9410

• UCREFRP Catalog Number: 9410
• Author: Wydoski, R. S. and E. J. Wick.
• Title: Ecological Value of Floodplain Habitats to Razorback Suckers in the Upper Colorado River Basin.
• USFW Year: 1998.
• USFW - Doc Type: Denver.
• Copyright Material: NO

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"point of no return" or "point of irreversible starvation" occurred
between 19 and 22 days.

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6.

The first food organisms of larval razorback suckers are diatoms,
rotifers, algae, and detritus. Soon afterward, razorback larvae begin to
select larger zooplankton organisms, primarily cladocerans and copepods.
Razorback larvae collected from shallow backwaters in Lake Mohave in the
Lower Basin and the Green River in the Upper Basin also ate early instar
chironomids and trichopterans. However, it must be recognized that fish
larvae as well as zooplankton and free-swimming benthic invertebrates are
captured in light traps, suggesting that the razorback larvae were
opportunistic in feeding on the concentrated benthic organisms. As
razorback larvae increase in size, they will select larger zooplankton
and small benthic organisms as food. Zooplankton and benthic
invertebrates are eaten by all life stages (larvae, juveniles, and
adults) of razorback suckers.

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7.

Nutrition and subsequent growth rate of larval fishes is extremely
important because smaller fish that are in poor condition (i.e., starved)
with limited locomotive ability are more susceptible to predation for a
longer period of time. Razorback sucker larvae that were deprived food
in the laboratory showed an initial increase in length as they utilized
remaining yolk reserves but they were significantly less in total length
and weight than larvae fed ad libitum at temperatures of 14, 18, and 23
C.

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Razorback sucker and other fish larvae, including razorback sucker,
exhibit compensatory growth and can recover quickly from short periods of
starvation if they encounter high prey densities before they reach the
"point of irreversible starvation".

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8. Zooplankton densities in the main channel never reached densities
required for larval razorback suckers to survive their critical period.
Zooplankton densities that were adequate for larval razorbacks during the
critical period were found in only two of the largest backwaters sampled
(Intersection Wash in the middle Green River and Millard Canyon in the
lower Green River). However, zooplankton densities, necessary for
survival during the critical period, were reached consistently in
floodplain habitats.

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9. Only a portion of the zooplankton or benthic invertebrate biomass is
available to razorback larvae since their mouths are gape-limited and
they tend to select the largest organisms that will fit into their
mouths. Aquatic organisms normally found in the water column such as
zooplankton also occur in benthic samples and benthic organisms that are
either free-swimming or emerging pupae also occur in the water column.
Larval fishes, including razorback suckers, feed on both benthic and
planktonic food organisms of the right size.

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10. Razorback suckers spawn in the spring on the ascending limb of the
hydrograph when extremely low densities of small food organisms first
used by larvae occur in the main river channel and backwaters, suggesting
that the life history strategy of this species evolved to utilize the
high productivity of floodplain habitats. Therefore, starvation may be
an important factor in survival of larval razorbacks during their
critical period.

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Drifting razorback larvae during the spring runoff are also highly
vulnerable to predation by nonnative fishes since razorback larvae
constitute the largest portion of drifting aquatic organisms entering
backwaters used by nonnative fishes.

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