Behavioural response of fishes to increasing pH and dissolved oxygen: field and laboratory observations



  • 1 Freshwater fish assemblages were sampled quantitatively before, during, and after naturally produced conditions of elevated pH and dissolved oxygen (DO) that occurred for 2–6h in two dense macrophyte beds. Field observations were followed by 3–4-h laboratory behavioural experiments in which banded killifish (Fundulus diaphanus), bluegill (Lepomis macrochirus), and striped bass (Morone saxatilis) were exposed to gradually 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. verticillata and Vailisneria americana) bed, but beds did not differ significantly in fish species richness or biomass. Within beds, diel changes in fish species composition, richness, density and biomass were minor.
  • 3 In the laboratory, avoidance of pH levels exceeding 9.5 was exhibited 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 beds, when high pH levels (9.5–10.0) are accompanied by high DO levels (200–260% saturation).