Biological Opinions - Bureau of Reclamation

By late July, surviving larval suckers have metamorphosed into age-0 juveniles. Water quality
has become highly dynamic, with wide daily swings in DO, pH, and total ammonia, which cause
stress in the fish. Water temperatures peak at this time, reducing the capacity of the water to
hold DO in solution. Higher temperatures also raise energy demands of fish, thus adding stress.
Cyanotoxins can also be present at this time and, when concentrations are high, they damage the
gut and liver, impacting the health of the fish, and leading to stress or mortality. Parasites,
including protozoans like Tricodina, the copepod Lernaea, and the trematode Neascus, are also
attacking the juveniles, adding additional stress and mortality. When fish are highly stressed and
water temperatures high, protozoan parasites can multiply quickly, causing death in a few days.
Additionally, in August and September, lake levels are declining and preferred habitats where
food might be most abundant are disappearing, perhaps causing the juveniles suckers to relocate
to areas where food might be less abundant. This movement also expends energy and further
stresses the fish, and could increase their exposure to predators, especially fish-eating birds. Of
those juveniles that survive, many end up at the south end of the lake and are entrained at the
Link River Dam or in the forebay of the A Canal. Consequently, by early fall, very few
juveniles survive in most years to enter the adult population 4 to 7 years later.
Table 7.9 Threats to the LRS and the SNS in UKL.
Threat
Entrainment at
Link River
Nature of
Threat
Mortality and
loss from
population
Life Stage
Affected
Mostly affects
larvae and age-0
juveniles
Primary Effect
Studies show
this occurs
annually, but
extent varies
among years
Mitigating
Factor(s)
Will only
affect larvae
and juveniles at
south end of
UKL
References
Gutermuth et
al. 2000a, b
Low
Dissolved
Oxygen
Concentrations
High Total
Ammonia
Concentrations
Mortality or
stress and
reduced
productivity
Mortality or
stress and
reduced
productivity
Juveniles and
adults mostly
due to timing
Juveniles and
adults mostly
due to timing
98
Good evidence
that this led to
die-offs in
1990s
New analyses
show water
quality exceeds
LC 50 values
more than any
other parameter
Lethal
conditions are
variable in time
and space and
are unlikely to
cover large
areas of the
lake;
fish should be
able to avoid
affected areas
to some degree
Lethal
conditions are
variable in time
and space and
are unlikely to
cover large
areas of the
lake;
fish should be
Perkins et al.
2000b
B. Martin,
USGS, pers.
comm., 2012