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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11. In addition to being effective predators on larval endangered fishes,
juvenile nonnative fish species are more than likely significant
competitors with larval and juvenile endangered fishes.

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Competition by two species occurs when food resources are limited, the
food is shared, and one of the species is adversely affected.
Competition among freshwater fish species is often difficult to document
because these fishes lack specialization in food habits so that much
overlap occurs in their food habits. However, the extremely low
densities of zooplankton during the spring runoff that serve as food for
larval razorback suckers and the high percentage of nonnative fishes in
backwaters of Upper Basin rivers provide evidence that competition may
also reduce survival of razorback larvae.

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12. Laboratory and pond studies conducted at the Dexter National Fish
Hatchery, New Mexico documented that about 2 months are required for
larval razorback suckers to achieve 25 mm TL (1 in). Flooding of
terraces or depressions for a short period of time (i.e., 7 to 10 days)
will not be adequate for larval razorback suckers to reach 25 mm TL when
they would no longer be vulnerable to abundant adult red shiners based on
size of the mouth-gape from studies conducted through Utah State
University. However, gape size is irrelevant to predation by the fathead
minnow because this species attacked catostomid larvae as a school, tore
the prey into pieces, and consumed the pieces in a Pacific Northwest
study. It is not known if other small nonnative fish species in the
Upper Basin are capable of tearing larvae into pieces and consuming the
pieces.

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Red shiners and fathead minnows compose 90% or more of the fish that
occupy backwater habitats in Upper Basin rivers based on the Recovery
Program's Interagency Standardized Monitoring Program. When levees were
removed to reconnect floodplain habitats with Upper Basin rivers,
nonnative fishes quickly colonized and dominated these habitats in the
Upper Colorado and middle Green rivers.

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13. Starvation of larval razorback suckers during their critical period from
the loss of productive floodplain habitats through regulated streamflows
combined with a high vulnerability to predation by nonnative fishes
appear to be the most important factors limiting recruitment in the Upper
Basin.

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14. Reconnecting floodplain habitats with rivers in the Upper Basin is
expected to benefit razorback suckers since these habitats will provide
adequate quantity and quality of food organisms that are required by
larval razorback suckers to survive their "critical period". Larvae and
juveniles of other fishes including the other endangered species
(Colorado squawfish, humpback chub, and bony tail) are also expected to
benefit from zooplankton and benthic macroinvertebrates that enter the
main channel and backwaters from floodplain habitats.

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Predation and competition from nonnative fishes on native fish larvae and
juveniles can be reduced in floodplain habitats with high densities of
zooplankton and benthic food organisms that can serve as alternate food
items. Floodplain habitats with rooted aquatic vegetation or other
structure also provide protection to razorback larvae and juveniles that
readily use such cover when available. In contrast, there is little to
no survival of larval razorback suckers in the present low velocity
habitats (primarily backwaters without cover) in Upper Basin rivers.

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15. Historically, the continuum concept and the flood pulse concept both
applied to nutrient cycling in the turbid, unproductive rivers of the
Upper Colorado River Basin. However, fragmentation of Upper Basin rivers

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