9678

• UCREFRP Catalog Number: 9678
• Author: Tyus, H. M. and C. A. Karp.
• Title: Habitat Use and Streamflow Needs of Rare and Endangered Fishes
• USFW Year: 1990.
• USFW - Doc Type: Flaming Gorge Studies - Draft Consolidated Report Preliminary.
• Copyright Material: NO

in Island Park or Jensen backwaters. ... Among the backwaters sampled, those
larger backwaters with narrow connections to the river, with a lower exchange
rate and a greater retention time (BA 300.5 and BA 251.0), generally had
higher densities of zooplankton.
... Fluctuations in riverflow that increase water level in backwaters
result in importation of riverine nutrients and POM into backwaters, as well
as resuspension of organic material from the inundated periphery of the
backwater with possible leaching of nutrients from organic material increase
turbidity which may provide cover for the native fish that evolved in this
ecosystem, and reduce the likelihood of predation on Colorado squawfish by
nonnative fish. Decreasing water levels in backwaters resulting from reduced
riverflows-may result in export of nutrients and biota to the river. The
extent of export of nutrients and biota to the river cannot be addressed at
this time.
Concentrations of major nutrients (nitrate-N, ammonia-N, and phosphorus)
generally increased in backwaters from upstream at Island Park to downstream
at Ouray. ... Ouray backwater nutrient concentrations exceeded river
concentrations. Higher nutrient concentrations in Ouray backwaters compared to
Island Park and Jensen backwaters and the river at Ouray may indicate internal
nutrient recycling within these backwaters or a response to attenuated
riverflows compared to upstream wherein nutrients transported into the
backwaters from the river during rising water levels are retained as riverflow
decreases, or leached from the inundated backwater shoreline. The less severe
action of inundation and draining in Ouray backwaters caused by attenuated
riverflows may reduce the tendency for violent mixing of water in backwaters,
and reduce export of POM and nutrients during draining.
Average macroinvertebrate abundance in backwaters decreased downstream
in 1987, but was greater in Ouray backwaters in 1988, compared both to 1987
and to upstream backwaters. However, average macroinvertebrate dry biomass
increased progressively downstream in both 1987 and 1988, with greatest dry
biomass present in Ouray backwaters, particularly.the reference site at river
mile 251. In many samples, chironomid larvae comprised over 90% of the benthic
fauna.
Benthic algae and detritus ... comprised a large portion of the stomach
contents of the young suckers collected. Predatory chironomid larvae,
although few in number in the backwaters sampled, likely prey on the grazing
chironomids. Food web interactions at this lower trophic level may result in
nutrient recycling within some Green River backwaters.
Food habit studies of 16 species of native and nonnative fish collected
from backwaters, indicated some dietary overlap due to heavy utilization of
chironomid larvae by some young fish <20mm TL. The 14 Colorado squawfish <20mm
TL collected in 1987 and 1988 consumed mostly chironomid larvae, while
Colorado squawfish >20mm TL showed evidence of piscivory, but continued to
consume chironomid larvae. As the Colorado squawfish grow and include larval
fishes, primarily red shiner, in their diet, along with chironomid larvae,
dietary overlap with other fish species diminishes because of the expanded
food resource. The stomachs of some young fish, such as the native suckers,
contained mostly algae. Few fish species other than Colorado squawfish and
Gila spp. consumed larval fish. After Colorado squawfish become piscivorous,
red shiners, fathead minnows, occasional catostomids, and other introduced
fish species are found in the diet suggesting less dependence on the
relatively abundant chironomid larvae.
14