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<br />-0.50C <br />5.0 mgll <br /> <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br /> <br />Chemical characteristics were measured using a Hydrolab Surveyor 2. Water temperature (:I: <br />0.01 OC), dissolved oxygen (mgll), conductivity (J.tS/cm), and pH were measured in the main channel <br />adjacent to the backwater, then along the deepest portion of the backwater beginning at the mouth <br />and working toward the back. Typically, four to six measurements were taken within each backwater <br />when free water was present. Fewer were taken when ice was present, depending on the extent of <br />the ice- cover. <br /> <br />3.4 Physical and Chemical Habitat Criteria <br /> <br />Minimum physical and chemical habitat requirements were designated and used as criteria to <br />detennine if a backwater could support fish through the winter (Table 2). The primary physical <br />variables of importance to fish in backwaters are size of the backwater and depth of available or free <br />water. An area of 30m2 was used as the minimum size criterion for backwaters, and there was no <br />maximl.lm size criterion. This criterion is consistent with the ISMP and is considered the minimum <br />size backwater likely to persist through normal daily flow fluctuations. The criterion of 0.3 ft (9.0 <br />em) offree water was used as the minimum amount of water useable by fish, either between ice cover <br />and the river substrate or between ice layers. Free water of less than 0.3 ft was not considered <br />suitable habitat because of the constricted space and the likelihood of a supercooling effect (Ashton <br />1983), which can be lethal to fish (Lagler et al. 1962). <br /> <br />Table 2. Summary of minimum phYSical and chemical habitat requirements for <br />backwaters sampled during the Winter Ice Study. <br /> <br />PARAMETER MINIMUM VALUE <br /> <br />Physical Parameters <br />Surface Area <br />Free Water Depth <br />Chemical Parameters <br />Temperature <br />Dissolved Oxygen <br /> <br />30 m2 <br />0.3 ft {9.0 cm} <br /> <br />Wintertime changes in chemical characteristics of backwaters are primarily related to changes in <br />temperature and dissolved oxygen. Minimum temperature requirement for backwater habitats was <br />set at -0.50C. Supercooled water below -0.5oe is near the freezing point depression Cd') of most <br />freshwater teleosts or modern bony fishes (Lagler et al. 1962). Freezing point depression is a <br />measure of the solute content of a solution, and hence its water diffusion pressure through tissue <br />membranes. Most freshwater te1eosts have a 'd' for blood and plasma of about -0.570C, which means <br />that body temperature (water temperature) below minus 0.50 C may cease water diffusion in the <br />circulatory system, and lead to the death of the fish. <br /> <br />The second chemical variable of importance to fish in backwaters is dissolved oxygen. Ice formation <br />can isolate backwaters and reduce water exchange with the mainchannel, hence, depleting the amount <br /> <br />6 <br />