Thesis - Nutrients, Cormorants, and Rainbow Trout in an Urban Lake, Reno, NV December 2008

This thesis, authored by Tom Skiles and published in December 2008, addresses some issues of Virginia Lake. Because this urban lake is a valuable resource to the residents of the Reno area, results are intended to provide information to those who manage its use and resources. Fish populations and water quality issues are addressed, specifically.

Abstract:
There are two chapters in this thesis, apart from this chapter (Introduction) and Chapter Four: Discussion and Final Remarks. Chapters Two (Nutrient Contributions By An Avian Community To An Urban Lake, Reno NV) and Three (Impact of Double-crested Cormorants On Rainbow Trout Stocked In An Urban Lake) were developed as independent manuscripts, though the research for each was conducted at the same place, Virginia Lake, Reno, NV. Chapter Two is an investigation of nutrient contributions by an avian community to an urban lake in Reno, NV. Chapter Three is an investigation of the impact of double-crested cormorants on rainbow trout stocked in the same lake, Virginia Lake.
Constructed in 1938, Virginia Lake is a 9 ha impoundment lake within the city limits of Reno, which, in recent times, has been plagued with poor water quality. In addition, the Nevada Department of Wildlife has suspected double-crested cormorants of depleting the stocked rainbow trout fishery, though it was unclear if stocked fish numbers were depleted primarily by predation or water quality, including excessively warm temperatures and periods of eutrophia within the lake, including depleted concentrations of dissolved oxygen in water.
The intent of our study, which was supported with funds from the Nevada Department of Wildlife and the Nevada Agricultural Experiment Station, was to examine the relative effects of each type of stress (predation and unfavorable aquatic environment) on stocked fish populations. At the outset, our study focused on mixing dynamics and phosphorous loading to the lake, including intensive measurement of temperature supplemented by other indicators of water quality in the lake. As the study progressed, we adopted the following objectives:
(1) to estimate the allochthonous total phosphorus (TP) loads derived from bird droppings,
(2) to compare bird-derived TP loads with loads from influent Truckee River water and total phosphorus (TP) mass present in the lake,
(3) to estimate survival of stocked rainbow trout, and
(4) to estimate harvest of these fish by anglers and double-crested cormorants.
Our studies for Chapters Two and Three relied on two primary methods: a census and sampling approach to estimating the type and number of birds resident at Virginia Lake over a 9 month period and a novel sampling approach to estimating predation losses among newly stocked trout, based on mark-recapture modeling and recovery of tags from tagged trout in a cormorant nesting area. These approaches are meant to provide insights useful to those trying to manage Virginia Lake to ensure that the lake remains an attractive resource for urban residents. We hope the results and recommendations contained in this document are useful to the agencies (the City of Reno Department of Parks and Recreation and the Nevada Department of Wildlife) and urban residents who appreciate this valuable resource.