Fluctuations in Numbers among Freshwater Crayfish, Potamobius pallipes Lereboullet

Abstract

Large fluctuations have occurred in the numbers of P. pallipes in the Ock, Thame, and Windrush Rivers, S. England, since at least 1885, and usually at about 13-14 yr. intervals. While the cause is unknown, the evidence points to disease as the main agency, possibly due to Thelohania contejeani or Bacillus pestis astaci, responsible for considerable epidemics on the continent. The periodicity in numbers foreshadows the epidemics. In 1930, crayfish were scarce in the Ock after sudden disappearance in 1928; they are still abundant in the Thame and the Windrush where epidemics are to be expected during the present period. Similar mortality occurs in river fish, but the data are insufficient to connect this with crayfish cycles. Repopulation takes place from side streams in which a few usually survive the epidemic; also by artificial restocking.