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Gray Whale Calf Occurrence in the Alaskan Arctic, Summer and Fall 2013, with Comparisons to Previous Years (continued)

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Figure 4. ASAMM gray whale calf sightings in 2013 compared to 1982-2012 gray whale calf sightings, on- and off-effort per month, with all months shown.

To compare gray whale calf sighting rates among years, data were limited to on-effort sightings. In 2013, there were 114 sightings of 194 gray whales in the study area; 37 of these individuals were calves. Sighting rates (whales per unit effort, WPUE) were calculated for gray whale calves as the number of calves sighted on-effort per on-effort kilometer (km) surveyed per month in order to make a comparison across years and correct for survey effort.

When on-effort gray whale calf sighting rates were compared across years, the rates were significantly higher in 2012 and 2013 (Table 2). Therefore, despite the additional survey effort in 2012 and increased circling in 2012 and 2013, there were likely more calves in the northeastern Chukchi Sea in 2012 and 2013 than in previous years. During the Southwest Fisheries Science Center’s (SWFSC) annual surveys of the gray whale northern migration off California, from February to May, relatively high numbers of gray whale calves were sighted in 2012 and 2013 compared to previous counts dating back to 1994 when the surveys began (see the SWFSC’s Gray Whale Studies – Calf Production website). It is possible that conditions were favorable for gray whale foraging in 2011-13, and many females were able to accumulate sufficient energy reserves to conceive in 2011 and 2012 and give birth in 2012 and 2013. Another possibility is that other habitats where gray whale cow-calf pairs have been documented in the past, such as along the Chukotka Peninsula, may not have been as favorable to cow-calf pairs in 2012 and 2013.

Table 2. ASAMM gray whale calf sighting rate (WPUE) from on-effort sightings (n = number of calves) and on-effort kilometers flown (km) per month per year. The survey area encompasses survey blocks 12-22.
  July August September October Total
Year km n WPUE km n WPUE km n WPUE km n WPUE km n WPUE
2009 5279 5 0.0009 3949 0 0.0000 7155 0 0.0000 5726 0 0.0000 22109 5 0.0002
2010 7646 0 0.0000 4253 0 0.0000 5218 0 0.0000 4081 0 0.0000 21198 0 0.0000
2011 5286 7 0.0013 7363 4 0.0005 10427 1 0.0001 3057 0 0.0000 26133 12 0.0005
2012 11250 50 0.0044 7644 5 0.0007 9446 0 0.0000 8819 0 0.0000 37159 55 0.0015
2013 5534 36 0.0065 10612 9 0.0008 10683 1 0.0001 1776 0 0.0000 28605 46 0.0016
Total 34995 98 0.0028 33821 18 0.0005 42929 2 0.0000 23459 0 0.0000 135204 118 0.0009

Gray whale calf on- and off-effort sightings in 2013 were primarily nearshore along the Alaskan coast and ranged from north and east of Barrow to Point Lay, with particularly high numbers offshore of Wainwright, in a cluster between Barrow and Point Franklin, and offshore and south of Point Hope (Fig. 4). The majority of calf sightings were <25 km from shore in shallow waters <50 m deep, with scattered sightings out to 77 km from shore in waters up to 56 m deep. In July, 15 calves were sighted in 10%-75% broken floe sea ice; sea ice receded from primary gray whale habitat in the study area in August, and no calves were sighted near sea ice in August or September. Distribution of calves in 2013 was similar to gray whale calf distribution in previous survey years.

The northeastern Chukchi Sea contains summer foraging habitat for adult and juvenile gray whales; gray whale calf distribution was similar to that of non-calves and overlapped areas where adults were feeding. It is possible that the shallow, nearshore waters also provided some protection from predatory killer whales. Killer whale numbers may be increasing in the Arctic as a result of an increase in gray whale calf abundance. Killer whales have been documented in the northeastern Chukchi Sea by several research groups, including ASAMM, and by Alaskan villagers. The Arctic Whale Ecology Study (ARCWEST) research cruise observed a killer whale predatory attack on a gray whale calf near Wainwright on 2 September 2013 (NMML, unpublished data; B. Rone, pers. comm.).

The continuation of broad-scale aerial surveys in the northeastern Chukchi Sea in summer and fall (particularly July) is necessary to assess the importance of this area to gray whale calves. Improving the understanding of gray whale distribution, abundance, behavior, and migration timing in this region will assist in monitoring climate-change impacts to gray whales and assist in decision-making to minimize impacts from petroleum exploration, development, and production and other anthropogenic activities.

By Amelia Brower




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