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Scientific Evaluation or Biological Opinlons on endangered snd'Thmntencd Vl0ca In the Klamath River Aasin: Tnteriin Ropnrt (2002) <br />hapJ ' a'. npp. eeWn�7 iew.�(Iy1WIk +LM1Vlh+mina,pomi, n�ywpne zly�z rr+l The Nul.mal neudemv qSoknLr, WI Pl;hR (C"04 <br />Xr�DANgMD AND THBEAT")M F15M IN THE XLAMAIH RIVER BASIN <br />mortality in any year of low water during the 1990s might in fact suggest an association <br />the reverse of the one postulated in the biological opinion, although the evidence is <br />statistically inconclusive. The USFWS itself has found no association of mass mortality <br />with lake levels (USFWS 2001, II1.2.70). Intensified eutrophication now affects the <br />charactetistics of the lake every year, and thus may constitute a threat to the suckers <br />regardless of interannual variation in water level. <br />250 <br />zoo <br />c 150 <br />100 <br />50 <br />Chlorophyll a vs. Lake Elevation <br />* year of high mortality <br />t year of strong recruitment <br />• 97* <br />.92 090 498 <br />091t •96s •95* <br />• 94 •93 <br />4138 4139 4140 4141 4142 4143 <br />Lake Elevation (feet) <br />Figure 4. Relationship of chlorophyll a and median August lake level in Upper <br />Klamath Lake between 1991 and 1998. Chlorophyll data are averages as <br />reported by Welch and Burke 2001. Recruitment and mortality information <br />is as reported by USFWS 2001. <br />Higher water levels potentially could be supported on grounds of improved <br />survival of fry or juveniles rather than suppression of adult mortality. Higher water <br />lovely could reduce the likelihood that spawning areas around the lake would be <br />dewatered and could be favorable to fry or juveniles. Abundance of juvenile suckers has <br />been monitored since 1991 on the basis of seining (Simon et al, 2000x). This <br />information, which must be used cautiously because it is not quantitative, indicates low <br />abundances of juveniles in the drought years 1992 and 1994 but not in drought year 1991, <br />Abundances also were low in non - drought years 1997 and 1998. Simon et al. (2000x) <br />have reported generally declining abundance during the non - drought interval 1995 -1998. <br />They have also shown (Simon et al. 2000a, b) that the abundance of age 1+ suckers <br />consistently has been very low, suggesting a bottleneck at this life stage, but <br />interpretation of the data is complicated by very low efficiency for catching fisb older <br />than one year. Overall, the study of young fish shows no clear pattern associated with <br />lake level. <br />The most reliable current information on recruitment is through analysis of age - <br />class structure of adult suckers (LJSFWS 2001, M. 2., page 43). This data record is not <br />14 <br />966 -d 660/1ZO'd 9EZ-1 999899BEDE s30dnOsu 1Vaf M 104 -WOdd W h ZOOZ- 80 -93d <br />