Humpback whale with irregularities (humps) on their back, which is the origin of the whale’s name.
Photo: Leif Nøttestad
The humpback whale has dominated so far with 41 sightings and minimum 103 animals. The group size has normally been between 2-4 individuals, sometimes large adults with a small calf. The humpback whale is a stout, thick-bodied whale weighing an average of 30,000 kg (up to 48,000 kg) and is approximately 14 m long (up to 18 m). The name humpback whale originates from the irregularities (humps) on their back (Figure 1). The fluke is white on the underside with distinct markings and scars, which makes the scientists capable of distinguishing individual animals from each other by means of digital photo-identification (Figure 2). We have also collected such valuable photos during this expedition that will be included and analysed in a large international database on humpbacks. Perhaps some of the individuals have previously been sighted and photographed off Cape Town during the winter months? Then we are able to tell something about the large-scale migration patterns in the Southern Hemisphere for these fascinating creatures.
Humpback whale diving for krill in the Southern Ocean, with the characteristic white underside of their flukes. Photo: Leif Nøttestad
During the southern hemisphere winter the southern populations leave Antarctica as the krill bloom declines and head north toward the equator at the same time that the northern populations are heading to Arctic waters. Not every member of the population migrates however. Humpbacks in our own waters in the Northern Hemisphere do not have any contact with their close relatives on the other side of the world. Marine mammals do not cross the Equator on their way north or south between their respective feeding and wintering/breeding grounds. Such borders do not exist for some seabird species such as the Arctic tern and the Sooty shearwater. They easily fly the tremendous trip from Arctic regions to the Antarctica (~ 15 000 km) and back again, in order to take advantage of the long days in summer with midnight sun for active feeding and breeding.
Map of humpback whale sightings, krill distribution and surface temperatures along the cruise tracks from Cape Town to the Astrid-Ridge close to Dronning Maud Land on the Antarctic continent, and northwards to Bouvet Island and beyond. Illustration: Øyvind Tangen
The positions of the sighted humpback whales together with concentrations of krill from acoustic recordings are shown in Figure 3. The temperature distribution close to surface is also shown along the cruise track. Most humpback whales were concentrated in cold waters as we can see from the large-scale overview on the map. In warmer areas we did not see any humpback whales, except just outside Cape Town. This is due to the fact that the productive areas or habitats in the Southern Ocean are not equally distributed, but rather patchily concentrated and linked to favourable oceangraphic conditions called fronts. Here, water masses with different nutrient levels, temperature and salinity meet and create favourable upwelling conditions for the whales. The humpbacks aggregated in these frontal regions and especially close to the coastline of the Antarctic mainland (see map), where temperatures went below 0ºC. In these areas we also see that krill are quite abundant and concentrated in dense schools favourable for hungry whales.
An interesting observation was the close association between drifting icebergs and small groups of humpback whales (Figure 4). Active plunge feeding at the surface was repeatedly observed within close proximity of icebergs. The krill are presumably attracted to drifting icebergs due to elevated phytoplankton concentrations as food for krill underneath and in near proximity of the icebergs. This in turn attracts humpbacks to the scene, since they have Antarctic krill on the top of their menu list.
A group of humpbacks swimming close to drifting ice bergs. Photo: Einar K. Loshamn
Several groups of whales approached very close to G.O.Sars and seemed quite curious, while the vessel was stationary or slowly moving for oceanographical measurements and biological sampling. Most marine mammals did not seem to avoid G.O.Sars. Maybe this was not a surprise, given that the nearest boat from us was the German research vessel ”Polarstern”, more than 1000 nautical miles (ca. 2000 km) distance away. We cannot really say it is crowded with boat traffic in this area of the Southern Ocean. These animals can easily hear and locate sounds from wires, nets, propeller and especially the ”symphony” from all the acoustic instrumentation emitting sound continuously at frequencies ranging from 18 to 330 kHz. It is worth mentioning that density of seawater (~1030 kg/ m3) is approximately 800 higher than in air (~1,2 kg/m3), and sound propagate about 4.3 times faster (~1480 m/s) and travel much longer distances in the ocean than on land. Sound emitted at low frequencies produced by whales can reach distances > 100 km in the ocean. Then it is easier to understand why sound is so important for many animal groups in the ocean including marine mammals for hunting, migration, avoidance, communication and reproduction. When light is limited, sound is the solution. Many whale species use sound (biological sonar) to “see” underwater, in a similar way as we apply modern echosounder and sonar technology enabling us to study fish, krill and other organisms down to deep dark ocean. Marine mammals have indeed evolved clever ways of solving daily challenges in the Southern Ocean. However, they probably need most of them to survive in this harsh environment.
Written byLeif Nøttestad, PhD, senior scientist at the IMR in Norway, working on behaviour, ecology and management of herring, mackerel and bluefin tuna. Leif is on this expedition involved in studies of fish and marine mammals, sonar investigations of krill and public outreach work on film and photo on board G.O.Sars in the Southern Ocean.
Øyvind Tangen. Senior engineer at the IMR, Norway. His main interest is science on Norwegian spring-spawning herring and Atlantic bluefin tuna. Øyvind is on this expedition involved in fish samples, database work and making maps and figures.