8067

• UCREFRP Catalog Number: 8067
• Author: Nance, E. L.
• Title: Planktonic and Benthic Invertebrate Densities in Three Backwater Habitats on the Lower Green River Final Report.
• USFW Year: 1997.
• USFW - Doc Type: Grand Junction, CO.
• Copyright Material: NO

the quality of potential food items remains in question. There are no references in the
literature which corroborate benthic feeding of nematodes by larval razorback suckers.
Microcrustacean populations, a major food item utilized by larval fish, were dominant in
the plankton samples, but were virtually absent from the benthic samples, suggesting that
they are available to fish only in planktonic forms. Other food preferences such as
chironomid larvae were also notably absent from both planktonic and benthic samples,
nor were emergent adults observed. Chironomid populations most likely develop later in
the season when higher water temperatures are more favorable to their development.

The often patchy distribution of zooplankton, especially the microcrustacea, needs to be
addressed. Five vertical tows and core samples seemed adequate to describe
zooplankton populations in both Anderson and Holeman. However, the Millard
backwater was much larger, deeper, and had many more microhabitats to provide for
zooplankton development. While the invertebrate counts presented here are generally
consistent across the five tows, there were instances where it was obvious that local
populations of zooplankton in certain areas of the backwater were sampled while others
were missed. The data also suggests that vertical zooplankton migrations during the night
did not occur in the Millard backwater. Whether this is also true in the Millard backwater
where zooplankton populations were higher, needs to be explored.

The large question that remains is whether or not larval razorback suckers are benthic
feeders, and if they are, can they subsist on a diet comprised mainly of nematodes? In a
study in Lake Mohave, Langhorst and Marsh (1986) found amorphous organic matter,
cladocerans, rotifers and copepods to be the dominant gut contents in larval razorbacks.
They made no mention of nematodes in either their planktonic or benthic samples. Other
questions that need to be addressed are gape size of larval fish in relation to the size of
food items available when they begin appear in these backwaters; whether or not larvae
are visual feeders; and what are the effects of competition with the huge populations of
non-native larval fish which also utilize these backwaters. Finally and most importantly,
further research needs to elucidate where razorback larvae forage, what food items are
present in their gut, and there needs to be an overall finer resolution between larval food
habits and food availability.

In conclusion, it is recommended that the Millard canyon backwater be the focus of any
future management strategy research, whether nutrient supplementation, exclosures, or
larval confinement studies. Not only is Millard larger in size and volume, it is also
easiest to access by boat early in the season before river levels rise. In terms of scale
alone, Millard can provide larval fish a greater opportunity to feed and/or escape
predation. Finally, densities of zooplankton were highest here, silt loading from the main
river channel seemed to settle more quickly, and unlike Anderson and Holeman, there are
no established campsites in the area where river traffic can impede research efforts.

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