7019

• UCREFRP Catalog Number: 7019
• Author: Hawkins, J. A.
• Title: Age and Growth Of Colorado Squawfish From the Upper Colorado River Basin, 1978-1990.
• USFW Year: 1991.
• USFW - Doc Type: Fort Collins, CO.
• Copyright Material: NO

INTRODUCTION

Age and growth information is essential for understanding fish populations. Age and
growth studies evaluate collective growth rates and age composition. Establishing a "typical
growth rate can help identify genetic and environmental factors that enhance or impede growth.
Although constrained by genetic limits, growth is influenced by environmental factors such as
temperature, photoperiod, habitat, and food. Knowing the age at which an individual or group
undergoes a certain activity can facilitate an understanding of life history. Age compositidn
shows the relative percentage of each age group within a population. Age series may be used to
assess population structure and changes in mortality. These factors can help assess current
population status and are critical for monitoring and managing a species.

Length and weight data can also characterize a population. The relationship of weight to
length can be either descriptive or predictive. A mathematical relationship is often used to
convert from one measure to the other, usuaIly by calculating the expected or average weight at a
given length. The equation describing the relationship can be used to differentiate populations,
subgroups, or other smaller taxonoIlJic units (Le Cren 1951). This relationship can also be used to
calculate condition which is often interpreted as an indicator of individual Or population
well- being (Bolger and Connolly 1989; Tesch 1968).

Humans are naturally curious about animals that are either large or old. Many published
papers on Colorado squawfish Ptychocheilus /ucius state that it is North America's largest minnow
(e.g. Haynes et al 1984; Tyus 1990). MilIer (1961) suggested an historical maximum length of 6
feet (1.8 m) and a weight of 100 pounds (45 kg) for Colorado squawfish in the lower Colorado
River. Early explorers in the 1800s reported catching Colorado Rjver salmon (Colorado
squawfish) by angling that ranged about 30 to 36 inches (762 to 914 mm) long (Dellenbaugh
1962). Photographs from the early 1900's support the historical Occurrence of large fish (Vanicek
1967; Minckley 1973; Seethaler 1978). Whether these large fish were common in the past remains
unknown, but few fish over 31 inches (787 mm) long have been coIlected in the last 25 years.
Behnke and Benson (1983) suggested that declines of large forage fish such as the bony tail Gila
e/egans may have led to a decline of large Colorado squawfish.

Three previous studies were conducted on age or growth of wild Colorado squawfish.
These include part of a dissertation (Vanicek 1967) that was later published (Vanicek and Kramer
1969), part of a master's thesis (Seethaler 1978), and a mimeographed report (Musker 1981) to the
US Fish and Wildlife Service (USFWS). Vanicek (1967) performed the first age and growth study
on Colorado squawfish collected from the Yampa and Green rivers within Dinosaur National
Monument during 1964-1966. He used scales to age 167 fish and back-calculated average length
and growth per group. The scale method was validated by: I) comparison and agreement with
length-frequency distributions of 0+ to 11+ age groups, and 2) agreement with calculated lengths
of fish coIlected in different years. Vanicek (1967) noted a decrease in average growth rates of
Colorado squawfish from 1958 to 1965, based on average grow.th obtained from back-calculation.
No important differences in growth or year-class strength were noted between years with
different flow regimes. Larger, older fish were not weIl represented with only 38 adults aged.
Age validation was limited to age groups less than 11+, and validation techniques were not applied
to older fish where accuracy of the scale method often .decreases.

Seethaler (1978) aged 68 fish from the Colorado River and 63 from the Green and Yampa
rivers. These fish were collected during 1974-1976. Size at sexual maturity was determined by
macroscopically examining 147 Colorado squawfish from the University of Utah fish coIlection.
Seethaler (1978) compared these data with those of Vanicek (1967) and determined that fish grew
faster in 1974 to 1976 than in 1964 to 1966. Growth rates were apparently equal between years,
but differed between the Colorado River and the Green and Yampa rivers. Seethaler (1978) was
unable to detect the first annulus on Colorado squawfish scales and adjusted his age data
accordingly. Validation methods were not discussed, except that his findings were similar to
those of Vanicek (1967).