BEHAVIORAL-RESPONSE OF FISHES TO INCREASING PH AND DISSOLVED-OXYGEN -FIELD AND LABORATORY OBSERVATIONS

1. Freshwater fish assemblages were sampled quantitatively before, dur ing, and after naturally produced conditions of elevated pH and dissol ved oxygen (DO) that occurred for 2-6h in two dense macrophyte beds. F ield observations were followed by 3-4-h laboratory behavioural experi ments in which banded killifish (Fundulus diaphanus), bluegill (Lepomi s macrochirus), and striped bass (Morone saxatilis) were exposed to gr adually increasing levels of: (i) pH; (ii) DO; and (iii) pH and DO. 2. Field results indicated that the macrophyte bed dominated by Hydrilla verticillata had a higher fish density than the mixed-species (H. ver ticillata and Vallisneria americana) bed, but beds did not differ sign ificantly in fish species richness or biomass. Within beds, diel chang es in fish species composition, richness, density and biomass were min or. 3. In the laboratory, avoidance of pH levels exceeding 9.5 was exh ibited by all species tested. No significant response was observed to either DO increase treatments, or to pH and DO increase treatments. 4. Results suggest fishes do not avoid areas, such as dense macrophyte b eds, when high pH levels (9.5-10.0) are accompanied by high DO levels (200-260% saturation).