AMLR 2003 Weekly
Report No. 6
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
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
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
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 4.5°C 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
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
A.
Jenkins send