Date of Award:

1992

Document Type:

Thesis

Degree Name:

Master of Science (MS)

Department:

Wildland Resources

Advisor/Chair:

Alis Launder

Abstract

I investigated the interaction of lake trout (Salvelinus namaycush) and their dominant forage fish populations, Utah chub (Gila atraria) and kokanee salmon (Oncorhynchus nerka), in Flaming Gorge Reservoir, Wyoming-Utah. Through bioenergetics modeling, I quantified the consumption dynamics of the lake trout population. From hydroacoustics analyses, I quantified the density and biomass of the two dominant forage fish populations.

In Chapter II, I report the results of the energetics analysis. The objective of this chapter was to understand the role of lake trout predation in recent changes in fish assemblage structure of the reservoir. Through lake trout diet analysis and exploration of forage fish growth rates, I quantified the duration of time that chubs and kokanee are vulnerable to lake trout predation. Faster growth rates of kokanee greatly reduce the duration of time that this species is vulnerable to predation relative to Utah chubs. Although chubs are more fecund than kokanee, this advantage in reproductive potential may not make up for differences in duration of vulnerability. I predict that kokanee will make up an even larger proportion of the total fish assemblage of the reservoir in future years.

In Chapter III, I compare annual estimates of lake trout consumption demand to biomass estimates of forage fish. I used vertical gill net sampling, beach seine surveys, and hydroaocustics to assess the distributions and biomasses of the Utah chub and kokanee populations. Biomasses of pelagic Utah chubs and kokanee were calculated to be 83 300 and 209 000 kg, respectively. Energetics analyses indicated that between 1985 and 1989 the lake trout population consumed 79 000 kg of chub and 196 000 kg of kokanee per year. These results suggest ' that forage fish populations should decline in future years. Annual consumption demand of lake trout between 400 and 600 mm (137 000 kg) exceeded biomass estimates of forage fish of useable size (22 000 kg), suggesting that this size-class of predator is currently food-limited. High occurrence of invertebrate prey taxa in the diet of small predators supports this food-limitation hypothesis. The lack of small pelagic forage fishes may reduce the ability of lake trout to recruit to sizes that are accessible to anglers and of value to the fishery.

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