Gammarus and Asellus populate almost...

Crustacea (Gammarus & Asellus)

Gammarus and Asellus populate almost all waters, but it is in the Sorgive and Chalck Stream, typically rich in aquatic vegetation, that they can become the dominant species. Fly-fishers know that they are always preyed on by fish, but their best time of use in fishing is when, due to the obvious seasonal cycles, the hatch of Ephemeral, Trichoptera and Plecoptera are scarce and also the bottom is less rich in mature larvae.

Vogel