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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EXECUTIVE SUMMARY

This report reviews the ecological value of floodplain habitats to recovery of
the razorback sucker and the anticipated responses of other endangered,
native, and nonnative fishes to floodplain habitat enhancement/restoration
activities in the Upper Colorado River Basin (Upper Basin). It is intended to
serve as a reference document for persons working on habitat enhancement
projects related to the Recovery Implementation Program for Endangered Fishes
in the Upper Colorado River Basin (Recovery Program) .

The conclusions summarize highlights of the information reviewed in the report
and the recommendations are based on rationale provided in the text that are
supported by the literature cited.

The report emphasizes the need for integration of all Recovery Program
elements, especially streamflow management, habitat development and
maintenance, management of nonnative fishes and sport fishing, and captive
propagation/stocking that must be done concurrently to achieve self-sustaining
populations (i.e., recovery).

Conclusions:

1.

The declining numbers of some endemic Colorado River fishes including the
razorback sucker is attributed to extremely low or complete lack of
recruitment. Although long-lived razorback sucker can spawn successfully
in some years and produce larvae, high mortality during the early life
stages limits recruitment in Upper Basin razorback stocks to the point
that they are no longer self-sustaining.

2.

Habitat alteration and nonnative fish introductions were considered to be
the two most important factors in the extinction of 40 native North
American fishes (27 species and 13 subspecies) during the past century.
These two factors also appear to be extremely important in the decline of
razorback sucker stocks in the Upper Basin and are undoubtedly related to
the decline of the other three endangered Colorado River fishes.

3.

Shifts in survival during early life stages of fish populations most
often result in a decline of populations in altered aquatic habitats.
Recruitment of fishes is curtailed primarily from mortality during the
larval stage from either starvation, predation, or both.

4.

Razorback larvae are 9-11 mm TL at swimup and larvae between 11 and 12 mm
TL with a mean age of 12-17 days predominated light trap captures in the
middle and lower Green River from 1992 to 1996. Approximately 20% of all
razorback larvae captured in the Green River were larger than 12 mm TL
with the two largest specimens at 20 and 24 mm TL. The oldest larva was
34 days old at capture. These results demonstrate that high mortality
occurs in the early life stages.

Similar results have been reported in the Lower Colorado River Basin.
Most razorback sucker larvae captured in Lake Mohave were less than 20
days old and averaged 11.4 mm TL where mortality from starvation was
estimated to be between 23% and 78% from 1992 to 1995, depending upon the
year of capture and nutritional index used. The remaining razorback
larvae succumb to predation by nonnative fishes, resulting in no
recruitment from natural reproduction in Lake Mohave.

5.

The density of zooplankton required for survival of larval razorback
suckers during the "critical period", when larvae are making the
transition from endogenous (yolk sac) to exogenous (mostly small
invertebrates) nutrition, was 30-60 organisms per fish per day based on
work completed at the Dexter National Fish Hatchery, New Mexico. The

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