9299

• UCREFRP Catalog Number: 9299
• Author: Anderson, R. and J. Mumma.
• Title: Aspinall Studies
• USFW Year: 1999.
• USFW - Doc Type: Annual assessment of Colorado Pikeminnow Larval Production in the Gunnison and Colorado Rivers, Colorado 1992-1996.
• Copyright Material: NO

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METHODS

Field Collections

Sampling for Colorado pikeminnow larvae was done using drift-net sampling at
three locations in 1992, at four locations in 1993 and at six locations in 1994, 1995 and
1996. The three stations sampled in all five years were: the Colorado River near Loma at
RK 247 (RM 153.7); the Colorado River just upstream of the Gunnison confluence at RK
275 (river mile 171.2); and the Gunnison River (below Redlands diversion) just upstream
of the confluence of the Colorado River (RK 3; RM 1.8). A station located on the
Gunnison River above the Redlands Diversion was established in 1993 at RK 8.8 (RM
5.5). Third stations were added to both the Colorado and Gunnison rivers in 1994. On
the Colorado River it was located upstream of Red lands Parkway bridge at RK 270 (RM
167.8) and on the Gunnison River the site was at Bridgeport at RK 47 (RM 29.3).

Sampling was done simultaneously at each site, and was conducted daily at dawn
throughout the month of July and into August depending on flow conditions. Passive,
near-shore drift-net sampling was conducted using two ichthyoplankton nets deployed
perpendicular to the shoreline at depths from 30 to 40 cm, and where velocities ranged
from 15 to 45 cm/s. The top of the net was positioned just below the water surface.
These nets were 0.5 m diameter, conical plankton nets mounted on 0.5 x 0.3 m
rectangular steel frames, and fitted with 33 cm-Iong, removable PVC buckets with a
threaded cod end (10 cm diameter). Each net had a 560 micron Nytex nylon mesh a
length of 4.0 m and an open-mesh to mouth ratio of 11 : 1. Each net was attached to two
steel rods driven into the substrate. Fish samples at all stations except Loma were field
processed and preserved in formalin. From 1993 to 1996 fish samples at Loma were
preserved in alcohol. Taxonomic identification and enumeration was done by CPMU
Larval Fish Lab in Fort Collins. Length was measured to the nearest 0.1 rom on all
Colorado pikeminnow larvae and length subsequently was used to estimate the age (days)
and the spawning date of each pikeminnow.

Data Analysis

Total drift was estimated for a one hour sample period by dividing the sample
count by the estimated proportion of flow sampled while the net was set. The percentage
of the total flow sampled was estimated as the ratio of the flow rate through the net to the
mean daily flow recorded at the gauge representing the sample site. Water volumes
passing through each net were calculated from start and finish velocity readings made with
either a Marsh-McBimey (Model 201) current meter or a General Oceanics Model 2030
flow meter suspended within the net mouth. Water temperature was measured at the start
of each sampling period.

Individual total length of all Colorado pikeminnow sampled were used in age-
growth equations developed by Haynes et al. (1985) to calculate the age (days post-
hatching) of each fish at capture. The date of spawning for that larvae was determined by

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