HYPOXIA AND HYPERTHERMIA TOLERANCES OF HEADWATER STREAM FISHES

We developed a reference base of relative hypoxia and hyperthermia tol erance rankings for many of the common fish species that inhabit small streams in Missouri. We also investigated the influences of geographi c variation and of fish size on these tolerances. Fish collected From the wild were acclimated to laboratory conditions over a 63-160-d peri od. They were then exposed to either progressively increasing temperat ures or to decreasing dissolved oxygen concentrations over a 4-6-h per iod. The temperature at which a fish lost equilibrium or the oxygen co ncentration at which it ceased ventilating was recorded as the end poi nt. No significant differences in critical maximum temperatures (hyper thermia tolerance) or critical minimum oxygen concentrations (hypoxia tolerance) occurred in any of the five comparisons between fish of the same species collected from different locations in Missouri. Neither the hypoxia nor hyperthermia tolerance values varied with fish size fo r any species. Among the 35 species tested, hypoxia tolerance means ra nged from 0.49 to 1.59 mg/L. Among the 34 species tested, hyperthermia tolerance means ranged from 34.9 to 3S.8 degrees C. The mean hypoxia tolerances of species were significantly, but poorly, correlated with their mean hyperthermia tolerances.