AMLR 2002 Weekly Report No. 4

 

3 February 2002

 

1. Current position is just off the east end of Elephant Island at Cape Valentine. The survey of bio-oceanographic conditions in the South Shetland Islands was finished this week with completion of the Joinville Island and South Areas. Personnel from the Copacabana field camp (W. Trivelpiece) and the Cape Shirreff field camp (R. Holt, M. Goebel and V. Vallejos) were brought aboard to conduct a census of Antarctic fur seal pups throughout the South Shetland Islands (see item 7).

 

2. Krill, salps and other zooplankton. Post-larval krill were present in 75% of 95 survey samples. The largest catch, from the South area, contained nearly 4000 individuals (1477 krill per 1000 m3). Other large catches (i.e., >1000 krill, estimated 400-700 per 1000 m3) were taken in all areas. These concentrations were located north of Livingston and King George Islands (Drake Passage), north of Joinville Island (Bransfield Strait) and northeast of Elephant Island area. Abundance differences among the four areas were not significant (ANOVA, P>>0.05 in all cases). Mean and median abundance in the Elephant Island area (39 and 8 per 1000 m3, respectively) were slightly higher than during the January 2001 survey (19 and 6 per 1000 m3). Juveniles, representing successful recruitment of the 2000/2001 year class, dominated the krill catches in all four areas. Krill >32 mm were rare in the South and Joinville areas where juveniles constituted 88-93% of individuals. Broader size ranges (16-60 mm) and more heterogeneous length-maturity stage compositions were represented in the West and Elephant Island areas. In the West area length distribution was polymodal with peaks around 22, 25, 31, 36 and 53-55 mm modes; 57% were juveniles, 17% immature and 26% mature stages. Reproductively mature males (M3b) constituted 6% and females 20% of the total; 84% of these females were in advanced stages, predominantly gravid (F3d). Small juveniles (20-30 mm) made up 46% of the Elephant Island catch. Older krill had lengths centered about a 41-42 mm mode and 20% of were >45 mm (i.e., >4 years old). Immature and mature stages comprised 9% and 45%, respectively. Females outnumbered males by 60%; most of these females (92%) were in advanced stages indicating active spawning in the area. The overall abundance and size/maturity composition indicated: extremely good proportional recruitment of the 2000/2001 year class; essential absence of recruits from the 1999/2000 year class (2 year-old krill); and markedly reduced numbers of krill from the highly successful 1995/1996 year class. Krill larvae were present in 23% of the samples. Greatest concentrations were in the Elephant Island area. The mean 36 per 1000 m3 was essentially the same as observed during January 2001. Calyptopis stages dominated, and resulted from spawning 30-45 days earlier (i.e., early to mid December). These observations, along with the adult spawning condition, indicate a normal spawning season and bode well for recruitment success of the 2001/2002 year class.

 

Salpa thompsoni was present in 88% of survey samples. Abundance was greatest in the Elephant Island (410 and 86 per 1000 m3) and South (201 and 71 per 1000 m3) areas and lowest in the Joinville Island area (184 and 2 per 1000 m3). Aggregate (chain) forms constituted 98% of the overall catch. Presence of extremely large aggregates indicate a particularly early onset of seasonal chain production. Despite a prolonged production period salp abundance in the Elephant Island was relatively low, suggesting that they were not being retained within the area during summer 2002.

 

Copepods numerically dominated the zooplankton in all four areas. The primary constituents, in order of abundance, were Calanoides acutus, Calanus propinquus and Metridia gerlachei. Greatest concentrations were in the West area (25400 per 1000 m3 mean, 12500 per 1000 m3 median) associated with the Oceanic influence. Mean and median copepod abundance values in the Elephant Island area were 1 to 2 orders of magnitude larger than observed during previous January surveys. Larvae of the euphausiid Thysanoessa macrura comprised the second most abundant taxon overall. Due to their predominantly offshore distribution they also ranked second in abundance to copepods in West and Elephant Island areas. Postlarval T. macrura, having a predominantly coastal distribution, ranked second in abundance to copepods in the South area. The overall impression is of high secondary production over much of the January 2002 survey area.

