Biological Opinions - Bureau of Reclamation

about 4,520 ft (1,378 m), and is effectively unavailable to adult suckers when water levels are
less than about 4,523 ft (1,379 m). Based upon the POR, there is a 20 percent probability (which
equals approximately 22 years out of 110 year POR) that lake levels will reach of 4,523 ft (1,379
m) or less during the year (Table 8.8). Thus, based on the POR, 20 percent of the time lake
levels during the term of this BiOp are likely to be at an elevation that is likely to cause adult
suckers to avoid the west lobe of the lake or expose them to increased risk of pelican predation.
At the proposed action minimum surface elevation of 4,520.6 ft (1,377.9 m) at the end of
September most of the east lobe is dry, except for the deeper pool nearest the dam into which
Willow Creek flows. Based on the POR, elevations this low should be rare, because they
occurred in the POR at a frequency of 5 percent (approximately 6 years out of 110 years).
However, because 2 of the 8 years in the POR when this happened were in the past decade (2004
and 2010), the incidence of low lake levels is likely to be greater during the term of this BiOp
than the POR suggests.
During droughts, the proposed action at Clear Lake is anticipated to adversely impact older
juvenile and adult suckers by reducing habitat availability, particularly lake surface area and
depth. When water depths are shallow, suckers could experience reduced body condition (i.e.,
be thin and have low fat reserves), have increased rates of parasitism, and be in poor health,
which can lead to low productivity and perhaps increased mortality (USFWS 2008).
Additionally, because in some years there is a large pelican rookery in Clear Lake, pelican
predation is also likely to increase due to shallow water depths, as mentioned above.
It should be noted that low water levels in Clear Lake were likely normal prior to the
construction of the Clear Lake Dam. In fact, much of the east lobe was a meadow that was used
to grow hay (USFWS 2002). Reclamation’s 1905 map of Clear Lake shows that the deeper area
of the east lobe was a marsh. Thus, historically, LRS and SNS in Clear Lake apparently had to
cope with and adapted to varying water levels.
The minimum lake elevation being proposed for Clear Lake (i.e., 4,520.6 ft [1,377.9 m]) has not
changed from minimums previously consulted on. Current monitoring data for SNS shows
evidence of frequent recruitment (i.e., multiple size classes are present; Hewitt and Janney 2011).
Therefore, it appears that droughts and resulting low lake levels, although are likely to have
adverse effects, has not resulted in population-level effects that we have detected and thus,
varying lake levels do not appear to be limiting the persistence of SNS in Clear Lake.
Current data for LRS indicates that there has been little recent recruitment in Clear Lake (Hewitt
and Janney 2011), as described in section 7, Status of the Species. The cause of this problem is
unknown. However, so called “recruitment droughts” are common among western lake suckers
(Scoppettone and Vinyard 1991); although the causes are unknown and all western lake suckers
are affected to some degree by water management. We don’t know exactly what is limiting LRS
recruitment but Project operations cannot be ruled out because there are several potential ways
that lake level management resulting in low lake levels could affect recruitment, including
drought stress and increased vulnerability to pelican predation. However, low lake elevations
below 4,523 ft (1,379 m) are likely to be uncommon events based upon the POR and therefore
not likely to be limiting the persistence of LRS in Clear Lake.
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