THE EFFECT OF WEED COVER ON JUVENILE SIGNAL CRAYFISH (PACIFASTACUS-LENIUSCULUS DANA) EXPOSED TO ADULT CRAYFISH AND NONPREDATORY FISH

In the laboratory we investigated the effects of weed cover on the act ivity, growth and survival of juvenile signal crayfish (Pacifastacus l eniusculus Dana) exposed to adult signal crayfish and white aspe (Leuc aspius delineatus Heckel), a non-predatory cyprinid. Juvenile crayfish showed no preference for habitats containing weed cover as opposed to habitats with no weed. However, real weed (Elodia sp.) improved juven ile growth and also reduced juvenile mortality. Both adult crayfish an d white aspe reduced juvenile activity and survival. Juveniles in cont rol tanks were more active at night. Juveniles became increasingly noc turnal in response to white aspe but were diurnal in response to adult crayfish. This suggests that non-predatory fish may affect juvenile c rayfish activity and distribution in a similar way as do predatory fis h. The results also suggest that adult crayfish influence juvenile sur vival indirectly by altering juvenile activity and growth as well as d irectly via cannibalism.