Biological Opinions - Bureau of Reclamation

vary by several orders of magnitude (Simon et al. 2012), and this variability is likely to have a
dramatic effect on entrainment rates. Additionally, estimated numbers of suckers entrained are
based on only a few years of data obtained in the late 1990s by Gutermuth et al. (2000a, b).
Because entrainment estimates are difficult to do and require extrapolations from short sampling
times to longer periods and from small samples to larger samples, the confidence limits of the
estimates are quite large.
Entrainment of larval suckers at the UKL outlet likely results from the interplay of multiple
factors that are incompletely known (Markle et al. 2009). Larval suckers have limited swimming
ability, are surface oriented, and therefore are vulnerable to down-lake transport by currents.
Modeling using data from measurements of currents in UKL (Cheng et al. 2005) indicates that
sucker larvae could be swept from spawning areas to the lake outlet in about 1 week (Reithel
2006, Markle et al. 2009). Most LRS and SNS larvae in UKL enter the lake along the eastern
shoreline, either from shoreline spawning or emigration from the Williamson River. This makes
them vulnerable to down-lake transport by the current that typically flows south along the eastern
shore of UKL to the lake outlet (Reithel 2006, Markle et al. 2009).
Information regarding UKL’s circulation suggests that larval suckers, particularly LRS larvae,
could also be retained in the wind-generated gyre (current) located farther offshore (Markle et al.
2009). Under prevailing northwest winds, the circulation in UKL is a clockwise gyre that
extends as far north as the shoreline between Agency Strait and Pelican Bay, and as far south as
Buck Island (Wood et al. 2006). This suggests that SNS larvae could be more vulnerable to
being entrained at the outlet of the lake than LRS larvae.
A Canal Entrainment Estimates
Although the A Canal is equipped with a state-of the-art fish screen meeting USFWS criteria,
approximately 50 percent of those that reach the fish screen pass are likely to pass through it and
are entrained into the canal system (USFWS 2008). This value is based on larval entrainment
evaluations at the A Canal fish screen (Bennetts et al. 2004). The other 50 percent of larvae and
all larger fish will be bypassed back to the upper Link River by a pump (typically from August
through October) or discharged by a gravity-operated flume to below the dam (typically April
through July). The pump bypass system uses a hidrostal pump that causes minimal injuries to
fish (Marine and Gorman 2005). The outlet of the pump-bypass flume is near the west bank of
the upper Link River, just downstream from the A Canal headgates and about 0.3 mi (0.5 km)
upstream from the Link River Dam.
Up to 1.6 million larval suckers could be entrained into the A Canal based on estimates
developed by Gutermuth et al. (2000a, b). However, that number assumes adult sucker
population sizes have remained constant since the late 1990s, which is not the case, as was
described above in the Status of the Species. Based on estimated changes in LRS and SNS
population sizes (Hewitt et al. 2011), and assuming no recruitment, the total number of adult
LRS and SNS in UKL has likely declined about 80 percent since 1998. Based on that, we
assume numbers of larvae present and in the lake and entrained at the A Canal has also decreased
because fewer adult females are now present and they would produce fewer eggs. Therefore, we
assume annual larval entrainment at the A-Canal is now 20 percent of what it was in 1998 and is
approximately no more than 320,000. Because this estimate is based on current LRS and SNS
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