Date of Award:

5-1986

Document Type:

Thesis

Degree Name:

Master of Science (MS)

Department:

Wildland Resources

Department name when degree awarded

Fisheries and Wildlife

Committee Chair(s)

Vincent Lamarra

Committee

Vincent Lamarra

Committee

Fred Post

Committee

Bill Helm

Abstract

The Bonneville cisco (Prospium gemmiferum), a small planktivorous whitefish, is an important part of the distinctive fish community of Bear Lake, Utah-Idaho. The Bonneville cisco plays a key role in the trophic structure by converting zooplankton to fish biomass and providing a major forage source for cutthroat and lake trout. Aspects of cisco feeding ecology studied include characterization of the zooplankton community composition and dynamics and cisco feeding habits and prey selection.

Composition and seasonal dynamics of the zooplankton community were determined for a fifteen month period during 1981-1982. The community was dominated by a calanoid copepod, Epischura, and a colonial rotifer, Conochilus. Cladocerans, primarily Bosmina and Diaphanosoma, comprised only a minor portion of the community, never exceeding five percent. The diel vertical distribution of the zooplankton was examined during five months in 1981. Zooplankton were concentrated in the epilimnion and metalimnion and no evidence of diel vertical migration was detected with the 10 m depth interval sampling scheme used. Some cladocerans were found to utilize refuges in the epilimnion and hypolimnion to avoid intense predation pressure by cisco.

Food habits and prey selection of cisco were examined during five months of 1981. Changes in the zooplankton community were reflected by changes in the food composition of cisco from previously reported studies in 1943. The dominant zooplankter, Epischura, remained an important food item for cisco. Departing from historical information, cladocerans like Diaphanosoma and Daphnia were major food items during periods of seasonal peak abundance. Cladocerans were consistently the most preferred prey items, being utilized in greater proportion than their abundance in the plankton. Also, cladocerans and copepods were nearly always preyed on in size-selective fashion.

Cisco were brought into the laboratory to determine feeding modes they used to capture prey. The reactive distance, or the minimum distance at which the predator can locate specific prey items, was determined for three sizes of Daphnia prey. Cisco were found to have superior prey location ability than other salmonids reported in the literature, having the ability near that of some centrarchids.

Characteristics of copepods and cladocerans such as their reproductive strategies, sizes, movement patterns, evasive capabilities and pigmentation were examined in relation to their importance in affecting cisco diets and prey selection. Cisco predation was clearly related to the vertical distribution and dynamics of some zooplankton prey.

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