Rainbow Trout (Onchorhynchus mykiss)
Current record: 34lb 12oz
Water: Loch Earn
Rainbow trout (Oncorhynchus mykiss) are native to North America but have been stocked so widely across UK stillwaters and rivers that they're one of the most commonly caught trout species here. The name comes from a broad reddish-pink stripe running along the lateral line, most vivid on breeding males, set against a blue-green to olive back scattered with dense black spots.
Resident freshwater fish typically run 1-5lb, but lake-stocked and anadromous forms can reach up to 20lb, worth knowing if you're fishing a water that stocks larger specimens specifically. They favour cold, clear, well-oxygenated water and swift currents where available, though UK stillwater fisheries obviously work around that with careful stocking and management.
Feeding is genuinely opportunistic: aquatic insects, crustaceans, small fish, even small mammals when the chance comes up, which is part of why they adapt so readily to whatever water they're put into.
Rainbows have a deserved reputation as the hardest-fighting trout species widely available, expect real acrobatics, jumps and hard runs are the norm rather than the exception once one's hooked. Fly fishing is the classic method, spinning, casting, and trolled lures all work too, and they'll take natural bait, live or dead, without much fuss.
They're also the backbone of UK trout farming, most stocked stillwater rainbows started life on a farm, and it's exactly that farmed origin that occasionally raises concerns when escapees mix with wild trout populations elsewhere, worth knowing if you're fishing a water near a wild brown trout river.