Melvin is a very unique lough with wild fish consisting of four varieties of trout, Atlantic salmon and Arctic char.

A survey conducted between 1977 - 1984 by Professor Andrew Ferguson and colleagues of the Department of Zoology at Queen's University Belfast applied a technique of genetic finger printing called gel electrophoresis, which determined that sonaghan (salmo nigripinnis ), gillaroo (salmo stomachius ) and Ferox (salmo ferox) constitute three distinct varieties of trout.

In fact so different genetically are these three varieties that Ferguson and his associates suggested that they could be considered full trout species.

The gillaroo trout

Gillaroo TroutLough Melvin is home to the Gillaroo or Salmo stomachius, a species of trout which eats primarily snails. The name "Gillaroo" is derived from the Irish language Giolla Rua, which means "Red Fellow". This is due to the fishes distinctive colouring. It has a bright buttery golden colour in its flanks with bright crimson and vermillion spots. The gillaroo feed almost exclusively on bottom living animals (snails, sedge fly larva and freshwater shrimp) with the exception of late summer when they come to the surface to feed.

The sonaghan trout

Sonaghan TroutThe sonaghan trout (Salmo nigripinnis) is another species of salmonid unique to Lough Melvin. It can have a light brown or silvery hue with large, distinctive black spots. Its fins are dark brown or black with elongated pectorals. Sonaghan are found in areas of open, deep water, where they feed on mid-water planktonic organisms such as water fleas ("Clandocera"), midge ("Chironomid") pupae and phantom ("Chaoborus") larvae. Sonaghan will be most readily located close to the surface over deep water. Fly-fishing with a team of wet flies fished in classic lough style (i.e. short, snappy casts from a boat drifting beam-on to the breeze) gives best chance of success. Sonaghan give a powerful and energetic fight out of all proportion to their size