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Although enhanced by irrigation in some areas, these data indicate that selenium exposures have <br />always occurred in the region. <br />The report by Hamilton (1998) also notes that the fish congregated in irrigation drainages that <br />exposed them to high levels of selenium that he believes harmed their reproduction. However, <br />reports that the fish used these areas go back to the 1920s (Quartarone 1995) indicating that such <br />locations probably have been used since irrigation began in 1915. <br />Considering the selenium concentrations observed in the early and present-day historical accounts <br />of irrigation water, it is worthwhile to consider data reported for an exposure in North Carolina. <br />Concentrations of selenium of 150 to 200 gg/L flowed into Belew's Lake raising the <br />concentration in the lake to 8-22 gg/L (Skorupa 1998). Immediate effects were observed, <br />including drastic declines in fish populations and increased numbers of deformed fish. However, <br />as described above, higher concentrations of selenium were observed. in the Upper Colorado <br />Basin, including within the rivers themselves, and neither deformed fish nor immediate <br />population declines were observed. (More discussion of contaminant levels that affect the fish is <br />provided in Sects. 3, 5, and 6.) <br />Hamilton (1998) suggests that the fish staged in the irrigation ditches before making their <br />spawning run and probably absorbed significant selenium. An alternate consideration is that <br />water levels and dilution would have been highest prior to the spawning run. The fish spawn in <br />response to spring floods when the selenium should have been diluted such that exposure may not <br />have been as great. According to Kidd (WestWater Engineering Inc. 1996), important rearing <br />habitat for razorback suckers "had a source of water other than the Colorado River itself, and did <br />not depend entirely upon the river flow to maintain their water level." Based on the comments of <br />Stephens and Waddell (1998) above, that could mean that selenium concentrations were typically <br />highest in their preferred habitats where they were formerly abundant. <br />1.5.1.2 Abundance and Decline of the Fish <br />The timing of the decline of the fish populations is debatable and apparently varies from one river <br />system to another and, perhaps, from one section of the river to another. A report reviewed by <br />Hamilton (1998) indicates that the endangered fish were rare in the Upper Colorado Basin after <br />1930, but in previous decades had been numerous (Dill 1944). On the other hand, a fisheries <br />biologist who commonly collected baitfish from backwaters in the Upper Colorado Basin reports <br />that razorback suckers were numerous and had to be "kicked out of the way" when wading <br />through some flooded bottomlands in the early 1970s (Kidd, G., Northwest Fisheries, personal <br />communication, May 25, 1999). Two 24-hour net sets caught 16 razorbacks at this location in <br />1974. Single net sets, after road construction altered this location in 1975 and 1976, resulted in <br />six and zero razorbacks, respectively (Kidd 1977). According to Mr. Kidd, razorbacks were <br />spawning and various age classes were present between 1971 and 1980, observations were <br />verified by another researcher (WestWater Engineering Inc. 1996). A member of the recovery <br />program also stated that there were frequent reports that razorbacks and squawfish were common <br />in the early 1970s (Pfeiffer, F., U.S. Fish and Wildlife Service, personal communication, Sept. 22, <br />1999). If the pikeminnow (squawfish) and. razorback sucker were relatively common-if not <br />abundant in the early 1970s in the Grand Valley-the question that arises is whether the <br />populations at this time were only big enough to be noticed but not viable. In other words, was <br />the apparent persistence of the fish long after high selenium exposures had. begun a result of their <br />longevity, or did the habitat changes in the 1970s deliver the critical blow to their populations?