Microhabitat use and recolonisation in white-clawed crayfish: application to conservation

Abstract

White-clawed crayfish Austropotamobius pallipes are listed as vulnerable on the International Union for the Conservation of Nature (IUCN) red list and British populations represent one of the greatest concentrations of this species in Europe. White-clawed crayfish still remain at risk in Britain due to habitat degradation, pollution, and non-indigenous crayfish and disease. This thesis investigates the microhabitat use of juvenile and adult white-clawed crayfish and quantifies the rate and pattern of recolonisation into an unoccupied reach of river.

The microhabitat study was carried out over a 1 km stretch of the River Wansbeck, Northeast England in 2008 and a 3.5 km stretch on the Hart Burn, a tributary of the Wansbeck, in 2009. In September 2008 there was a 1 in 115 year flood and the data collected before and after the flooding were compared. Crayfish recorded in the Wansbeck were separated into three age groups, 0+ (0 - 9.4 mm carapace length (CL)), 1+ (9.5 - 16.4 mm CL) and 2+ and older (16.5 mm+ CL). Crayfish recorded in the Hart Burn were separated into four age groups 0+ (0 - 7.4 mm), 1+ (7.5 - 13.4 mm), 2+ (13.5 - 22.4 mm) and 3+ and older (22.4 mm+).

Crayfish of different ages had different microhabitat requirements where young crayfish, in particular 0+ crayfish, were more restricted in the microhabitats they inhabited. Young crayfish were found in closer proximity to the bank, in smaller substrata and slower velocities than older crayfish. In both study sites substratum heterogeneity created suitable habitat for all ages of crayfish and in the Hart Burn, root habitats were found to be important for all ages of crayfish. Extensive flooding in 2008 was found to significantly reduce densities of larger crayfish, probably reflecting large-scale mortalities or major redistribution.

Recolonisation was studied in the lower 3 km of the Hart Burn after a mass mortality event occurred in May 2004, apparently due to a one-off acute pollution incident. The recolonisation data highlights the impact mass mortality events can have on crayfish populations. It took 4 years of no further pollution for the population to recover which relates to a recolonisation rate of 750 m year-1. The passive downstream drift of juveniles was the most rapid form of movement recorded and was responsible for the downstream population expansion in the first two years. Juvenile crayfish seem to have some control over their dispersal, as there was limited dispersal over periods of high flows and rapid dispersal over periods of low flows. Active upstream and downstream dispersal by adult crayfish was similar in rate.