Abundance and Distribution Surveys for Ice Seals Conducted from the USCG Icebreaker Polar Sea, 6-27 April 2008
Figure 4. Ship and helicopter tracks of the Polar Sea during the 2008 spring cruise. Click to enlarge.
Figure 5. Map showing the abundance and distribution of seals observed during the 2008 Polar Sea cruises. Click to enlarge.
Table 1. Number of pinnipeds observed during the Polar Sea helicopter surveys.
Species
Number Identified
Bearded seal
88
Ribbon seal
70
Ringed seal
21
Spotted seal
128
Unknown pinniped
155
Total Seals
462
Walrus
183
Sea lion
2
Total Pinnipeds
647
Three researchers from the National Marine Mammal Laboratory's (NMML) Polar Ecosystem Program were joined by an Alaska Native seal hunter to conduct aerial abundance and distribution surveys for the four species of ice seals (bearded, spotted, ribbon, and ringed seals) which are known to occur and breed in the eastern region of the Bering Sea during the spring and summer. The fieldwork was conducted from the U.S. Coast Guard icebreaker Polar Sea and a USCG 65-HH Dolphin helicopter from 6 April to 27 April 2008 (Fig. 4).
The Polar Sea entered the Bering Sea ice pack near Bristol Bay on 16 April 2008. Whenever the weather conditions were conducive to flying, between 0900 and 1500 (local apparent time), we conducted line-transect surveys from the USCG 65-HH Dolphin helicopter based aboard the icebreaker. Each flight consisted of two parallel 50 nautical mile (nmi) transect surveys spaced 10 nmi apart. The target altitude and speed for the aerial surveys were 400 ft and 85 knots. Each flight had three observers, including a flight mechanic who was trained in our methodologies and in identifying the different species, sexes, and age classes of ice seals.
Data from the flight mechanic will be used to identify potential sighting biases among the primary observers. All observers recorded information on the species, group size, and distance from the helicopter track line for each sighting. In addition, an externally mounted camera took digital pictures of the area beneath 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. In all, 462 seals (Table 1) were observed during 24.02 hours of survey effort covering 1,967.8 nmi of survey line on 19 flights over 11 days.
Preliminary analyses indicate some habitat partitioning among the four different species of ice seals, and it may be related to their foraging strategies (Fig. 5); this partitioning is also consistent with observations made from helicopter surveys conducted from the USCG icebreaker Healy in 2007. Bearded seals are benthic feeders and were most abundant in the shallow waters near the St. Lawrence Island polynya, where walrus (also benthic feeders) were also most abundant. Ribbon seals are known to forage at depths more than 500 m and were most abundant at the southern edge of the sea ice, close to the shelf break and proximate to deep water. Spotted seals feed throughout the water column over the Bering Sea shelf and tended to occupy the more interior areas of the pack ice. Finally, ringed seals tended to occupy the fast-ice habitat close to shorelines. Survey data from cruises planned for 2009 and 2010 will further contribute to a multiyear sightings database that will be used to calculate the springtime abundance and distribution of ice seals in the eastern Bering Sea.
By Michael Cameron, Erin Moreland, and Peter Boveng
