ECOLOGICAL IMPACT OF INTRODUCED CRAYFISH ON BENTHIC FISHES IN A BRITISH LOWLAND RIVER

Pacifastacus leniusculus (Dana), a native crayfish of western North Am erica, was introduced into the U.K. in 1976. Our study examined some i nteractions between P. leniusculus and benthic fish in a British lowla nd river, the River Great Ouse. In a river survey an inverse correlati on was found between the abundance of crayfish and the two dominant be nthic fishes, bullhead (Cottus gobio L.) and stone loach (Noemacheilus barbatulus [L.]) in six riffles. The benthic fishes were least abunda nt in the riffle nearest the original site of crayfish introduction an d gradually increased in abundance both up and down river as crayfish abundance decreased. The hypotheses that crayfish compete with bullhea ds and stone loach for shelter and prey on fish were tested by laborat ory experiments in an outdoor artificial shelters on the bottom. In co mpetition experiments 12 fish of one species were alternatively kept a lone and with 12 crayfish for 3-day cycles lasting a total of 12 days. The results showed that crayfish out-completed both fish species for shelter. Predation was measured by keeping 24 fish of each species alo ne and with 36 crayfish for 10 dyas respectively in the artificial str eam. The mortalities of both fish species were significantly higher wh en crayfish were present. The loss of fish could be partly due to the predation because cray-fish guts contained the remains of some lost fi sh and they were observed preying on both fishes in a tank. In the riv er crayfish lived at high densities reaching greater than or equal to to 20 m(-2) in riffles, and they continued to disperse. This may lead to a great reduction in benthic fish abundance if not local extinction s.