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Techniques for Determining the Availability of Food Items to Seaducks Wintering on the Chesapeake Bay, Maryland

Introduction: Historically, the Chesapeake Bay has been a major wintering area for seaducks.  Based on aerial surveys, three species of seaducks, surf scoters (Melanitta perspicillata), black scoters (Melanitta nigra), and long-tailed ducks (Clangula hyemalis), have shown major declines in recent years.  One possible explanation for this decline is a reduction of available food items.  Data obtained through satellite telemetry (Table 1.) shows a median depth of 19 and 21 feet for black and surf scoters, respectively. Combined with survey results (Figure 3.) and field observations, this data indicates that surf and black scoters likely feed at depths ranging from 10 to 30 feet in the mesohaline region of the Chesapeake Bay.  Survey results and field observations indicate similar preferences. These deep water areas are usually the first to be affected by low oxygen conditions, which may be a factor influencing the quantity and types of food items available to seaducks. Additionally, telemetry data indicates that as winter progresses, seaducks are forced to move into deeper water in search of preferred food items (Figure 1.).  Research is needed to describe the seasonal depletion and availability of seaduck food items as a possible explanation for seaduck population declines.

Objectives:
1.Describe the macrobenthic organisms present at seaduck feeding sites in the mesohaline regions of the Chesapeake Bay.
2. Correlate available food items with known food habits of seaducks.
3. Describe the sediment characteristics, salinity, and dissolved oxygen content at seaduck feeding habitats.
4. Correlate the available food items with the numbers of seaducks.
Figure 1: A graph showing the monthly average of scoter depth in the Ches Bay, MD
Figure 1.  Monthly average of scoter depth in the Chesapeake Bay, Maryland
Methods:  Two study sites were chosen from areas known for traditionally large seaduck populations (Herring Bay and Poplar Island).  Twelve sample points were randomly selected at each location using Geographic Information Systems (GIS) in water 10 to 30 feet deep (Figure 2.).  Coordinates of each point are programmed into a GPS unit to facilitate the location of each point.  At each sampling point, macrobenthic invertebrates are collected using 3 grabs from a Peterson dredge with a 0.052 m2 surface area.  Material sampled is rinsed through a 0.5 mm sieve and collected for analysis.  All non-organic material is discarded and only intact, viable organisms are used; shells and other remnants are not counted for benthic samples.  Benthic samples from each sampling point  are then identified and separated by species.  Bivalve species are measured along the anterior-posterior axis and separated into 5 mm size classes.  Each size class is then measured volumetrically and the number of individuals within each size class are counted. All other species are also measured volumetrically and counted.  In addition to invertebrate sampling, sediment characteristics, dissolved oxygen, and salinity are determined at each point.
Species
Sex
Number
Depth (ft)
Median (Range)
Distance from shore (km) Median (Range)
Surf Scoter
M
121
21 (1-66)
1.7 (0.01-6.3)
Black Scoter
M
79
19 (1-63)
2.1 (0.01-6.5)
Table 1: Habitat data of scoters in Chesapeake Bay as determined by satellite telemetry
A benthic sample showing Baltic Macoma
Sample containing Baltic Macoma
Figures 2 and 3: Location of Benthic Sampling Sites and boat surveys
A Peterson Dredge used for sampling
Peterson Dredge used for sampling
A benthic sample that contains mussels
Sample containing hooked mussel
David Kidwell and Matt Perry taking benthic samples
Kidwell and Perry  taking benthic samples
Boat surveys are conducted at each study area to determine the number of seaducks present.  Surveys are done by moving through each study area once in a sinuous pattern .  The species, number, and position of all seaducks observed are recorded by a pair of primary observers positioned at the bow of the boat.  To reduce the disturbance on the ducks, all counts are completed at a constant speed of 30 mph.  An independent observer is positioned at the center of the boat to monitor the position of flying ducks to minimize the chance of duplicate counting.
Benthic sampling began in winter 2004 and will continue through spring 2006.  Samples will be taken 5 times per year, late September, November, February, April, and July.  Each month will be analyzed against each other to determine the effects of predation, low-oxygen conditions if present, and recolonization at each site.  Statistical correlation techniques will be carried out to determine if there is a link between the benthic organisms sampled and duck density found at each site.
Preliminary results have shown that concentrations of seaducks are associated with areas with hard substrate, with hooked mussel (Ischadium recurvum) as the dominant benthic species.  Results of this study will be used to assist in the management of seaduck populations and habitat.
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