9458

• UCREFRP Catalog Number: 9458
• Author: National Park Service.
• Title: Workshop on Population Estimates for Colorado Pikeminnow and Humpback Chub.
• USFW Year: 2001.
• USFW - Doc Type: Fort Collins.
• Copyright Material: NO

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SAN JUAN RIVER COLORADO PIKEMINNOW POPULATION ESTIMATES

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By:
Dale Ryden
U.S. Fish and Wildlife Service
Colorado River Fishery Project
Grand Junction, Colorado
December 6, 2001

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Abstract

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A Schnabel multiple-census population estimate (following Van Den Avyle 1993) was performed,
based on captures and recaptures of adult Colorado pikeminnow in the San Juan River between 1991 and
1995. Associated confidence intervals were calculated using Poisson distribution tables (from Ricker
1975) to compensate for small numbers of captures/recaptures. This population estimate was verified
using a Petersen population estimate with Bailey's modification (following Van Den Avyle 1975) to
compensate for small numbers of captures/recaptures.
Between June 1991 and October 1995, only 17 adult Colorado pikeminnow were collected in the
entire San Juan River. However, population estimates could only be performed using 15 (88.2%) of
these adult fish collected on the 15 sampling trips performed between June 1991 and October 1995 (three
trips per calendar year) from river miles (RM) 136.6-119.2 (17.4 RM). The reason for this was that RM
136.6-119.2 was the only common section of the San Juan River sampled during all 15 sampling trips..
Population estimates could not be done for later years because after October 1995, only one wild adult
Colorado pikeminnow was captured in the San Juan River.
The Schnabel population estimate yielded a value of 19 adult fish, with 95% confidence intervals
(C.!.) = 10-42 adult fish (99% c.!. = 8-54 fish). The Petersen population estimate yielded estimates
ranging from 9-20 fish for the 14 sampling intervals involved.
It should be remembered that the study from which the data to perform these population estimates
was taken was not intended to be a population estimate study, but was rather a fish community
characterization study. The almost complete lack of wild Colorado pike minnow collections from 1996-
200 I has precluded a more all-inclusive and up-to-date population estimate from being performed.
Adult Colorado pikeminnow have been documented (via radio telemetry) to actively avoid capture
efforts (electro fishing and trammel-netting). In the San Juan River numbers of adult Colorado
pikeminnow are low, making random capture in the river's main channel rare. In addition, fish cannot be
trapped and sampled in off-channel habitats (backwaters, gravel pit ponds, sloughs, oxbows, etc.), which
the San Juan lacks. Given these facts, it is likely that until the number of adult Colorado pikeminnow in
the San Juan River is bolstered viaaugmentation, a relevant, riverwide population estimate may be
impossible to obtain.
While one of the assumptions ofthe Schnabel population estimate (0% mortality) was violated,
verification via the Petersen population estimate indicates that the values obtained via the Schnabel
population estimate were probably very close to the mark. Given that all but two adult Colorado
pikeminnow were collected from RM 136.6-119.2 between June 1991 and October 1995, those numbers
can probably be loosely extrapolated to the river as a whole. It is my belief that there were probably less
than 40 adult Colorado pikeminnow in the entire San Juan River as of October 1995. That total is
undoubtedly less today. However, at present, there is NO reliable population estimate for the number of
adult Colorado pikeminnow (wild and stocked) currently residing in the San Juan River.

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