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.

Last Modified: 12/07/2013
Modified by: Meng Zho...

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