Although they are unable to breed naturally in all but a few British rivers, the Rainbow trout has been an incredible success in the UK since first introduced from the United States in the late 1800s.

Thanks to an eagerness to feed and its habit of shoaling, the Rainbow Trout became a huge favorite of local fisherman. Its fast growth rate and tolerance of crowding in captivity made it a favorite of area fish farmers as well.

Indeed, the multi-hued “Oncorhynchus mykiss” is now widely used all around the world for fish farming and restocking of angling fisheries as it has since been introduced to every continent except Antarctica.

The only fly in the ointment for the British was two different strains of rainbow trout were initially used. The Kamloops strain (which actually originated in British Columbia) and the Shasta strain (from Northern California) were both given the honors of schlepping back to the mother land, but it turns out these strains became sexually mature at different times of the year.

The solution was the development of an egg treatment that produced farmed Diploid rainbow trout, which are all sterile females. This resulted in clean fish year-round, and as a bonus, these trout also matured faster.

The introduction of these “sexless” rainbows had another benefit for the angler; It opened the door for the abolition of the closed season for the fish, and fishery owners benefited from the faster growth rate soon resulting in a new national record fish.

As a result, a race began between fish farmers to grow bigger rainbow trout to meet the demands of the anglers, while another began between fishery owners to stock new jumbo trout to attract fishermen to their fisheries.

Selective breeding has since pushed the growth rate even higher. Coupled with intensive feeding programs and continued egg treatments, this selective breeding has produced today’s mammoth rainbows.

There is a rather confusing situation in the record fish list. Separate records now exist for all trout species – not just in the UK, but all over the world. There are now separate records for fish which have been recently stocked against fish that have been in the water for a period of time or fish that are “wild.” Which would you rather catch?

Note. The current British record for rainbow trout stands at 33Lb. 4oz.