Responses of seven species of native freshwater fish and a shrimp to low levels of dissolved oxygen

The tolerances of seven New Zealand freshwater fish species and one species of shrimp to low levels of dissolved oxygen were determined in the laborat ory by holding fish at dissolved oxygen levels of 1, 3, or 5 mg litre(-1) f or 48 h at 15 degrees C. Juvenile rainbow trout (Oncorhynchus mykiss) were also tested for comparison. All of the banded kokopu whitebait (Galaxias fa sciatus), juvenile torrentfish (Cheimarrichthys fosteri), adult and juvenil e common smelt (Retropinna retropinna), juvenile common bully (Gobiomorphus cotidianus), and trout were dead after 48 h exposure to 1 mg litre(-1). Ad ult and juvenile inanga (Galaxias maculatus), adult common bully (Gobiomorp hus cotidianus), and shrimp (Paratya curvirostris) had mortalities ranging from 27 to 80% at 1 mg litre(-1), whereas all of the elvers (Anguilla spp.) survived. Juvenile rainbow trout were the most sensitive of the species te sted, and the only species that had mortalities during exposure to 3 mg lit re(-1). No deaths of any fish occurred at 5 mg litre(-1). Time to 50% morta lity at 1 mg litre(-1) for common smelt (adult and juvenile) and juvenile c ommon bully were similar to those for juvenile rainbow trout. Most fish mov ed towards the surface within the first few hours of exposure to 1 mg litre (-1) Rainbow trout was the only species that showed consistent surfacing be haviour at 3 mg litre(-1) and rarely were fish observed to move towards the surface at 5 mg litre(-1). Within the limits of the lifestages and species tested, adequate protection is provided for native fish by adopting the Un ited States Environmental Protection Agency dissolved oxygen criteria for s almonid waters.