Semester

Spring

Date of Graduation

2006

Document Type

Dissertation

Degree Type

PhD

College

Davis College of Agriculture, Natural Resources and Design

Department

Wood Science and Technology

Committee Chair

James T. Anderson.

Abstract

Mute Swans (Cygnus olor) contribute to the localized decline in Submerged Aquatic Vegetation (SAV) through foraging in the Chesapeake Bay, Maryland. I assessed intensity of feeding activity of 868 Mute Swans in non-breeding flocks and breeding pairs through focal sampling during 2003 and 2004. I also assessed SAV reduction due to Mute Swan herbivory by measuring difference in percent cover, shoot density, and canopy height among 2 year controls (n = 54), 2 year exclosures (n = 54), and 1 year exclosures (n = 54) at 18 sites on the eastern shore during the same time period. Analyses of the time-activity budgets showed that feeding was the most predominant activity of Mute Swans. Mute Swans in flocks spent more time feeding than those in pairs and swans in larger flocks spent more time feeding than those in smaller flocks. Analyses also showed that Mute Swan herbivory caused reduction in percent cover, shoot density, and canopy height of SAV. At the end of the study, mean percent cover, shoot density, and canopy height in the controls were lower by 79%, 76%, and 40% respectively as compared to those in 2-year exclosures. Percent cover, shoot density, and canopy height increased by 26%, 15%, and 22% respectively between early and late seasons of SAV growth in exclosures, but decreased by 36%, 41%, and 18% in the controls. Non-breeding flocks that mainly occupied shallow water caused considerable (i.e., 75% to 100%) SAV reduction. On the contrary, breeding pairs mainly occupied moderate depth sites and caused less (i.e., 32% to 75%) SAV reduction.;I developed set of statistical models and selected the best one using Akaike's Information Criterion and spatial models using Geographical Information Systems to predict the Bay-wide SAV decline under the influence of Mute Swan herbivory. The spatial models indicated that 43% of the total SAV area had beds of widgeon grass (Ruppia maritima) or eelgrass ( Zostera marina) (or both) with higher (i.e., 40-100%) cover and such area potentially faces intense grazing pressure by Mute Swans. Statistical modeling suggested that although Mute Swan herbivory is not the most important factor influencing bay-wide SAV decline, it certainly contributes to SAV in the Bay. Therefore Mute Swan impacts should be incorporated into a larger framework of SAV protection in the Chesapeake Bay.;As flocks (especially larger ones) are more detrimental to SAV as compared to paired Mute Swans, I recommend that emphasis primarily be placed on removing adult Mute Swans in flocks, and secondarily on pairs. Considering the goal of the Atlantic Flyway Council to reduce Mute Swan Population in the Atlantic Flyway from about 14,000 birds in 2002 to 3,000 birds by 2013, I suggest that population of Mute Swans in the Chesapeake Bay, Maryland should be reduced from 3,600 individuals in 2002 to 342 individuals by 2013.

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