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ScIcntifc rvallaation of Biological opinions on Endsngued and ThroratonrA PIghas In the XIamath RIvcrBasin: TvItcri In Rcpolt (2002) <br />hnpyrax�.nnr crmr+ �c�n .,�rprlan�0. +?yin,imi�i +,abn�, ggjV qhr aUZ. 2+111 The Narinnaa Amicrm all r ghnr 2'crcd <br />EYALmATwN OF THE BI !2 ra OPINION ON SHORT 95E AND Los-tj 'R-fti �tS <br />consistent with the underlying assumptions of proposals for maintenance of higher water <br />levels. The strongest recruitment (as inferred from relative abundances of adult year <br />classes) observed over the last ten years was for 1991 (Figure 5), which falls within the <br />lowest 15% of lake levels since 1950. Furthermore, as shown by the continuing strength <br />of the 1991 year class in 1995 and beyond, the year class showed good survival through <br />the dry years of 1992 and 1994. While the use of emergent vegetation by fry is cited as a <br />reason for maintaining high water levels, the combination of high recruitment in 1991 <br />and low recruitment in other years (as inferred from year class data) casts doubt on the <br />importance of this factor, at least within the operating range of the 19909. Furthermore, <br />fry of the Upper Klamath Lake populations appear to use submcrged as well as emergent <br />macrophytes (Cooperman and Markle 2000), and thus may not be highly sensitive to the <br />reduction in access to emergent vegetation that occurs in dy years (Dunsmoor at al. <br />2000). Overall, the presumed causal connoctions between lake levels and recruitment of <br />the sucker! populations in Upper Klamath Lake do not have strong scientific support at <br />present. , <br />600 <br />500 <br />�400 <br />m <br />300 <br />8 <br />Z 200 <br />100 <br />0 <br />O Lost River suckers. <br />O Shortrose suckers <br />1994 1993 1992 1991 1940 1989 1988 1987 <br />Year Class <br />Figure 5. Relative strength of year classes for endangered suckers as reconstructed from <br />survey of mass mortality in 1995, 1996, and 1997 (combined). Source: USGS <br />and USFWS records. <br />Mortality possibly could be caused by multiple factors that interact with lake <br />level. For example, mortality of suckers is influenced by changes in water column <br />stability; an extended period of stability leading to declinc of oxygen near the bottom can <br />be followed by sudden mixing of the entire water column associated with a change in <br />weather (high wind velocity). Thus, interpretation of information on lake level is <br />complicated by the influence of weather. There is no evidence as yet, however, that the <br />significance of undesirable tnixing events is higher when lake levels are low than when <br />15 <br />966 -d 680 /BZO*d 98Z -1 8998998808 snnosm iVaI m 100 -Woad Wk ZOOZ -80 -SU <br />