Sampling UK Pacifastacus leniusculus (Dana, 1852): the effect of trapping on population structure

Populations of non-native signal crayfish, Pacifastacus leniusculus, are damaging to UK native species and habitats though their populations are expanding with no coherent framework in place for their control. This is partly the result of a literature gap on the effect of trapping on non-native crayfish population structure which this thesis will explore in order to add to the European literature. Population size structure analysis has been facilitated via the creation of novel samplers and an in-depth analysis of the effect of aperture on the size/life stage of crayfish sampled. Smaller trap apertures, the addition of refuge material and novel samplers increased the catch of juvenile crayfish. Sex was indeterminable for up to 50% of juvenile crayfish, with juvenile sex ratios potentially biased towards females. Conditions on the River Lark did not limit populations, though temperature varied significantly between sites whilst substrata, pH and biological oxygen demand did not. Three years of trapping and juvenile sampling enabled population analysis at a site level. The population at Lark Head (professionally trapped), had a consistent size structure from 2010 to 2012, whilst individuals at Barton Mills (community trapped) and the Plough (untrapped), showed size decreases over time. The proportions of adult to juvenile individuals, and males to females, were similar at all three sites in 2011 & 2012. Catch per unit effort, decreased at all three sites with the greatest reductions at trapped sites. There is no evidence that catch sizes, or the proportion of juveniles, increased with trapping in spite of one site being trapped by the community since 2001 and another trapped by professionals since 2005. This refutes inferences that trapping causes an increase in biomass due to a reduction in the number of cannibalistic and dominant large males, with size and sex bias in traps also not corroborated.