AMLR 2003 Weekly Report No. 6

16 February 2003

 

1. Our current position is approximately 56 nautical miles to the SW of Elephant Island where we are conducting the second of two surveys of bio-oceanographic conditions in the vicinity of the South Shetland Islands. The survey is divided into four areas: the South Area in the central portion of Bransfield Strait south of King George and Livingston Islands, the West Area north of King George and Livingston Islands; the Elephant Island Area encompassing the northern portion of the South Shetland archipelago; and the Joinville Island Area in the western portion of Bransfield Strait. Survey operations in the West Area were completed this past week. The weather and sea state conditions have been moderate to heavy; ice conditions have been kindly to this point.

 

2. Krill, salps and other zooplankton. Post larval krill were present in 22 of 25 IKMT samples collected in the West survey area. Mean and median abundance values (93 and 21 per 1000 m3) were more than 2 larger than the previous month. Krill size and maturity stage composition were also increased over those observed here in January. This resulted from loss of small individuals <25 mm in length and a substantial increase in numbers of individuals >40 mm. Juvenile, immature and mature stages, respectively, comprised 9%, 27% and 64% compared to 19%, 45% and 36% the previous month. Reproductively mature males comprised 30% of all collected and 31% of the mature females were in advanced reproductive stages. These marked changes result from seasonal onshore migrations by the different size/maturity categories. In contrast to last month, no krill larvae were collected in the West area.

 

Although mean salp (Salpa thompsoni) abundance was 2 higher than the previous month (124 +/- 261 vs. 60 +/- 69 per 1000 m3) the associated standard deviation and median value (20 vs. 34 per 1000 m3) indicates that this was due to distributional attributes (i.e., patchiness) rather than a substantial abundance increase. The solitary form was only a minor component of the salp catch (5%). The 30 mm median length, similar to that in January, and paucity of individuals <20 mm in length suggest that peak seasonal production was in early December.

 

As during Survey A, copepods numerically dominated West area catches. Like krill, median copepod abundance was >2 the January value due to seasonal ontogenetic processes. One species, coastally derived Metridia gerlachei, accounted for over 50% of mean copepod abundance.

 

Of note was the presence of adult myctophids (25 total) in 5 of 7 night samples collected in the West area; this contrasts markedly with Survey A when only 2 of 8 night samples had myctophids (3 total). Mytophids are an important prey item for fur seals but were not represented in their diets last month. As with krill and copepods, it is possible that the seasonal presence of these fish within our survey area (and in proximity to predator populations) is related to ontogenetic and/or migratory processes.

 

3. Krill biomass and dispersion. Krill were detected acoustically throughout the West Area with highest biomass densities mapped inside the shelf edge and to the north of Greenwich and Robert Island. A second area of increased krill density was detected to the north of King George Island. During leg 2 of the 2003 AMLR cruise, mean krill density for the West area is estimated to be 29.2 g/m-2. This is a negligible increase over leg 1 density estimates (27 g/m-2) and a more substantial increase over observed densities in this area during February of 2002 (3.2 g/m-2) and 2001 (16.3 g/m-2).

 

4. Phytoplankton. Chlorophyll concentrations in the West Area have increased considerably since Leg I. For 24 stations covered, 5 m chlorophyll was 0.38 +/- 0.56 mg m-3 and integrated (to 100 m) was 19.64 +/- 15.93 mg m-2. These values compared to 0.18 +/- 0.09 mg chl m-3 and 12.25 +/- 6.8 mg chl m-2 for 5 m and integrated values, respectively, measured for Leg I. Leg II values are slightly lower than the February average (1995 - 2002) of 0.83 +/- 0.83 mg chl m-3 for 5 m and 56.58 +/- 36.64 mg chl m-2 for integrated (to 100 m) chlorophyll concentrations (160 stations). Highest chlorophyll concentrations were observed in shelf and shelf-break waters (bottom depth less than 1000 m) north of King George Island, with 2.3 mg chl m-3 recorded at station 11-07. Lowest chlorophyll concentrations were observed for offshore stations. Six drifter buoys have been deployed this Leg. Signals are still being obtained from the 12 drifters deployed during Leg I.

 

5. Oceanography and meteorology. Winds predominantly from the NW to SW, between 10 and 30 knts with peaks at 40 knts. Also brief periods from the NE averaging 10 knts. A sharp atmospheric pressure increase from 980 to 1008 mbar, during Friday, and a decrease to 993 mbar during Saturday and Sunday, was accompanied by NW'ly winds between 25 and 40 knts and rough sea conditions (5 to 8 m swells). Mainly overcast skies with brief periods of sunshine and air temperatures ranging between 0.5 and 4.5C were experienced during the week. A full moon was observed during Saturday and Sunday nights, which registered on the surface PAR sensor of the SCS.

 

27 CTD stations were done in the West Area. As per the Water Zone classification tables, Type 1 (ACW) water was found in the northern stations, off the shelf. The five inshore stations were found to be mainly Type 4 (Bransfield Straight) water, with five Type 2 (Transition) stations along the self-edge and one (D18-12) on the SW shelf.

 

6. Predator diet studies. This week 11 fur seal scats were processed. As expected, all samples contained krill. Unlike the first three weeks of scats collected, however, week four's scats showed a large increase in the amount of fish consumed. 9 of 11 scats contained otoliths, and in staggering numbers. 1, 601 otoliths were removed and identified to species. Otoliths from E. antarctica outnumbered G. nicholsi nearly 100 to 1 in week four. Interestingly, of the two scats processed from week five, the reverse is true, with G. nicholsi drastically outnumbering E. antarctica. No other fish species have been observed. Lipids were extracted from twenty milk samples, and the lab procedure went normally. In addition, 34 fresh krill have been measured for total length, carapace length, and width

 

7. Bird and marine mammal observations. This report includes information regarding the west area survey. Systematic observations of vertebrate abundance and behavior were conducted during daylight hours in transit between stations. A total of 18 transits were sampled and 20 species of seabirds were recorded. Highlights included soft-plumaged petrel and common diving petrel. Wandering Albatrosses were conspicuous in the survey area as ship followers, and there was a considerable amount of turnover of individuals attending the ship. Grey-headed albatrosses, which were scarce in leg 1, were now relatively common in the study area. Marine mammal observations included: fur seal, Weddell seal, leopard seal, humpback whale, minke whale, southern bottlenose whale, and hourglass dolphin. Fur seals were especially associated with the shelf zone north of Livingston Island.

 

A. Jenkins send