Intellectual Merit: The krill surplus hypothesis argues that the near-extirpation of baleen whales from Antarctic waters during much the twentieth century led to significant changes in the availability of krill for other predators. Over the past decade, however, overall krill abundance has decreased by over an order of magnitude around the Antarctic Peninsula, in part due to physical forces, including the duration and extent of winter sea ice cover. Krill predators are vulnerable to variability in prey and have been shown to alter their demography in response to changes in prey availability This research will use novel tagging technology combined with traditional fisheries acoustics methods to quantify the prey consumed by a poorly understood yet ecologically integral and recovering krill predator in the Antarctic, the humpback whale (Megaptera novaeangliae). It also will use a combination of advanced non-invasive tag technology to study whale behavior concurrent with hydro-acoustic techniques to map krill aggregations. The project will (1) provide direct and quantitative estimates of krill consumption rates by humpback whales and incorporate these into models for the management of krill stocks and the conservation of the Antarctic marine ecosystem; (2) provide information integral to understanding predator-prey ecology and trophic dynamics, i.e., if/how baleen whales affect the distribution and behavior of krill and/or other krill predators; (3) add significantly to the knowledge of the diving behavior and foraging ecology of baleen whales in the Antarctic; and (4) develop new geospatial tools for the construction of multi-trophic level models that account for physical as well as biological data.
Broader Impacts: Whales are assumed to be a major predator on Antarctic krill, yet there is little understanding of how whales utilize this resource. This knowledge is critical to addressing both bottom-up and top-down questions, e.g., how climate change may affect whales or how whales may affect falling krill abundances. This program will integrate research and education by providing opportunities for undergraduate and graduate students as well as postdoctoral researchers at Duke University, the Florida State University and the University of Massachusetts at Boston. This project will also seek to integrate interactive learning through real time, seasonal and curriculum development in collaboration with the National Geographic Society as well as at the participating universities and local schools in those communities.
The major objective of this project was to use novel tagging technology combined with traditional fisheries acoustics methods to quantify the prey consumed by a poorly understood yet ecologically integral and recovering krill predator in the Antarctic, the humpback whale (Megaptera novaeangliae). The hydrography-zooplankton group focused on the physical and biological environment around krill aggregations to understand the causes for krill aggregations, specifically: Circulation fields transporting or retaining krill aggregations Phytoplankton and mesozooplankton as the diet fields for krill The field research was conducted in the 200 and 2010 May-June cruises of this project, and also in the 2013 May-June cruise in the same region from the NBP 1304 cruise (Figure 1). The significant discoveries include: 1) Discovery of Gerlache Strait Warm Current persistently supplying heat in austral fall and winter In May-June 2009, 2010 and 2013, a persistent northeastward current in the Gerlache Strait (referred as to Gerlache Strait Warm Current) was found which is consistent with finding from drifter measurements during the 1989-1992 RACER project (Zhou et al. 2001). This current delivers the relative warm shelf water originated from the Antarctic Circumpolar Current (ACC) into the Gerlache and western Bransfield Straits, which may set up the necessary physical environment for the biological hotspots in this region. The onshelf intrusions of the Upper Circumpolar Deep Water (UCDW) has been considered as being driven by topographic feature such as trough, and the curvature of shelf slope. In our recent analysis, a new mechanism is discussed that the strong westerly wind storms may lift the UCDW onto the shelf through Ekman pumping mechanism (Zhou et al. 2013). 2) Discovery of krill-whale hot spots in embayments of the Gerlache Strait High concentrations of krill were found in embayments along the Gerlache Strait together with large whale aggregations in all 2009, 2010 and 2013 cruises (Figure 3). Similar high concentrations of krill were also found in Wilhelmina and Charlotte Bays in a 1992 austral winter cruise. If the physical environment of temperature and ice conditions is critical to krill and whales, the variation of the Gerlache Strait Warm Current may significantly affect the survivorship of krill in this area. 3) Krill mortality The temporal change of the krill distribution in Wilhelmina Bay from the 2009 May-June survey is shown in Figure 4. The total krill biomass estimates are 2.6, 1.6 and 1.0 M ton in the survey periods of May 01-03, 18-20 and 28-31 2009, respectively. The decrease in biomass leads to the krill loss rate approximately -0.015 day-1 which is similar to the rate of mesozooplankton biomass decrease approximately -0.017 day-1 (Zhou et al. 2004). At this mortality rate, the remaining krill biomass will be approximately 20% by the August-September period (Figure 5). Comparing to such a large mortality, the grazing loss by whales is insignificant. 4) Krill diet: Grazing of krill on mesozooplankton in fall and winter seasons The inverse relationship between krill and mesozooplankton may indicate the grazing of krill on mesozooplankton (Figure 6) (Espinasse et al. 2012). In Figure 6, Station A was outside of the krill aggregation while Station B was within the krill aggregation. All samples were taken at the night when krill migrated to the surface layer. The differences in vertical distributions of mesozooplankton imply grazing effects of krill that mesozooplankton were grazed in the surface layer by krill at night.
