Abstract

Catch per unit effort for Procambarus clarkii Girard in Lake Naivasha in late 2000 was very poor at 0-3.5% of the abundant catches in late 1999 and did not recover substantially by 2003. The population crash of P. clarkii in Lake Naivasha from 1999 to late 2000 can be summarised as the decline of an exotic species in an unstable exotic food web. Exotic water hyacinth (Eichhornia crassipes) mats (which normally fringe the papyrus reeds at the edge of the lake) play an important role in P. clarkii recruitment and population dynamics, sustaining very high densities of P. clarkii. There was decimation of and subsequent lack of recovery of the water hyacinth mats from late 2000 to 2003 due to their biological control by the exotic alien coleopteran, Cyrtobagus eichhorniae. This, coupled with the water hyacinth mats’ importance to P. clarkii for recruitment and as refuges from predation may explain why P. clarkii populations plummeted in late 2000 and subsequently did not increase to 1999 levels from 2001 to 2003. Thus, one exotic species, Cyrtobagus eichhorniae, was decimating a second exotic species, E. crassipes, with detrimental impacts on a third exotic species, P. clarkii. Part of the cause of the population crash of P. clarkii along the eastern and southern shores of Lake Naivasha in late 2000 may also have been due to a drought which decimated key littoral habitat along these shores. The drought is related to the ‘El Nino’ event and afflicts the East African highlands on a regular basis. The relatively low numbers of P. clarkii recorded in Lake Naivasha to 2003 was reflected in an upsurge in aquatic macrophytes and associated mollusca in the lake.

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