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Polar Ecosystems Program

Abundance and Distribution Surveys for Ice Seals Aboard the USCG Healy and the Oscar Dyson, 10 April – 18 June 2007

Species Shipboard  Helicopter
Bearded seal 225      320     
Ribbon seal 204      217     
Ringed seal 46      24     
Spotted seal 436      778     
Unknown pinniped 205      228     
Walrus 329      283     
TOTAL 1,445      1,850     
Table 1.  Number of seals and walrus observed during the Healy shipboard and helicopter surveys.

Researchers from the National Marine Mammal Laboratory’s Polar Ecosystems Program (PEP) were joined by four Alaska Native seal hunters to conduct shipboard and aerial abundance and distribution surveys for the four species of ice-breeding seals (i.e., bearded, spotted, ribbon, and ringed seals) which are known to occupy the eastern region of the Bering Sea in spring and summer.

The fieldwork was conducted during two cruises (10 April–12 May and 16 May–18 June 2007) on the U.S. Coast Guard icebreaker Healy and a cruise (3–30 May 2007) on the NOAA ship Oscar Dyson (Fig. 2).


figure 2, click to enlarge
Figure 2, click image to enlarge

figure 3, click to enlarge
Figure 3, click image to enlarge

Whenever the Healy was moving and within 750 m of sea ice, between the hours of 0600 and 1800 local apparent time, at least two observers were posted on the bridge to record the presence of seals and walrus. Information on the species, group size, and distance from the ship’s trackline (as calculated using angle measurements from inclinometers) was recorded along with sea-ice type and concentration, weather, and visibility. Where possible, the age, sex, and molt stage of animals were also recorded. In all, 1,116 individual seals (Table 1 above) were observed during 131 hours and 50 minutes of survey effort covering 1,116.0 nautical miles (nmi) of survey line.

Whenever the Healy was near ice and the weather conditions were conducive to flying, between 0900 and 1500 local apparent time, we also conducted line-transect surveys from a helicopter based aboard the icebreaker. Each flight had 2-3 observers and was flown at an altitude of 400 ft and a speed of 95 knots. A camera mounted on the airframe took digital pictures of the area underneath the helicopter every 2 seconds. These images will be analyzed for the presence of seals and to identify the type and percent cover of sea ice. As with the shipboard surveys, information on the species, group size, and distance from the helicopter’s trackline was recorded.

In all, 1,567 seals (Table 1 above) were observed during 48 hours and 55 minutes of survey effort covering 4,414.4 nmi of survey line on 44 flights.

We also conducted line-transect surveys opportunistically from the Oscar Dyson’s bridge. We recorded 296 seals or groups of seals while on effort, of which 47% were spotted seals, 22% were ribbon seals, less than 1% were bearded seals, and the remainder (30%) could not be identified to species, usually because of distance from the observer.

Preliminary analyses indicate some habitat partitioning among the three most abundant species of ice seals in spring
(Fig. 3). This partitioning may be related to their foraging strategies. Bearded seals are benthic feeders and were most abundant in the shallow waters near the St. Lawrence Island polynia, where walrus (also benthic feeders) were also most abundant.

Ribbon seals are known to forage at depths over 500 m and were most abundant at the southern edge of the sea ice and close to the shelf break, in close proximity to deep water. Spotted seals feed throughout the water column while over the Bering Sea shelf and tended to occupy the more interior areas of the pack ice.

These associations tended to break down later in the season as the reduction of the ice field limited areas available for hauling out. Survey data from cruises planned for 2008 and 2009 will contribute further to a database that will eventually be used to calculate the springtime abundance and distribution of ice seals in the eastern Bering Sea.

By Michael Cameron and Peter Boveng

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