Immigration rates of signal crayfish (Pacifastacus leniusculus) in response to manual control measures

1. Crayfish are amongst the most frequently introduced non-native aquatic organisms, with well-documented negative effects on a large number of freshwater taxa. Many crayfish-control strategies make use of manual removal by trapping, a method known preferentially to remove the largest individuals, leaving the juvenile population almost entirely untrapped. 2. A predicted outcome of trapping bias in riparian habitats is that removed individuals could be replaced by large crayfish immigrating from surrounding, untrapped, areas. We tested the hypothesis that removal by trapping of American signal crayfish from a UK river would result in increased rates of immigration, and increased distances moved, of crayfish from untrapped areas. 3. We studied four stretches of the River Windrush each 1 km in length and divided into three sections; a 250 m long upstream section, a 500 m middle section and a 250 m downstream section. At two sites (removal sites) signal crayfish were trapped and removed from the 500 m middle sections, at the other two (non-removal) they were marked and returned. All crayfish captured in the upstream and downstream sections were marked and returned. 4. Probability of capture was higher for larger individuals with both chelae intact, and larger crayfish were more likely to immigrate from the upstream and downstream sections into the middle. The percentage of captured crayfish immigrating into the middle sections was the same (3.7%) in both removal and non-removal sites. However, the mean distance that crayfish moved when immigrating was significantly greater at removal sites (239 m) than at non-removal sites (187 m). 5. These results imply that removal of large individuals may have reduced the potential for interference competition by increasing the relative competitiveness of the immigrating individuals and permitting them to make larger movements. Consequently, the impact of manual removal strategies, both on the signal crayfish population and other biota affected by them, is likely to be reduced at the point of removal, but to extend at least 200 m beyond the trapped length of river.