Common Goldeneye on the Great Salt Lake: Abundance, Food Habits, and Nutrient
Reserves during Winter
Josh L. Vest, Department of Wildland Resources, Utah State University, 5230 Old Main
Hill, Logan, Utah, 84322 USA E-mail: [email protected]
Michael R. Conover, Department of Wildland Resources, Utah State University, 5230
Old Main Hill, Logan, Utah, 84322 USA E-mail: [email protected]
John T. Luft, Utah Division of Wildlife Resources, 1594 West North Temple, Salt Lake
City, Utah, 84114, USA E-mail: [email protected]
Extended Abstract: The Great Salt Lake
(GSL) is a hypersaline (>5% salinity)
terminal lake located in northern Utah and is
a dominant water feature in western North
America. Along with its associated
marshes, the GSL is an important resource
for millions of migratory waterbirds due to
its size, location within a predominately
xeric environment, and abundant
invertebrate biomass. Indeed, an estimated
3–5 million waterfowl annually migrate
through the GSL area. Although waterfowl
abundance in the GSL system peaks during
the fall migratory period (August–October),
anecdotal reports suggest some species,
including common goldeneye (Bucephala
clangula), may use the hypersaline GSL as
wintering habitat. However, waterfowl
population estimates from the GSL are
lacking and the value of the GSL as foraging
habitat to waterfowl is unclear given its high
salinity (≥3 times marine environments).
Furthermore, relatively few studies have
evaluated physiological condition of
waterfowl from hypersaline environments.
Our objectives were to 1) estimate
population indices of common goldeneye
(hereafter goldeneye) on the GSL through
winter, 2) evaluate dietary composition of
goldeneye wintering on the GSL, and 3)
evaluate sources of variation in body mass
and nutrient stores of goldeneye wintering
on the GSL.
We designed and conducted aerial surveys,
employing a stratified random sampling
design, to estimate population indices ( Iˆ ) of
wintering goldeneye on the GSL. Surveys
were conducted once monthly November–
April during winters 2004-05 and 2005-06.
Goldeneye population indices peaked during
January surveys in both winters ( Iˆ 2005 =
44,335; SE = 11,982; Iˆ 2006 = 43,617; SE =
10,188). Peak population indices during
January on the GSL equated to >4% of the
combined continental breeding population of
Barrow’s (B. islandica) and common
goldeneye. However, variance estimates
associated with Iˆ were large (CV ≥ 0.23).
Population indices were marginally higher
(P = 0.05) in March 2006 ( Iˆ = 28,436; SE =
6,951) when lake elevations were higher
(0.4 m) and conditions generally colder
relative to March 2005 ( Iˆ = 13,388; SE =
3,275).
We collected goldeneye (Nfemale = 313, Nmale
= 288) throughout the South Arm of the
GSL during winters (November 19–April 5)
2004-05 and 2005-06. We obtained external
morphological measurements, weighed
carcasses (g) with and without plumage and
ingesta contents, and determined molt
intensity indices for each bird. We sorted,
dried, and weighed esophageal contents
from each bird and used multivariate
analyses of variance to evaluate variation
(sex, age, year, winter time period) in
aggregate percent biomass (APB) of major
food groups consumed. We obtained
estimates (g) of total body lipid content and
ash free lean dry mass (AFLDM) from a
random subset of carcasses (nfemale = 175,
nmale=168). We used an information
theoretic approach to evaluate variation in
masses of ingesta free plucked body mass
and nutrient stores of goldeneyes during
winter on the GSL. We developed a set of
10 candidate models that included
combinations of intrinsic factors (structural
size, age, molt intensity), extrinsic factors
including lake surface elevations and
effective temperatures (Tef ; i.e., the ambient
temperature [°C] corrected for the effect of
wind speed), and linear and quadratic
temporal variables (i.e., collection date).
Overall, APB of goldeneye diets were
comprised of 68% brine fly (Ephydridae)
larvae, 19% wetland plant seeds, 9%
freshwater invertebrates, and 4% brine
shrimp (Artemia franciscana) cysts. APB of
Ephydridae larvae was lowest during late
winter (i.e., March) 2004-05 (36% ±
6%; x ± SE) when warmer temperatures
freed freshwater marshes from ice; APB of
wetland plant seeds was greatest (52% ±
6%) during this time. APB of Ephydridae
larvae was high (≥ 58%) throughout all
winter periods in 2005-06 when lake
elevations were higher (0.4 m) and winter
conditions generally colder relative to 200405.
All best approximating and competing
models explaining variation in female body
and lipid mass and AFLDM contained the
Lake Elevation variable. GSL lake surface
elevations were positively related to female
body mass ( β̂ Lake =117, SE = 22), lipid mass
( β̂ Lake = 53, SE = 16), and AFLDM ( β̂ Lake =
12, SE = 4). All best approximating models
explaining variation in male body and lipid
masses and AFLDM contained the Lake
Elevation variable. GSL surface elevations
were positively related to male body mass
( β̂ Lake = 110, SE = 28), lipid mass ( β̂ Lake =
37, SE = 13), and AFLDM ( β̂ Lake = 7, SE =
2.1).
Our results suggest the GSL is an important
wintering area for a significant number of
common goldeneye in western North
America. Ephydridae appear to be a
significant component of their winter diet,
especially when access to freshwater
habitats may be limited. Unfortunately,
density estimates of Ephydridae larvae are
lacking and their population ecology is
poorly understood in the GSL.
Regional and local drought conditions were
more severe prior to winter 2004-05
resulting in lower lake elevations, relative to
2005-06, and likely influenced the positive
relationship we observed between goldeneye
body mass and nutrient stores and GSL
elevation. Reduced body mass and nutrient
stores have been related to regional drought
conditions in other waterfowl. However,
salinity is inversely related to GSL
elevations and may have also influenced
observed body mass and nutrient store
patterns. Salinity ranged from 17–14%
during the winter with low lake elevations
(2004-05) compared to 15–12% during
2005-06. High salinity concentrations (1420%) have been associated with reduced
Ephydridae abundance and quality in other
regions. Thus, salinity effects on
Ephydridae could influence goldeneye
foraging behavior, nutrient stores, and GSL
use.
Water delivery and quality are of increasing
conservation concern due to increased
human demand and changes in precipitation
and hydrologic patterns in western North
America. Reduced freshwater inputs to the
GSL will lead to increased salinity and may
approach thresholds that limit halophile
invertebrate abundance or quality. Further
research is needed to elucidate relationships
between lake elevations, salinity,
Ephydridae density and population biology,
and subsequent impacts on goldeneye
wintering ecology in the GSL.