The enclosure trap, a new tool for sampling juvenile crayfish

When managing crayfish populations, it is important to have access to reliable data on population densities and growth of juveniles. Present sampling methods are mostly active and often labor intensive, do not always yield quantitative data, and may harm the crayfish. A new passive method is described which aims to avoid these disadvantages. It uses a 0.09 m2 trap which is set on the river or lake bottom, well in advance of expected hatching of crayfish eggs. The trap has a substrate which juveniles colonize and a net that fences them in and captures them when the trap is lifted. In field tests, the new trap provided data on mean density (29·m-2) and mean total length (22 mm) of young-of-year signal crayfish which were comparable to data from suction sampling. The enclosure trap may provide data for recruitment studies and become a tool in crayfish research in general.