Brown trout supplies have been depleted as a result of fishing pressure, and the Prysor Angling Association has decided to augment the wild species with introduced rainbows. These fish have adapted well to their new environment, with a good overwintering survival rate.

Llyn Trawsfynydd is well-known for its top-of-the-water action and high-quality, hard-fighting fish. Brown trout in the wild average about 1 pound, however browns up to 7 pounds have been taken. Spawning occurs in the Afon Prysor's higher reaches.

Throughout the season, rainbow trout are supplied at regular intervals, with an average weight of 1.10 to 1.12 lbs and a few bigger fish introduced. Fish weighing between 4 and 8 pounds are captured on a daily basis. Numerous doubles have been captured throughout the years, with the fisheries record now standing at 17.12 lbs.

The lake contains a school of perch, as well as a few rudd and some large pike. In 1992, a 42-pound grass carp was killed after it was struck by a propeller. This fish had been in the lake for at least 13 years, having escaped from a cage housed by the CEGB for experimental reasons in the 1980s. However, no Grass carp have been seen since 1992.

Llyn Trawsfynydd is a multi-method lake with many spaces for bank anglers to fish bait or fly. There are numerous designated fly fishing spots on the riverside. The season begins on February 1st and runs through December 20th, with rainbows accessible year-round and browns available just during their season, which runs from April to the end of September.

A fleet of 40 boats equipped with outboard engines is available for fly fishing exclusively; no places are off-limits. Caution is essential owing to subsurface outcropping of boulders that may easily trap an unwary fisherman, particularly when the lake is not fully stocked. Metal pins are used to label the majority of harmful rocks. These underwater rocky outcrops provide excellent habitat for fish.

With an average depth of of 12 feet, there is little need to fish with fast sinking lines, however they cannot be ruled out entirely, since the majority of the sport is topwater utilising floating or intermediate lines.

Various ways are used to entice the fish, the most common being tiny lures, palmered flies, buzzers, and dries, while some anglers prefer larger lures; it all depends on personal choice and the time of year and current hatches.