AMLR 2004 Weekly Report No. 3


25 January 2004


1. Our current position is approximately 30 miles south of Elephant Island where we are conducting the first of two surveys of bio-oceanographic conditions in the vicinity of the South Shetland Islands. The survey is divided into four strata: the West Area north of King George and Livingston Islands; the Elephant Island Area encompassing the northern portion of the South Shetland archipelago; the Joinville Island Area in the western portion of Bransfield Strait; and the South Area in the central portion of Bransfield Strait south of King George and Livingston Islands. Survey operations in the West Area and half of the Elephant Island Area were completed this past week. Heavy weather and seas inhibited progress during portions of the week.


2. Krill, salps and other zooplankton. Over 4000 krill (Euphausia superba) have been collected in 31 of 49 total Elephant Island Area samples. Overall abundance so far is ca. 4 times greater than that in the West Area. The overwhelming majority of krill (90%) are longer than 35 mm and represent the highly successful 2000-2001 and 2001-2002 year classes. Males and females are equally represented. About half of the mature females have mated and demonstrate ovarian development (stage 3C). This, in conjunction with relatively small concentrations of early larval stages, indicates that peak seasonal spawning has not yet occurred. As in the West Area, the paucity of small juvenile and immature sizes suggests minimal recruitment success from last year's (2002-2003) spawn. As in the West Area, copepods and the salp, Salpa Thompsoni, have similar abundance and numerically dominate the zooplankton assemblage. Metridia gerlachei, a coastal species, represents about half of the copepods so far collected in this area.  Post larvae of the euphausiid, Thysanoessa macrura, follow copepods and salps in overall abundance. The high latitude salp, Ihlea racovitzai, is regularly collected near the island shelf break zones and may indicate the presence of westward flowing slope water.


3. Krill biomass and dispersion. Mean krill biomass density in the West Area was 26 g m-2, similar to last year's estimate for this stratum of 27 g m-2. Krill were not as evenly distributed as last year with highest densities in the western portion of the West Area along the shelf break near Livingston Island. Highest densities of krill in the Elephant Island area were mapped near the shelf break northwest of Elephant Island. In contrast, myctophid fish were mapped in the eastern portion of the West Area and in the Elephant Island Area north of the shelf break.


4. Phytoplankton. Phytoplankton concentrations in the West area averaged 0.3 +/- 0.4 mg chl-a m-3 for 5 m samples (31 +/- 20 mg m-2 for chl-a concentrations integrated through the water column to 100 m). Chl-a concentrations were higher within the shelf and shelf-break region (0.5 +/- 0.5 mg m-3 and 42 +/- 24 mg m-2) compared with pelagic stations (0.2 +/- 0.3 mg m-3 and 25 +/- 15 mg m-2) for 5 meter and integrated values, respectively. These concentrations are slightly lower than historical (1997-2003) values of 1.1 +/- 0.9 and 0.4 +/- 0.5 mg chl-a m-3 for 5 m concentrations (50 +/- 36 and 30 +/- 25 mg m-2 for chl-a integrated to 100 m) for shelf/shelf break and pelagic waters, respectively. Satellite imagery (Modis Aqua) revealed bloom development along the northwestern coasts of the Shetland Islands and western coast of Elephant Island.


5. Oceanography and meteorology. 45 CTD stations were occupied in the West and Elephant Island Areas. According to the Water Zone Classification table, there was a less clearly defined distinction of the classical Water Zone 1 (ACW) type waters on the offshore stations in the West Area, than in previous years. Many stations in this area displayed a mixing of Water Zone 1 and 2 waters. This mixing was also evident in many of the shallower inshore stations, to the north of the islands, where the distinctions between Water Zones 2, 3 and 4 were not as distinct as in previous years. The 200 m horizontal temperature plot showed the intrusion of <0ēC water between King George and Elephant Islands. Predominantly northwesterly winds, averaging 20 knts, were experienced during the week. A barometer drop from 1003 to 965 mbar during Monday resulted in a strong westerly wind on Tuesday, peaking at 45 kt. Air temperatures ranged between 1 and 4ēC, and periods of clear skies were experienced.


6. The instrument buoy moored off Cape Sherriff, continued to send data to the base during the week, for 5 minutes, every 15 minutes. The island personnel reported the real-time data was “very, very exciting!” as they had been able to observe changing aggregations of acoustic targets detected by the 50 and 200 kHz echosounders fitted in the buoy.  


7. Predator diet studies. 35 milk samples have been processed for lipid extraction. Transterification will be performed at the SWFSC in March. To date, 5 Gymnoscopalus braui and 15 Electrona antarctica myctophid fish have been caught in the net tows for lipid extraction studies as well as krill. 17 fur seal scat samples have been processed from Cape Shirreff from weeks 1-4 of this season. All samples contain krill and only a few contain otoliths from myctophid species of G. nicholsi, E. carlsbergi and E. antarctica. None of the scats contain any squid beaks. Additionally, 190 live krill have been sexed, measured for total length, removed carapace length and removed carapace width for a validation study.


8. Bird and marine mammal observations. As of January 25, We have conducted 57 transits between stations. 18 species of seabirds were recorded in the West Area, and 24 in the Elephant Island Area. Blue Petrels and Antarctic Prions were very common during transits over the South Shetland Trough and in offshore waters over the Shackleton Fracture Zone. These species, which are usually more abundant in proximity to the convergence zone in the Drake Passage, are more abundant and spatially contagious this year than in either leg of AMLR 2003. A total of 16 Soft-plumaged Petrels have been recorded in the northwest section of the Elephant Island Area. This species was not detected within the survey area in Leg I of AMLR 2003. Other unusual species include Thin-billed Prions (4), Common Diving Petrel (2) and Arctic Tern (1). Five species of albatrosses have been recorded. Wandering and Royal Albatrosses have been regular ship followers, and on many occasions individual birds have been observed following for over 12 hours. White-chinned petrels were common in the Elephant Island Area, and on different occasions two groups numbering 29 and 11, were observed sitting on the water. Chinstrap Penguins and Cape Petrels continue to be numerous. Cape Petrels have been observed capturing prey throughout the survey area. Chinstrap penguins are also abundant, and so far the peak abundance was encountered on transits in the vicinity of Elephant Island. The following marine mammals have been observed during this portion of our survey (in order of abundance): Cetaceans: Humpback (adult and juvenile), Fin, Minke, Southern Bottlenose, and Killer Whales; Pinnipeds: Antarctic Fur Seal. Fur seals have been observed in small groups (2-5) in the offshore waters of the Elephant Island transects. On station line 04, north of the east end of Elephant Island, five separate groups (2-3 individuals) of Fin, and Minke whales were observed traveling north. On the same station line, in transit between 04-05 and 04-06, two Killer Whales were observed between Clarence and Cornwallis Islands.


Submitted by Roger Hewitt