Biological Opinions - Bureau of Reclamation

which are more likely to occur offshore than SNS juveniles (Terwilliger 2006, Simon et al 2011,
2012). Habitats used by age-0 juveniles include vegetated and unvegetated areas with apparently
no particular substrate size, including fine substrates such as mud (Buettner and Scoppettone
1990; Simon et al. 2000, 2009; Terwilliger 2006; Hendrixson et al. 2007a, b; Burdick et al.
2009a). However, there is evidence that the juvenile suckers use rocky substrates, such as
gravel, more frequently than fine-grained substrates like mud (Terwilliger 2006; Simon et al.
2009). Access to diverse substrates might increase survival by enabling juvenile suckers to find
more food or avoid predators if environmental conditions affecting the distribution of food or
predators change through the summer. Additionally, water quality might vary over different
substrates because of the presence or absence of currents and the DO demand by organic-rich
sediments, which vary by location in UKL (Wood 2001). In general, rocky substrates in UKL
are found nearshore where sediments are swept away by waves and currents (Eilers and Eilers
2005). Because of the increased circulation and lower levels of organics in these sediments,
rocky areas should, in general, have higher levels of DO than those areas where mud
predominates.
The habitat diversity needs for age-0 juveniles of these species are unclear, but when lake levels
drop below about 4,140.0 ft (1,261.9 m) during August, vegetated wetland habitats become
dewatered, and as the lake recedes below 4,138.0 ft (1,261.3 m), rocky substrates become
increasingly scarce as nearshore habitats transition to mud (Simon et al. 1995, Bradbury et al.
2004, Eilers and Eilers 2005). Thus, as lake levels recede below 4,140.0 ft (1,261.9 m) and
especially below 4,139.0 ft (1,261.6 m), age-0 juveniles have fewer available habitats and could
be forced to move into areas where conditions (e.g., food, water quality, or predation) are less
favorable, which could have negative effects on their fitness and survival. At the lowest
modeled elevation at the end of August (i.e., 4,138.4 ft [1,261.4 m]), there would be almost no
habitat diversity and age-0 juvenile suckers would have to use muddy substrates.
Although we do not have data showing how habitat diversity affects survival of age-0 juveniles,
it is reasonable to assume if habitat becomes limiting it would affect survival. Because LRS age-
0 juveniles tend to use off-shore habitats where mud substrates dominate (Terwilliger 2006;
Simon et al 2010, 2011), they are less likely to be affected by low lake levels. However, because
SNS juveniles are more likely to use inshore areas and a greater diversity of substrates
(Terwilliger 2006; Simon et al 2010, 2011), they are more likely to be adversely affected by low
lake levels. Adverse effects are most likely to occur at elevations below 4,139.0 ft (1,261.6 m) in
August. Four of the 31 modeled years (13 percent) have elevations at or below 4,139.0 ft
(1,261.6 m) in August (Table 8.1). Under those conditions, SNS age-0 juveniles are likely to
experience low survival.
During September and October, age-0 juveniles appear to leave nearshore areas as the lake
elevation is nearing its annual minimum (Buettner and Scoppettone 1990, Terwilliger 2006). It
is not understood whether this seasonal movement by juveniles is related to decreasing
availability of nearshore habitats resulting from declines in lake surface elevations (USFWS
2002), or other causes, such as a biological response to other natural environmental cues or
changes in physiological demands during late summer (USBR 2007). In general, seasonal fish
migrations are thought to maximize fitness by increasing food availability, reducing predation, or
avoiding harsh environmental conditions (Brönmark et al. 2010).
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