9489

• UCREFRP Catalog Number: 9489
• Author: Mueller, G.
• Title: The Role of Stocking in the Reestablishment and Augmentation of Native Fish in the Lower Colorado river Mainstream ‎(1998-2002)‎.
• USFW Year: 1998-2
• USFW - Doc Type: 03-288,
• Copyright Material: NO

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transition was relatively simple, they did not have to waste precious energy learning these skills. Hatcheryreared
fish on the other hand did not possess these skills. Miller (1954) showed that stream reared trout out-performed
hatchery-reared trout and similar results have been reported for animals used by terrestrial programs (Conant,
1988; Griffith and others, 1989). Physical and behavioral stress associated with the transition demands time and
tremendous energy reserves. Stressors are accumulative in nature and can lead to poor performance and severe
cases lead to chronic fatigue and death (Wydoski and others, 1976).

Habitat Preference

The second obvious factor deals with the habitat selected by each species. The upstream portion of the study
area is represented by channelized river while the lower reach contains vast expanses of off-channel wetlands.
Spawning razorbacks prefer the latter while flanneImouth suckers remain upstream in the channelized portion of
the river year-round (Figure 5.1).
FlanneImouth suckers represent the most common sucker in the Colorado River basin. They. are most
abundant in large to medium sized tributaries that are higher gradient and are characterized by cooler
temperatures, rockier substrates, and are channelized. Adults avoid reservoirs or backwaters and are considered a
stream fish.
FlanneImouth suckers have successfully adapted to the physical habitat changes currently found immediately
downstream of Davis Dam. Historically the lower river was shallow, warm, sandy, and turbid. Operation of
Davis Dam has scoured the immediate river bed, leaving the channel armored with rock and large gravels. Today,
the river more closely resembles a higher gradient tributary. Flannelmouth distribution suggests they concentrate
in this area and tend to avoid downstream reaches dominated by drifting sand, warmer temperatures, and
numerous backwaters that are more common near Lake Havasu. Their preference for channel habitats would
reduce their exposure to predators such as largemouth bass, sunfish, and bullhead (Figure 5.2).
Habitat preference for razorback suckers is different, reflecting a dependence for floodplain wetlands that were
more abundant prior to human settlement. The vast expanses of To pock Swamp, once estimated at 1,740 ha, is
now isolated from the river by a levee (Beland, 1953). It is easy to understand why off -channel habitats served
as nurseries for their young and why they were preferred by adults (Bradford and Gurtin, 2000) (Figure 5.1).
Razorback suckers were essentially a slack water species, which helps explain their ability to spawn almost
anywhere. Today, remnants of those habitats are crowed with resident nonnative predators (Figure 5.2).

1m Flannelmouth sucker .. Razorback sucker I

Upper 40
km

Lower 40
km

o

1

2

3

4

Fish/100 m2 of trammel net

Figure 5.1. Graph showing the relative abundance of razorback and flannelmouth suckers in the upper and lower
40 kIn of river found between Davis Dam and Lake Havasu, 1999-2002.

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