AMLR 2009 Weekly Report
No. 2
11-19 January 2009
Oceanography
The CTD
system, with its 11 bottle carousel, dissolved oxygen
(DO) sensor, fluorometer, transmissometer,
altimeter and PAR sensor, was tested during the acoustic equipment calibrations
in Admiralty Bay. A further 24 casts were done successfully across the West
Area, with only station 11-05 being abandoned due to conditions being too rough
to attempt a CTD cast. Water samples were taken at predetermined depths at all
stations and salinity measured with a Portasal salinometer, to compare with the data collected by the CTD.
Water samples were also processed to measure dissolved oxygen using Winkler
titrations. A total of 10 stations were sampled where dissolved oxygen ranged
from <4 ml/L to > 8ml/L. These samples are used to verify the stability
of the CTD’s DO sensor performance. Northwesterly winds averaging 20 knts were experienced along Lines 1 to 5 of the Western
Area. A barometer drop to 975 millibar on January 16th, saw the wind swing to the Northeast, peaking at 45 knots,
accompanied by rough seas, rain and grey skies.
West Area
Krill and Zooplankton
Generally
small numbers of postlarval krill were present in 18
of the 23 IKMT net samples collected in the West Area. The largest catches of 110 and 122
individuals (22-23 per 1000 m3) occurred over the South Shetland
Island shelf. The overall mean and
median concentrations in this area (6 and 2 per 1000
m3) were similar to the low values recorded here during the January
2005 and 2006 surveys. The krill
were predominantly older, mature individuals. Lengths ranged from 20 to 56 mm, but the median and modal
lengths (48 and 50 mm) represent three-year old krill from the 2005/06 year class.
Males outnumbered females by 2:1 and most of the females (72%) were in
advanced maturity stages indicating active mating and spawning during the
survey period. The virtual absence
of larval krill in the samples suggests that this is the first major seasonal spawning
effort here.
Salpa thompsoni numerically dominated the
zooplankton collected in the West Area.
This salp occurred in all but one of the
samples with overall mean and median values of 1077 and 112 per 1000 m3. The aggregate chain form constituted
nearly the entire catch, with 95% of individuals less than 26 mm and a 15 mm
median and modal length. These
relatively small lengths suggest a delayed onset of chain production (e.g.,
late November) compared to previous years.
Copepods,
the pteropod Limacina helicina and postlarval Thysanoessa macrura
followed S. thompsoni
in abundance, with respective mean and median concentrations of 594 and 129 per
1000 m3, 122 and 116 per 1000 m3 and 120 and 64 per 1000
m3. With the exception
of L. helicina,
the overall composition and abundance of the zooplankton assemblage, including
krill, as well as krill demography in the West Area were quite similar to
conditions monitored in this area during January 2005.
Acoustic
estimates of Krill
Estimates
of krill biomass (g/m2) along transects on the West Shelf were very
low. Mean krill biomass was < 0.5g/m2. Total krill biomass,
estimated using the three frequency krill algorithm,
and the SDWBA technique showed that biomass was <21,000 tons. This is an
extremely low value, but similar to the values observed in 2001, 2005 and 2006.
Considerable amounts of other scatterers were present
in the water. These scatterers included salps, Limacina, and the
small euphausiids not well delineated by the techniques
we use. Interestingly, little of the acoustic backscatter seemed to be
attributable to Myctophid fishes. However, final analysis of the data have not been conducted, and definitive
estimates of occurrence and distribution will await the completion of the
survey.
Seabird and
mammal observations
Data on the
distribution, abundance and behavior of seabirds and mammals were collected
during underway ship operations in the West strata. Twenty-one transects were collected
totaling approximately 540 nautical miles of survey effort. The seabird
community consisted primarily of (percentage-wise): Cape Petrels, Chinstrap
Penguin, Southern Giant Petrel, Blue Petrel, Antarctic Prion,
Antarctic Fulmar, White-chinned Petrel, Black-browed Albatross, Wilson’s Storm
Petrel, Black-bellied Storm Petrel, Gray-headed Albatross, and Wandering
Albatross. In addition, 8 Royal Albatross (last recorded in the 2006 AMLR
survey), 3 Antarctic Petrels, and 5 Soft-plumaged Petrels were observed.
Seabird
feeding aggregations (primarily Cape Petrels) were patchily distributed and
were located in four locations. A known feeding aggregation ‘hotspot’, detected
during previous AMLR surveys north of King George Island, contained numerous
feeding birds. Another dense feeding aggregation was located ~40 nautical miles
north of Livingston Island.
A total of
24 Fin Whales (16 sightings), 12 Humpback Whales (9 sightings), and 2 Minke Whales (2 sightings) were observed. In addition, 25
Long-finned Pilot Whales were recorded during transit to the northern edge of
the West strata near the Shackleton Fracture Zone.
Phytoplankton
Phytoplankton
data will be presented in next weeks report for both the West Shelf and the
Elephant Island Areas.