AMLR 2006 Weekly Report No. 4

5 February 2006

We are currently sampling as part of the NSF sponsored AMLR supported nearshore study off Cape Shirref, SSI. We have completed the broad scale acoustic survey for krill biomass. As part of the nearshore survey the “Yuzmor” has been sampling krill along 3 transects during the night, and are acoustically sampling krill along10 transects over and area of approximately 400 nmi². At this time we have completed one pass through the entire grid, and will begin the second pass tomorrow morning. This sampling will continue until 9 February when the Yuzmor will anchor off cape Shireff and recover the nearshore team, moorings, and personnel before departing to Admiralty Bay for post cruise acoustic calibration.

Oceanography and meteorology. The reliable performance of the CTD, carousel, auxiliary sensors and onboard salinometer verification system, across the 129 casts of the main survey area, and the beginning of the Cape Sherriff inshore survey area, resulted in no lost survey time, due to equipment maintenance or repair. The week’s processed CTD data across the Bransfield Strait showed a predominance of Water Zone 4 type waters, except at the western end of the strait, where a mixed area of low surface salinity/warm surface temperature Water Zone 4, 3 and 2 types were sampled.

Monday and Tuesday saw Northwesterlies averaging <10 knots, but a barometric pressure trough of 1003 to 987 to 1005 millibars across Wednesday and Thursday, accompanied by a wind shift to the northeast, produced wind speeds averaging 32 knots, peaking at 42 knots on Thursday afternoon and a drop in air temperature to < 1°C.

Krill and Zooplankton – Joinville Island and South Survey Areas. Like the West and Elephant Island Areas, modest numbers of postlarval krill (Euphausia superba) were broadly distributed across Bransfield Strait, where they were present in 5 of 6 Joinville Island and all 20 South Area net samples. Concentrations encountered in the Joinville Island Area (95 and 15 per m³ mean and median values) exceeded those in the Elephant Island Area (24 and 11 per m³) with the largest Survey A catch (510 per 1000 m³) located here over the Joinville Island shelf. Smaller concentrations in the South Area (26 and 8 per 1000 m³ mean and median) were similar to those of the Elephant Island Area.

As usual, the krill age-maturity composition in Bransfield Strait was much more diverse than in the other areas and represented the inclusion of smaller and younger individuals. Lengths in the Joinville Island Area ranged from 23-55 mm, with modes at 32, 42 and 47 mm roughly corresponding to 1-, 2- and 3-year old krill. Half of the krill were < 42 mm in length. Accordingly, juveniles and immature stages comprised 19% and 29% of the total while the other 52% were mature stages. Males and females were equally represented, with 90% of mature females in advanced reproductive stages (i.e., with developing ovaries, gravid or spent). Gravid stages constituted 20% of total individuals here, including virtually all 2-year old (36+ mm) females collected.

Within the South Area lengths ranged from 26-59 mm with 34, 42 and 50 mm modes and a 48 mm median. Here juveniles represented only 5% and immature stages 15% of the total compared to 80% mature forms. This was due primarily to relatively large numbers of large mature krill associated with input of water from Drake Passage into northwestern Bransfield Strait.

Predominantly early stage calyptopis 1 stage krill larvae were found indicating a late December-early January spawn. The median concentrations in Joinville Island Area (14 per 1000 m³) were similar to those in the Elephant Island Area (19 per 1000 m³) while lower concentrations in the South (16 and 0 per 1000 m³ mean and median) were similar to those in the West Area. As in the Elephant Island Area, larval krill concentrations appeared associated with complex fronts and gyres in the eastern portion of the large Survey Area.

In contrast to previous years, Salpa thompsoni was virtually absent from Bransfield Strait confirming its current association with water north of the Southern Antarctic Circumpolar Current Front (SACCF). The other salp species Ihlea racovitzai, a marker for Weddell Sea water, was also rare reflecting the relatively limited presence of Weddell gyre (Zone 5) water this field season.

Copepods comprised the most abundant zooplankton taxon in Bransfield Strait, but with median concentrations (360 and 770 per 1000 m³) nearly an order of magnitude lower than in the West and Elephant Island Areas. The coastal species Metridia gerlachei was most abundant followed by small “other” unidentified species. Mean abundance of postlarvae of the euphausiid Thysanoessa macrura ranked third after copepods and larval krill in the Joinville Island Area while in the South Area it ranked second after copepods, followed by that of chaetognaths and high latitude euphausiid Euphausia crystallorophias. Of note was the presence of pelagic juvenile Antarctic silverfish, Pleuragramma antarcticum, in 50% of the Joinville Island and South Area samples. The distribution of these fish across south-central Bransfield Strait suggests input from Gerlache Strait located to the southwest.

