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Evidence of striped bass natural reproduction in Lake Powell was first discovered in 1979
and annually thereafter. Initially, it was thought
that reproduction was confined to the Colorado
River above the lake where river current could
suspend eggs and larvae. The spawning site was
in or below Cataract Canyon, a 19-km gorge
containing 23 sets of rapids. Striped bass apparently used less than 20 km of river above the
reservoir because ripe striped bass adults were
18 gustaveson and blommer
collected below but not above Cataract Canyon
during spawning season (Persons and Bulkley
1982).
During spring 1979, striped bass spawning was discovered in the lower reservoir near
the dam. In spring of most years, prespawning striped bass aggregated near Glen Canyon
Dam. These fish seemed attracted to the current created as water was drawn through the
dam penstocks. With time and warming, the
aggregation left the 165-m-deep dam forebay
and moved to nearby coves where spawning occurred. Most fish left the prespawning staging
area simultaneously during early May as water
surface temperature reached 16–19°C, which
is the generally accepted peak spawning temperature range (Setzler et al. 1980). Spawning
sites were located near the dam where floating
masses of dead (unfertilized) eggs were clearly
visible. J. D. Bayless (South Carolina Wildlife
and Marine Resources Department, personal
communication) found that unfertilized eggs
would float but that immediately after fertilization, eggs would sink 0.3 m in 27 s. Settling
rate slowed with time, but eggs still descended
0.3 m every 60 s some 24 h after fertilization
(Gustaveson et al. 1984).
An oxygen-temperature profile taken in
spawning coves near the dam showed oxygen
levels of 8.4 mg/L near the surface and 13.2
mg/L on the substrate in 9 m of water. Siltation
was insignificant in these coves on the rock and
sand substrate during the brief 48-h incubation
period (Hardy 1978). Thus, eggs settling on the
bottom of Lake Powell had adequate oxygen
for normal development.
Collection of larval striped bass 18–22 mm
total length (TL) with midwater trawl and meter tow net samples confirmed successful reproduction of striped bass near the dam. The collected larvae were determined not to be derived
from the river upstream of the reservoir. Prolarvae are capable of swimming at 4 d of age and
could be expected to travel great distances if assisted by strong mainstream reservoir currents.
However, studies of density currents using total
dissolved solids as an indicator showed a weak
density current in the reservoir that could not
have assisted in moving larvae from the inflow
any closer to the dam than 190 km (D. Merritt, U.S. Bureau of Reclamation, personal communication). Instead, striped bass larvae were
captured 295 km downstream from the inflow.
If spawning occurred only at inflow areas and
larval fish were distributed by reservoir currents, then larval fish should have been found
throughout the reservoir. In contrast, there was
a preponderance of young striped bass at two
distinct locations, the inflow and again at the
dam. It was evident from these analyses that
these young striped bass found near the dam
were the result of successful in-reservoir spawning (Gustaveson et al. 1984).
Over the years, it has been confirmed that
striped bass spawning occurs annually in Lake
Powell, Lake Mead, Lake Mohave (Liles 1985),
and Lake Havasu. Eggs and larval striped bass
also pass through the dams to provide additional recruits to downstream lakes and canals
as water is delivered from the Colorado River
to locations in California, Nevada and Arizona
(Stewart and Burrell 2013).[4]
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