec, Canada, August.

Yukihiro Fukuda / Japanese giant salamander (Andrias japonicus) moving upstream to spawn, Japan, August