Acoustics and biomass of krill. We have completed preliminary estimation of the biomass of krill in the Bransfield Strait, in subareas of Joinville and in the South area using the EK-60. Only two transects were completed in the Joinville area during daytime. Thus the biomass estimate 234 K tons, has high variability (CV=35%) associated with it. Mean krill density varied widely between the two transects (2.2 and 22 g m²) contributing to the high variability as well. In the southern area biomass was low just 134 K tons, and mean density of krill was 5 g/m². These numbers continue to be very low and suggest that biomass in the surveyed area was less than a million tons of biomass present in the South Shetlands. We will estimate krill biomass using the SDWBA approximation in the final report.

Phytoplankton. Continued high biomass was encountered in the Joinville Island and South Areas. In the Joinville Island Area, 5 meter chlorophyll averaged 0.98 ± 0.66 mg m^-3 (5 stations), with integrated values of 64 ± 25 and 38 ± 11 mg chl-a m^-2 for integration to 100 m and 1% PAR, respectively. In the South Area, 2.35 ± 1.39 mg chl-a m^-3 averaged at the surface (19 stations), and 100 m and 1% PAR integrated values of 101 ± 51 and 53 ± 12 mg m^-2, respectfully. Highest chl-a concentrations were found in the central and northern portions of the entire Bransfield Strait where 2.63 ± 1.25 mg m^-3 was the average at the surface (17 stations) and 103 ± 53 mg m^-2 averaged as integrated to 100 m. The lowest phytoplankton biomass recorded was measured along the peninsular shelf area where 0.7 ± 0.26 mg m^-3 was averaged for 5 m water (7 stations), and 70 ± 26 mg m^-2 was the average for 100 m integrated values.

Ancillary projects have measured in-situ radiometric, backscattering and fast repetition rate fluorometry data from 18 stations, with samples taken for 27 CHN and HPLC pigment, and 10 photosynthesis verses irradiance curves. 6 stations with 18 samples have been taken for particulate and dissolved iron, iron speciation and organic ligands. Also, 33 radium samples have been analyzed, plus 74 thorium-234 for vertical transport analysis. Approximately 175 samples have been collected for near-future nutrient analysis of different depths from various stations within the entire survey grid.

Predator Diet Studies. Lipids have been extracted from 48 Antarctic fur seal milk samples. A total of 41 scat samples from weeks 1-6 have been processed to date. Thus far, five scats have contained otoliths with week 6 starting to show a higher concentration of otoliths. The otolith of G. nicholsi is dominant in week 6 scats. The nearshore net tows are finding G. nicholsi at almost every station which is concurrent with the findings on the fur seal scats. Krill total length, calculated from the carapace length and width continues to range between 36-60 mm with the majority of total lengths averaging between 50-57 mm. The fish that are caught during the nearshore survey nightly net tows will be saved and used in the bomb calorimeter for total energy concentrations (at the SWFSC). In addition to the 150 frozen krill already measured, an additional 100 krill will be measured this week to continue the regression equations and discriminate function analyses.

Seabirds and Mammals. Data on the distribution, abundance and behavior of seabirds and mammals was collected during underway ship operations in the Joinville and South strata. Four transects were collected in the Joinville area, totaling 90 nautical miles of survey effort. Southern Fulmars were abundant during transit between stations, and one Antarctic Petrel was observed. Twelve transects were collected in the South area, totaling 250 nautical miles of survey effort (70% of South area). Southern Fulmars were very abundant along transects, and were frequently observed resting on the water in large rafts numbering from 20 to 50+ birds. They were usually encountered along the northern and southern shelf break regions in Bransfield Strait, especially near surface slicks. Feeding aggregations by Cape Petrels were common along transects in the vicinity of King George Island. On the westernmost transect in the South area (Station 15-15 to 17-13), numerous groups (size ranging from 20 to 30+) of Black-browed and Gray-headed Albatrosses were observed rafting and surface seizing for prey. These observations were the greatest densities of albatrosses observed during AMLR 2006. An exceptional sighting of a Snow Petrel was observed, and enjoyed by all on 1 February. Humpback Whales were the numerically dominant marine mammals encountered during the South survey period. Furthermore, on at least five transects, more than 20+ sightings were recorded of groups averaging 3 animals. Breaching by Humpbacks was common, and on several occasions 2 or more individuals were observed breaching simultaneously. On transit to station 09-11 south of Admiralty Bay, 5 Killer Whales were encountered.

C. Reiss sends.