Growth and reproduction of the introduced crayfish Pacifastacus leniusculus in a British lowland river

Abstract

Signal crayfish, Pacifastacus leniusculus (Dana), a native of North America, were introduced into the River Great Ouse, the major lowland river in mid- to eastern England in 1984. The present paper reports the growth and reproduction of a naturalised signal crayfish population in the river. Crayfish moulted between April and October. Males of individuals >55 mm carapace length (CL) and females of individual >45 mm CL moulted only once a year while younger ones moulted more than once. Mature males grew faster than mature females by a larger per moult increment (MI) and more frequent moult. For crayfish >30 mm CL, the MI increased with CL and then decreased as crayfish grew larger, while the percentage moult increment (PCMI), the annual moult increment (AMI), and the annual percentage moult increment (APCMI) decreased with increase in CL. The mean AMI of males (36–70 mm CL) ranged from 11.7 to 5.1 mm in CL, that of females (41–70 mm CL) 8.4 to 3.4 mm. Both males and females matured in their third year. The smallest ovigerous female was 36.3 mm CL. In 1994, the mean number of pleopodal eggs per ovigerous female sampled at the end of April was 158. A recruitment of 70 juveniles per m 2 was estimated in the original pool of crayfish introduction.