7354

• UCREFRP Catalog Number: 7354
• Author: Wydoski, R. S.
• Title: Biological Review of Endangered or Imperiled Fish in the Upper Colorado River Basin.
• USFW Year: 1985.
• USFW - Doc Type: Washington, D. C.
• Copyright Material: NO

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recovery process. In addition, if work is not progressing in a manner that will
meet measurable objectives, either changes must be made in the study design or
the study should be terminated or postponed. Other work that can be classified
as "nice to know" should be placed on a low priority. Success of the recovery
program must demonstrate positive results and be accompanied by rational
criteria to maintain support in the cooperative endeavor.
The following topics are offered to the Biological Subcommittee for
consideration based on my review of the published literature and unpublished
reports as well as my discussions with various investigators:
LIFE HISTORY AND ECOLOGICAL REQUIREMENTS OF RARE COLORADO RIVER FISH
o Evaluate techniques for external sexing of adult fish, aging to determine
the age composition of existing stocks, and marking of small fish for
identification during monitoring the success of the recovery efforts.
Rationale - Various techniques must be developed to better
understand and interpret the life history and ecology of the
rare fish. Interpretation of distribution and movement of
adult fish would be greatly enhanced if the sex of the fish
were known. A number of techniques for external sex
determination have been described for a variety of fish
species. Some of these techniques may be directly applicable
to the rare fish or would be applicable with modification.
Although it is known that the Colorado squawfish and
razorback sucker live to an old age, the age of these species
cannot be determined easily after they reach sexual maturity
when growth is very slow or does not occur. Finally,
numerous investigators have concluded that curtailment of
reproduction is one of the main reasons for the low numbers
of some of the UCRB fish. Available data support the
phenomenon of larval drift in the Colorado squawfish and the
published literature also demonstrates that this phenomenon
is common for riverine species. The ability to mark small
fish to determine the extent of their dispersion through
passive drift during the early larval stages and through
active movement afterward would add to an understanding of
the dynamics during the early life stages. It would also
provide insight into the significance of this dispersal
mechanism in the ecology of the rare fish.
Considerations - Obviously the first consideration is that
this information can be obtained without killing the rare
fish. Sex determination has been done externally by
developing simple relationships on the length of certain fins
or by examination of the genital openings with a probe. In
the Gila sp., there appears to be a relation between the
length of the pectoral fin and sex. Males appear to have
pectoral fins that extend beyond the origin of the pelvic
fins while females have shorter pectoral fins that do not
reach the origin of the pelvic fins. In some catostomids,
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