Occurrence of Humpback, Fin, and Minke Whales in the Eastern Chukchi Sea, 2008-2015: Population Recovery, Response to Climate Change, or Increased Effort? (cont.)
A total of 65,479 km were flown on-effort in the eastern Chukchi Sea from 1979 to 1991 (Fig. 2). Most of that effort was in October. Circling from transect effort was not differentiated from off-effort survey modes from 1979 to 1991, so the total effort includes transect effort only. A total of 185,938 km were flown on-effort (including circling from transect effort) in the eastern Chukchi Sea from 2008 to 2015. Effort was fairly evenly distributed across all months, with the least amount of effort in October.
The sole sighting of large whales from 1979 to 1991 (three fin whales in 1981) was not made by a primary observer, so there is no sighting rate for large whales for this early time period. Sighting rates from 2008 to 2015 were calculated for the eastern Chukchi Sea by month for each species (Fig. 3). The highest sighting rates for humpback and fin whales were in September, and sighting rates were also relatively high for fin whales in August. Minke whale sighting rates were highest in August. When sighting rates were calculated for each species per subarea (northeastern Chukchi Sea and southcentral Chukchi Sea, see Fig. 1), the importance of the southcentral Chukchi Sea becomes obvious (Fig. 4). For all three species of large whales, sighting rates were orders of magnitude greater in the southcentral Chukchi Sea compared to the northeastern Chukchi Sea.
Figure 2. Summary of ASAMM effort in the eastern Chukchi Sea, 1979-1991 and 2008-15. Effort from 1992 to 2007 was negligible.
Figure 3. Fin, humpback, and minke whale sighting rates per month, in the eastern Chukchi Sea, 2008-15. Sighting rate, using only the sightings collected by primary observers during on-effort survey mode, provides an estimate of relative abundance.
Figure 4. Fin, humpback, and minke whale sighting rates for the northeastern Chukchi Sea and southcentral Chukchi Sea subareas, July-October combined, 2008-15. Sighting rate, using only the sightings collected by primary observers during on-effort survey mode, provides an estimate of relative abundance.
Fin, humpback, and minke whales occur in the northern Bering Sea, and there is ample evidence of historical occurrence in the western Chukchi Sea, but there are relatively few records for the eastern Chukchi Sea (summarized in Clarke et al. 2013, Oceanography). Increasingly common occurrence of these large whales in the eastern Chukchi Sea in recent years (2008-present) has been documented by this study and corroborated by concurrent visual and acoustic studies (e.g., NMML’s Arctic Whale Ecology Study (ARCWEST), funded by BOEM; NMML’s Chukchi Acoustics, Oceanography, and Zooplankton (CHAOZ, CHAOZ-X) studies, funded by BOEM; the Chukchi Sea Environmental Studies Program (CSESP); and Shell Oil).
The apparent lack of large whales (other than bowhead or gray whales) in the eastern Chukchi Sea prior to 2008 may be related to one or more phenomena. Populations may be recovering from illegal commercial whaling that occurred in the North Pacific as recently as the 1970s. Additionally, large whale distribution and habitat use are likely related to water masses that collectively produce sharp temperature and salinity gradients, which can aggregate zooplankton, fishes that feed on zooplankton, and large whales that feed on fishes. Variations in water mass incursion, combined with earlier and greater loss of summer sea ice and other effects of climate change, may be enticing large whales to expand farther into the Chukchi Sea.
Finally, the potential effect of increased survey effort cannot be ignored. Survey effort during August and September 1979-91 was not extensive, particularly in August ( Fig. 1). Survey effort in October 1979-1991 actually exceeded that for October 2008-15. Most fin, humpback, and minke whales were seen (and sighting rates were highest) in August and September 2008-15, indicating that fin, humpback, and minke whales may be migrating south from the Chukchi Sea by that time. It is possible that large whales were present in the eastern Chukchi Sea, particularly the southcentral subarea, when aerial surveys were conducted in 1979-1991 but were not detected due to infrequent survey effort during the months they would most likely have occurred.
The majority (54%) of fin, humpback, and minke whales in 2008-15 were seen after the aircraft diverted from its flight path to investigate a sighting. And about one-third of the fin, humpback, and minke whale sightings were in close proximity to other species of large whales, including gray and bowhead whales. ASAMM survey protocol incorporates regular circling of large whale sightings to verify species, determine group size, and confirm the presence or absence of calves. Aerial or ship surveys that use passing mode, moving continuously along a transect line without diverting, may be more likely to misidentify species or erroneously assume that all whales in a group are the same species.
The ASAMM project will continue documenting marine mammal distribution and relative abundance in the eastern Chukchi and western Beaufort Seas in summer and fall 2016. If the past 7 years are any indication, we are in for another exciting and interesting year.
By Janet Clarke, Megan Ferguson, Amelia Brower, Amy Willoughby, and Christy Sims
