8142

• UCREFRP Catalog Number: 8142
• Author: Horn, M. J.
• Title: Nutritional Limitation of Recruitment in the Razorback Sucker ‎(
• USFW Year: 1996.
• Copyright Material: NO

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17

temperature, and following a few days of additional development swim-up and begin

actively feeding (Minckley and Gustafson 1982; Marsh 1985). After swim-up, larval

razorback suckers actively seek out current and readily move into the downstream drift

(Harold Tyus, University of Colorado, personal communication).
Although substantial numbers of larvae are produced every year in lower

Colorado River reservoirs, the populations of adult fish have been in decline since the

early 1900's and there has been little if any documented recruitment in the past several

decades (Dill 1944; Minckley et al. 1991). The razorback was formerly one of the most

abundant fish in the basin, found throughout the Colorado River and all its major

tributaries (Holden 1980; Minckley et al. 1991). Today, its range is restricted in the lower

basin to an estimated 25,000 fish that inhabit Lake Mohave, a few scattered individuals

along the lower river, and at most several hundred individuals scattered throughout the

upper basin (McCada and Wydowski 1980; Lanigan and Tyus 1989; Paul Marsh, Arizona
State University, personal communication). It was listed as an endangered species in
1991 (USFWS 1991).

There are three general hypotheses to explain the lack of recruitment success.

One of these is nutrition. Cessation of annual flooding and alteration of seasonal flow

patterns and channelization have affected productivity in riverine sections and decreased

the utility of connected backwaters as habitat. Daily flow fluctuations have reduced the

productivity and availability as refugia of many shoreline habitats. In reservoirs, larvae

hatch into habitats very different than those historically. Plankton densities are typically

lower and more variable in Lake Mohave during spring than in adjacent backwaters