Biological Opinions - Bureau of Reclamation

survey records until the 1900’s, it is likely that these once superabundant species began to
decline in numbers around the turn of the 20 th century concurrent with significant destruction and
degradation of sucker habitat. Later, from the 1960s to the early 1980s, recreational harvests of
suckers in UKL progressively decreased (Markle and Cooperman 2002), which reflected further
declines in the LRS and SNS populations and led to their listing under the ESA in 1988. From
1995 to 1997, water quality-related die-offs killed thousands of adult suckers in UKL (Perkins et
al. 2000b). Over that three-year period, more than 7,000 dead suckers were collected and many
other dead suckers were likely present but not detected.
More recently (between 2002 and 2010), the abundance of LRS males in the lakeshore-spawning
subpopulation in UKL decreased by 50 to 60 percent, and the abundance of females in UKL
decreased by 29 to 44 percent (Hewitt et al. 2012; Figure 7.2). It is not clear if the riverspawning
subpopulation of the LRS in UKL has increased or decreased between 2002 and 2010
because of improvements in sampling methodology part way through the study that give the
appearance of a large influx of individuals, but it is likely that this population decreased
proportionately similar to the spring-spawning population (Hewitt et al. 2012).
Capture-recapture data indicate that the UKL SNS adult population decreased in abundance by
64 to 82 percent for males and 62 to 76 percent for females between 2001 and 2010 (Hewitt et al.
2012). Although the adult populations of both species in UKL have declined substantially, the
SNS adult population is at a greater risk of extirpation from UKL than LRS because it had
declined to a greater degree and there are approximately 10 times LRS in UKL than SNS (Hewitt
et al. 2012). If the trend from 2001 through 2010 continues for the SNS in UKL we may expect
that roughly 1,000 will remain by the end of the term of the BiOp in this water body. However,
the risk of extirpation becomes even more likely given that the relatively advanced age of most
individuals in UKL will likely result in an acceleration of declining trends during the BiOp term
as individuals begin to succumb to old age.
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