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Cetacean Assessment and Ecology Program

Occurrence of Humpback, Fin, and Minke Whales in the Eastern Chukchi Sea, 2008-2015: Population Recovery, Response to Climate Change, or Increased Effort?

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In the past, large whales observed during surveys in the eastern Chukchi Sea were generally black or gray, bowhead whale or gray whale. Fairly simple, fairly straightforward. But that has changed in recent years.

From 1979 to 1991, aerial surveys for marine mammals were conducted by the Naval Ocean Systems Center and its contractors in the northern Bering, eastern Chukchi, and western Beaufort Seas. These surveys, funded by the Minerals Management Service (MMS), were the precursor to the Alaska Fisheries Science Center’s National Marine Mammal Laboratory’s (NMML) current Aerial Surveys of Arctic Marine Mammals (ASAMM) project funded by the Bureau of Ocean Energy Management (BOEM). While data were collected on all marine mammals, the emphasis for the early surveys was bowhead and gray whale distribution and abundance, so the temporal and spatial focus was largely concentrated in areas where those species would likely occur.

From 1992 to 2007, aerial survey effort in the eastern Chukchi Sea was negligible as the MMS-funded aerial surveys were limited to the western Beaufort Sea. In 2008, the aerial survey component of the Chukchi Offshore Monitoring in Drilling Area (COMIDA) project funded by the MMS was initiated, with surveys conducted by NMML in late June, most of August, and late October. Starting in 2009, COMIDA effort was expanded to cover mid-June or early July through the end of October, a level of effort that has continued to be supported by BOEM via the ASAMM project. ASAMM surveys are conducted using protocols similar to the earlier MMS-sponsored surveys (e.g., two dedicated observers, one data recorder, survey altitude of 305-457 m; see Clarke et al. 2015), albeit with a few differences.

Figure 1. Eastern Chukchi Sea study area with fin, humpback, and minke whale sightings, July-October 2008-2015, and the fin whale sighting from July 1981. All sightings are plotted, regardless of survey mode; on-effort sightings by primary observers are designated with a star. Each sighting may be of one or more whales. The northeastern Chukchi Sea subarea is outlined in magenta; the southcentral Chukchi Sea subarea is outlined in umber.

Surveys from 1979 to 1991 were conducted almost exclusively in twin-turbine Grumman Gooses, at survey speeds of 260-278 km/hr and without the benefit of bubble windows, so downward visibility was restricted. Surveys from 2008 to 2015 were conducted in twin-turbine DeHavilland Otters and Turbo Commanders, at survey speeds of 204-223 km/hr, and all aircraft were outfitted with bubble windows so downward visibility was unrestricted. All data from 1979 to 2013 (2014-15 data will be added when data reviews are complete) are archived in the same historical database to facilitate analyses. Sighting rates (number of “on-effort” whales per “on-effort” km surveyed, where “on-effort” is defined as data collected during transect and circling from transect survey modes) were calculated to compare relative abundance between areas and time periods. Sighting rate calculations used only the sightings from primary observers because primary observers’ sole responsibility was to search for sightings, unlike pilots or data recorders who were secondary observers with other, higher priority responsibilities.

In the eastern Chukchi Sea (lat. 67°-72°N, long. 157°-169°W), literally hundreds of bowhead whales and thousands of gray whales were seen from July to October 1979-91, but sightings of other large whales were rare. No humpback whales or minke whales were seen, and there was only one sighting of three fin whales, in July 1981 (Fig. 1).

From 2008 to 2015, large whale sightings in the eastern Chukchi Sea were again dominated by bowhead and gray whales. But, unlike the early years, sightings of humpback, fin, and minke whales were no longer rare (Fig. 1). Humpback whales (55 sightings of 97 whales) were seen from July through October (primarily in September), distributed from lat. 66.9°N to 71.2°N, at distances from shore ranging from 1 to 145 km and at depths ranging from 7 to 61 m. Fin whales (41 sightings of 67 whales) were seen primarily in August and September and had the most limited distribution, from lat. 67°N to 69.5°N. Fin whales were seen at distances from shore ranging from 24 to 140 km and at depths ranging from 28 to 52 m. Minke whales (24 sightings of 27 whales) had the most extensive distribution, from lat. 67.1°N to 71.9°N, and were seen from July through September. Minke whales were commonly seen very close to shore (<1 km offshore) but were also seen farthest from shore (170 km), at depths ranging from 3 to 60 m. Fin, humpback, and minke whales were often seen in close association with other cetacean species, including gray whales. Observed behaviors included diving, feeding, milling, resting, rolling, swimming, and tail slapping. Two fin whale calves were seen in 2012, and one humpback whale calf was seen each year in 2014 and 2015.

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