Biological Opinions - Bureau of Reclamation

m) (1,380.93 m) (1,381.21 m) m)
50
4,530.41 (1,380.87
m)
4,531.28
(1,381.13 m)
4,532.28
(1,381.44 m)
4,532.05 (1,381.37
m)
During droughts, suckers concentrated in shallow water are likely to experience increased rates
of disease, parasitism, and bird predation (USFWS 2008). It is also reasonable to assume that
the resulting high densities of fish could deplete the food supply, causing additional stress, loss
of productivity, and possible mortality. In 1992, when Clear Lake elevation reached a minimum
of 4,519.40 ft (1,377.51 m) in October, suckers showed signs of stress by the following spring,
including low body weight, poor gonadal development, reduced juvenile growth rates, and high
incidence of external parasites and lamprey wounds (Reclamation 1994). At higher lake levels
in 1993 to 1995, overall fish body conditions improved, with increased body weight and fewer
external parasites and lamprey wounds observed (Scoppettone et al. 1995).
Bird predation on LRS and SNS in Clear Lake appears substantial. For example, in 2010 and
2011, there was evidence that 20 percent of suckers fitted with radio transmitters were consumed
by either pelicans or cormorants. Because this number was based only on transmitters recovered
from nesting colonies, and transmitters might have been deposited elsewhere, this value is
considered the minimal predation rate (Hewitt, USGS, pers. comm. 2012) experienced by
suckers during this time period.
Bird predation is likely to be more intense during periods of low water levels because the
shallow depths would enable pelicans to reach suckers in depths of less than 3 ft (less than 1 m).
Additionally, suckers are vulnerable to bird predation during spawning migrations, especially if
flows in Willow Creek decline sharply during migration, stranding suckers and making them
more visible. Although SNS might be most vulnerable to bird predation because of their longer
migration in Willow Creek, the larger size of LRS could make them more vulnerable throughout
the year because they are more easily detected. Additional studies are needed to determine the
full effect of bird predation on these populations.
Prolonged drought coupled with irrigation diversions, seepage, and evaporation results in a
substantial reduction in lake surface area and depth, and likely poses a threat to LRS and SNS.
Missing year-classes is likely evidence of these threats. Other potential threats at Clear Lake
include entrainment and stranding below the dam once irrigation diversions are terminated, but
no studies have been done to document these, so their effects are unknown.
7.11.2 Gerber Reservoir
The only listed suckers known to be present in Gerber Reservoir and its tributaries are SNS. The
primary known threat to SNS populations in Gerber Reservoir is an extended multiple-year
drought that would result in low lake levels that could initiate a fish die-off during the late
summer and fall, or during prolonged ice cover conditions in the winter (USFWS 2008);
however, these conditions have not occurred to date. During 1986 through 2004, irrigation
releases measured through Gerber Dam were 31,000 acre-feet from April through October, with
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