Second North American Sea Duck Conference - Patuxent Wildlife ...

SECOND NORTH AMERICAN SEA DUCK CONFERENCE
ON THIN ICE:
FORAGING ENERGETICS OF SPECTACLED EIDERS
IN THE PACK ICE OF THE BERING SEA
Samantha E. Richman and James R. Lovvorn
Department of Zoology & Physiology, University of Wyoming, Laramie, WY; cruciger@uwyo.edu
The spectacled eider (Somateria fischeri), a threatened species, winters in pack ice of the Bering
Sea. In dives to 40-70 m for benthic invertebrates, the high energy costs of foraging are offset by
high benthic biomass. However, there is evidence that the dominant clam prey has changed from
Macoma calcarea to Nuculana radiata, perhaps affecting the foraging energetics of the eiders. To
assess the relative foraging value of M. calcarea vs. N. radiata, we studied differences in nutrient and
energy content, digestibility, and intake rates for differing clam sizes, areal density, and depth in the
sediments. Because sea ducks have been observed using mainly foot propulsion for short, shallow
dives and wing propulsion for long, deep dives, we measured the metabolic costs of these different
swimming modes with respirometry. To avoid using a Threatened species for experiments, we used
common eiders (Somateria mollissima) for digestion studies and white-winged scoters (Melanitta
fusca), the same size as spectacled eiders for diving studies. For the prey size range comprising 93%
of the eiders’ diet (18-30 mm), M. calcarea including shells was lower in ash, and higher in nitrogen,
lipid, and energy, than N. radiata. Digestibility was 76% for M. calcarea vs. 67% for N. radiata. For
scoters foraging on clams buried in sand in an aquarium 1.8 m deep, intake rate (number/s) decreased
by 31% when burial depth in the sediments was increased from 4 to 7 cm. Oxygen consumption
during propulsion by wings plus feet vs. feet alone was measured for scoters diving to 2 m in water
at 8 ºC. Total dive costs were 33% lower for mainly wing-propelled dives than for foot-propelled
dives. Stroke rates were higher in foot-propelled dives; however, vertical speed (m/s) was higher
in wing-propelled dives. Thus, wing propulsion lowers dive costs and increases vertical speed,
yielding longer bottom time for the same total dive time. Our results show that impacts of long-term
benthic change on eiders depend not only on shifts in total clam abundance, but also on differences
in digestibility and intake rates for differing sizes and burial depths of clams. Required food intake is
reduced by very efficient wing propulsion.
50 ANNAPOLIS, MARYLAND, USA NOV. 7-11, 2005