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Cetacean Assessment and Ecology Program 

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

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Oct-Nov-Dec 2013
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Figure 2. Gray whale mother with calf underneath. Photo by Vicki Beaver, 2013 ASAMM survey, NMFS Permit No. 14245.

 

The gray whale (Eschrichtius robustus) (Fig. 2) is a baleen whale that lives in the eastern North Pacific and Arctic Oceans and can reach 15 m in length. The Eastern North Pacific stock of this species makes a seasonal migration along the western coast of North America, from calving and breeding grounds in the lagoons of Baja California, Mexico, to primary summer feeding grounds in the northern Bering and Chukchi Seas. These seas contain multiple foraging areas for gray whales due to high primary and secondary production, resulting in high densities of benthic prey. Gray whales typically arrive in the northern feeding grounds with the breakup of sea ice in June and begin their return migration south in October when sea ice starts to form. Gray whale calves make the spring migration with their mothers and are weaned at 7-9 months of age while on the summer feeding grounds.

Occurrence of the Eastern North Pacific stock of gray whales has been documented on summer feeding grounds in the northeastern Chukchi Sea by the Aerial Surveys of Arctic Marine Mammals (ASAMM) project and its predecessor projects, the Bowhead Whale Aerial Survey Project (BWASP) and the Chukchi Offshore Monitoring in Drilling Area (COMIDA) marine mammal aerial surveys. These broad-scale aerial surveys are conducted in the northeastern Chukchi and western Beaufort Seas (68°-72°N and 140°-169°W) by the National Marine Mammal Laboratory (NMML) and are co-managed and funded by the Bureau of Ocean Energy Management (BOEM). Surveys have been conducted in the western Beaufort Sea portion of the study area (140°-157°W) since 1979 and in the northeastern Chukchi Sea portion of the study area (157°-169°W) from 1982 to 1991 and 2008 to 2013. The goal of the ASAMM project is to investigate the distribution and relative abundance of marine mammals in the western Beaufort and northeastern Chukchi Seas during the open-water (ice-free) months of July-October, when various species are undertaking migrations to seasonally occupied habitats both within and adjacent to the study area. Transect flightlines generally lie perpendicular to the coastline, cutting across isobaths, prevailing currents, and expected gradients in marine mammal density. Beginning in 2009, a coastal transect between Point Barrow and Point Hope was regularly flown 1 km offshore and parallel to the coast. This article focuses on effort in survey blocks 12-22 (Fig. 3), encompassing the western half of the ASAMM study area (154°-169°W), because that is the area in which gray whales are normally encountered.

Survey effort in the study area has consistently occurred from July through October 2009-13, but the number of kilometers flown has varied by year due to study objectives and prevailing weather conditions. Survey effort in 2013 was greater than in 2009-11, but less than in 2012. In 2012 and 2013, survey effort was higher due to summertime beluga surveys in the northeastern Chukchi Sea and expanded Beaufort Sea surveys, which began in mid-July instead of August or September. In 2013, survey effort was lower than in 2012 due to three factors: intermittently poor weather conditions; the government shutdown, which suspended survey effort from 1 to 19 October; and the absence of additional surveys to target belugas. The 2012 beluga surveys were sponsored by the Alaska Beluga Whale Committee (ABWC) and increased survey coverage in the northeastern Chukchi Sea and the western Beaufort Sea in the first half of July. Survey effort was designated as “on-effort” (transect and circling from transect), “off-effort” (search and circling from search), or “deadhead.” The 2013 field season spanned 2 July to 28 October. There were 90 survey flights initiated for a total of ~51,000 km flown on- and off-survey effort (Fig. 3). Approximately 40,000 km were on-effort and ~25,000 transect km were in the western part of the ASAMM study area (blocks 12-22).

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Figure 3. ASAMM on- and off-effort flightlines and survey block numbers, July-October 2013.

In 2013, there were 174 sightings of 281 gray whales on- and off-effort in the study area; 57 of these individuals were calves (Fig. 4). Some calf sightings were likely of the same calf on multiple days. A sighting of a white-colored gray whale mother with a normal gray-colored calf was documented on 7 July 2013 near Point Lay and again on 21 July between Point Franklin and Barrow; both sightings were likely of the same unusually white-colored gray whale mother and her calf. In 2012 and 2013, 67 and 57 calves were seen, respectively; prior to this, the years with the greatest number of calf sightings were 1982 and 2011, with 18 calves seen each year (Table 1). There was a higher proportion of calves to total gray whales sighted (20%) in 2013, compared to other years with calf sightings, which ranged from 1% to 13%.

Table 1. ASAMM gray whale calf sightings per month in the northeastern Chukchi Sea for each survey year, 1982-91 and 2008-13. Surveys were rarely conducted in this region from 1992 to 2007.  
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Gray whale on- and off-effort sightings spanned all months surveyed in 2013; calves were sighted in July, August, and September. The month with the highest total number of gray whale sightings and gray whale calves was July: 47 calves were sighted in July, 9 calves were sighted in August, and 1 calf was sighted in September. July has historically been the month with the highest numbers of total gray whale and gray whale calf sightings (Table 1); it is likely that gray whale cow-calf pairs begin migrating south after July. It is also possible that calves grow large enough that they are no longer identified as “calves” by September-October, particularly if they have been weaned from their mothers.

The increase in calf sighting numbers in 2012 and 2013 may be somewhat related to increased effort in circling mode. Designation of circling effort was incorporated into the survey database in 2009, and in 2012 and 2013 more circling on cetacean sightings was initiated in an attempt to more accurately estimate group size and determine whether calves were present. About 10% of the total non-deadhead survey time in 2012 and 2013 was spent on circling, compared with 5% in 2011. More survey time spent on circling has led to more calves being detected and recorded while on circling: 67% of calves were sighted after circling was initiated in 2013 and 43% in 2012 compared to 22% in 2011 and 0% in 2009 (no calves were sighted in 2010). (continued)


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