David Hall / Split level of a Sockeye salmon (Oncorhynchus nerka) in shallow water; Adams River, British Columbia, Canada. October.

David Hall / Split level of Sockeye salmon (Oncorhynchus nerka) during migration; Adams River, British Columbia, Canada. October.

David Hall / Sockeye salmon (Oncorhynchus nerka) migration, Adams River, British Columbia, Canada. October.

David Hall / Sockeye salmon (Oncorhynchus nerka) migration, Adams River, British Columbia, Canada. October.

Nick Hawkins / Chum salmon (Oncorhynchus keta) migrate up a small river near Bella Bella, British Columbia, Canada. September

Nick Hawkins / Atlantic salmon (Salmon salar) under river ice. Quebec, Canada. October.

John Waters / Slow motion clip of a Brown trout (Salmo trutta) breaching to predate an adult Mayfly (Ephemoptera) emerged on the surface, River Kennet, Hungerford, Berkshire, England, UK, June.

John Waters / Slow motion clip of a Brown trout (Salmo trutta) predating an adult Mayfly (Ephemoptera) struggling to take off from water, River Kennet, Hungerford, Berkshire, England, UK, June.

Nick Hawkins / Atlantic salmon (Salmo salar) migrating for spawning in river, Gaspe Peninsula, Quebec, Canada, October.

Nick Hawkins / Two Atlantic salmon (Salmo salar) silhouetted against sky, Gaspe Peninsula, Quebec, Canada, October.

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

John Waters / Pair of Brown trout (Salmo trutta) spawning, with others eating eggs, River Kennet, Berkshire, England, UK. March.

John Waters / Female Grayling (Thymallus thymallus) pursued by several males, River Kennet, Berkshire, England, UK. March.

John Waters / Pair of Brown trout (Salmo trutta) spawning, with another male trying to fertilise eggs River Kennet, Berkshire, England, UK. February.

John Waters / Small group of Rainbow trout (Oncorhynchus mykiss) swimming in the River Kennet, Berkshire, England, UK, March.

John Waters / Small shoal of Brown trout (Salmo trutta) swimming and feeding in the River Kennet, Berkshire, England, UK, March.

Loic Poidevin / Grizzly bear (Ursus arctos horribilis) mother watching cub catching salmon, Katmai National Park, Alaska, USA, August

Loic Poidevin / Grizzly bear (Ursus arctos horribilis) cubs with salmon, Katmai National Park, Alaska, August. Small reproduction only.

Loic Poidevin / Grizzly bear (Ursus arctos horribilis) cubs with salmon, Katmai National Park, Alaska, August. Small reproduction only.

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

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

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.

Brandon Cole / Sockeye salmon (Oncorhynchus nerka), male sockeye salmon fighting up a shallow stream on its migration to spawning beds. Adams River, British Columbia, Canada, October 2014

Brandon Cole / Sockeye salmon (Oncorhynchus nerka) interacting in shallow river, males chasing each other while female waits over gravel suitable for depositing her eggs after fertilization. Adams River, British Col...

Brandon Cole / Sockeye salmon (Oncorhynchus nerka) from behind whilst on migration to spawning ground. Adams River, British Columbia, Canada, October 2014

Graham Eaton / Salmon / Trout fish (Salmo sp) jumping a waterfall on the Afon Lledr, Betws Y Coed, Wales, October

Ann & Steve Toon / Osprey (Pandion haliaetus) catching trout, Rothiemurchus estate, Cairngorms, Scotland, UK, July

Charlie Summers / Black Bear (Ursus americanus) with Pink Salmon (Oncorhynchus gorbuscha) catch, Anan Creek, Alaska, July.

Erlend Haarberg / Stoat (Mustela erminea) in white winter coat, trying to steal fish (trout) hanging on transparent fishing line. Vauldalen, Sor-Trondelag, Norway, April

Chris & Monique Fallows / Pink Salmon (Oncorhynchus gorbuscha) migrating in small shallow river, Prince William Sound, Alaska, USA.

Michel Roggo / Brown bear (Ursus arctos beringianus) fishing for Sockeye salmon in the Ozernaya River, Kamtchatka, Russia, August

Alex Mustard,Alex Mustard / Rainbow trout (Oncorhynchus mykiss) under trees, Capernwray Lake, Lake District, England, UK.

Alex Mustard,Alex Mustard / A female (in front) and male Sockeye salmon (Oncorhynchus nerka) over eggs in their spawning river. Adams River, British Columbia, Canada, October.

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 / Sockeye / Red Salmon (Oncorhynchus nerka) on spawning migration; males surrounding a female. Adams River, British Columbia, Canada, October.

Michel Roggo / Sockeye / Red Salmon (Oncorhynchus nerka) on spawning migration. Adams River, British Columbia, Canada, October.

Terry Whittaker / Atlantic salmon (Salmo salar) leaping a waterfall to reach spawning grounds upstream, River Endrick, Trossachs National Park, Scotland.

Terry Whittaker / Atlantic salmon (Salmo salar) leaping a waterfall to reach spawning grounds upstream, River Endrick, Trossachs National Park, Scotland.

Nick Hawkins / Atlantic salmon (Salmo salar) male guarding it's nest, known as a 'redd' from precocious juvenile salmon parr that may try to fertilize the females eggs. St. Mary's river, Nova Scotia,...

Nick Hawkins / Brook trout (Salvelinus fontinalis) gather to spawn, St. Mary's river, Nova Scotia, Canada. October.

Nick Hawkins / Sea run Brook trout (Salvelinus fontinalis) gathering to spawn, St. Mary's river, Nova Scotia, Canada. October.

Nick Hawkins / Sea run Brook trout (Salvelinus fontinalis) gather to spawn, St. Mary's river, Nova Scotia, Canada. October.

Nick Hawkins / Brook trout (Salvelinus fontinalis) gathering to spawn, St. Mary's river, Nova Scotia, Canada. October.

Nick Hawkins / Brook trout (Salvelinus fontinalis) gathering to spawn, St. Mary's river, Nova Scotia, Canada. October.

David Hall / Split level of Sockeye Salmon (Oncorhynchus nerka) and Chinook Salmon (Oncorhynchus tshawytscha) during the salmon run; Adams River, British Columbia, Canada. October.

David Hall / Sockeye Salmon (Oncorhynchus nerka) and Chinook Salmon (Oncorhynchus tshawytsha) (right) during the salmon run; Adams River, British Columbia, Canada. October.

David Hall / Sockeye salmon (Onchorhynchus nerka); Adams River, British Columbia, Canada. October.

David Hall / Migrating Sockeye salmon (Oncorhynchus nerka); Adams River, British Columbia, Canada. September.