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PROCEEDINGS OF THE DESERT FISHES COUNCIL 1992 - VOLUME MV <br />reached 31°C/87F°, fish would become very active, gilling faster, and taking forays to the surface where they <br />would skim their bodies along the surface. Perhaps this splashing cooled the water. After about 20 minutes at this <br />temperature the fish would calm down, but gilling rate would remain elevated. <br />First, I conducted a preliminary run by heating just one tank of 10 fish (see Table 2). Two fish died in control <br />tanks during this first week in aquaria. The average daily maximum temperature was 33°C/92°F. A total of three <br />fish died when the temperature reached its peak of 34°C/93°F, on the second and 4th days. Lockhart reported <br />similar results in 1980, when death occurred at 33.8°C/93F° for fish acclimated at 21.5°C/71°F. <br />The second run included all ten tanks and 100 fish, with tank treatment randomly assigned. Of the heated <br />tanks, the average daily maximum temperature was 32°C/90°F, and a total of 10 fish died. The highest overall <br />temperature reached during this run was 35°C/95°F, after which 5 of the 10 deaths occurred. <br />The third run included 4 adult tanks and one juvenile (or 1 year old). tank. The average daily maximum <br />temperature was 31°C/87.5°F, overall maximum for the run was 32/90°F, and no mortalities occurred during this <br />treatment. <br />There appears to be a direct relationship between average maximum temperature and mortality. Of the 13 fish <br />that died, the mean maximum temperature was 34°C/93 °F, while that of survivors was 31.5 °C/88.6°F, a significant <br />difference (t = -6.577, p < 0.01, df 53). <br />Mean <br />Trial Max Temp n Mort Max Temp <br />I 33.5°C 10 30% 34°C <br />II 32°C 60 16% 35°C <br />III 31°C 40 0% 32°C <br />Table 2 Results from experimental high temperature treatment for control of Ichthyophthiriasis, at <br />Dexter National Fish Hatchery and Technology Center during spring of 1992 (see text for further <br />explanation). <br />Discussion <br />No treated fish got Ich this year, however no control fish got Ich this year, either. Therefore, at best the results <br />of this experiment on the effectiveness of treating Ich-infected woundfm with heat are inconclusive. Some <br />explanations come to mind: It may be that the last malachite/formalin treatment finally did get rid of the parasite. <br />However, after so many unsuccessful trials, this is unlikely. Another possibility is that the usual procedure of <br />recirculating the raceway water gives the tomites a better chance of encountering a host. Continuous flushing (as <br />was done this year) may decrease the chance of such an encounter. Also, water quality would be enhanced by <br />continuous flushing, and therefore minimize that stress on the fish. <br />Although inconclusive about Ich, conducting this experiment did provide some additional insight into the <br />biology of these little-known fish. The woundfm's ability to withstand high temperature is surely an advantage in <br />the Virgin River system. Woundfin have been collected there at 36°C/97°F (Deacon, 1987). The maximum <br />temperature tolerated by woundfm in this experiment was 35'C/95 *F, while in Deacon's CTM experiment (1987) <br />it was 39.5°C/103°F. The difference in maximum temperatures obtained in these experiments may be explained <br />by the rate at which temperatures were raised. In this experiment, temperature was raised at an average rate of <br />0.03 °C per minute, while in Deacon's experiment it was raised 1 °C per minute. As woundfin can withstand higher <br />temperatures when raised quickly, this would imply a phsiological mechanism in place to buffer against the effects <br />of variable water temperatures. <br />Even with this thermal treatment and intensive handling this year, the fish still spawned. On the first days of <br />treatment I observed spawning behavior in the aquaria, and after all fish had been cycled through the primary <br />holding aquarium, I found fry. Since the last adults were removed from that aquarium on June 7, the fry were <br />spawned sometime prior to that. On July 7, fry were dipnetted from the raceway of treated fish. This continued until <br />Sept 23, for a total of 257 fry from treated fish. It is possible that some fry could have escaped our nets, making <br />this recruitment number a conservative one. Only 1 fry was observed in the raceway of untreated fish during the <br />same period. At Dexter National Fish Hatchery, fry are usually collected between May 18 and June 20 (DNFHTC <br />files), a period of 4 1/2 weeks. This year's experimentally delayed spawning season was at least 15 weeks long, <br />and therefore the longest recorded season of production at Dexter. Although Lockhart (1980) found peak gonad <br />35