 

3. Krill biomass and dispersion. Upon completion of Survey A, krill biomass densities were estimated as 10 to 28 g m-2, 19 to 40 g m-2, 7 g m-2, and 15 g m-2 for the West, Elephant Island, Joinville Island, and South areas, respectively. In comparison, biomass densities for the West, Elephant Island, and South areas of 17 g m-2, 16 g m-2, and 13 g m-2 were estimated during Survey A of 2001. Greatest krill biomass densities are located halfway between King George Island and Elephant Island, and north of Elephant Island. Other areas of high densities were the regions northwest of King George Island, north of Livingston Island, and northwest of Snow Island. These areas were consistently located along the 500 m isobath. The Joinville Island area was relatively low in krill abundance with respect to the West, Elephant Island and South areas.

 

4. Phytoplankton. The pattern for surface chlorophyll concentrations in the Bransfield Strait and Joinville Island Area closely follows the types of water, with low values found for Weddell Sea water and high values for Bransfield Strait water. 5m chlorophyll averages 0.84 +\- 0.78 mg m-3, and integrated (100 meters) averages 47 +\- 28 mg m-2 for the entire area (31 stations). The Bransfield Strait region closest to the Shetland Islands (7 stations) averaged 1.89 +\- 1.00 mg chl m-3 as compared to 0.36 +\- 0.27 mg chl m-3 for those stations closest to the peninsula (10 stations). The most phytoplankton rich area of the entire first leg were for stations 11-11, 09-09 and 12-12 having highest 5 meter chlorophyll concentrations of 3.2, 2.8 and 2.4 mg m-3, respectively. The lowest chlorophyll concentrations of the first leg were found near the Weddell Sea (Stations 02-13, 04-11, and 04-13), having 0.08 +\- 0.01 mg chl m-3. Upon completion of the survey grid, 100 high pressure liquid chromotography, 31 particulate organic carbon, 101 particulate absorbance, and miscellaneous particulate fluorescence and particulate size distribution samples of the water column were collected for analysis at Scripps Institution of Oceanography. These samples were in addition to 7 deployments of an integrated optics package, 31 casts of a free-fall profiling radiometer, and 92 CTD casts that incorporated a fluorometer and both a blue and a red transmissometer.

 

5. Oceanography and meterology: Overall, as in previous years, the southern part of the survey is mainly type IV (Bransfield water) with an intrusion of type V (Weddell water) from the south-east. The north-east axis through the center of the survey (including the island chain) is dominated by transition water (types 2 & 3 identified) meandering into the north. The north-western area is influenced by Drake Passage water and the southern boundary of the Antarctic Circumpolar Current (ACC). The presence of type I water is less clearly defined with the characteristic winter water (WW) subsurface temperature minimum not as pronounced nor as cold as the previous season and at a slightly higher salinity. Although the 1.8C isotherm marking the southern boundary of the ACC cuts through the survey area from west to north-east, meandering in the area NW of Elephant island, the temperature-salinity (TS) curves from CTD casts north of this are not conclusively type I and the tentative conclusion is that the area surveyed north of the island archipelago is mainly transition water.

 

Moderate North Westerly to Westerly winds (<20knts) for most of the week with some periods of stronger Easterlies (up to 30 knts). Air temperatures were mostly above 1C, with a minimum of 0.5C and a brief peak of 7C. Overcast and foggy days were interspersed by four partly cloudy days. The highest barometer reading for trip so far was recorded this week, being 1007mbars.

 

6. Predator diet studies. Scats from weeks 1-4 have been processed (38 scats and 3 regurgitations). In the regurgitation sample processed this week, whole undigested krill were measured according to CCAMLR Standard Length Measurement in addition to carapace measurements in order to ground-truth the regression used to relate carapace size to body length. Prey samples of myctophid fish have been identified by species, measured for Standard Length, weighed, and sex has been determined as well as stomach fullness and contents. Each myctophid had their otoliths removed as a future identification guide. Samples were then homogenized and frozen for further studies.

 

7. The 2002 Survey of South Shetlands fur seal colonies began on 30 January. The first census was of San Telmo Island, the largest single colony in the survey, begun with the best of weather conditions. Our weather held for two days and we were able to census Window Island, Start Point and Cape Smith (last surveyed in 1986/87), Desolation Island, and the Fildes Peninsula, King George Island. On 1 Feb we landed at Stigant Point and tried to make a landing later in the day at Turret Point on the south coast of King George Island. We chose to investigate Turret Point because of an anecdotal report of fur seal pups there. If confirmed, Turret Point, King George Island, would be the first re-colonization of a south coast site in the South Shetlands. Unfortunately, our visit was cancelled due to high winds. Yesterday and today we surveyed the Seal Island and Cape Valentine group of colonies. Thus far, most sites appear to be stable and have similar numbers of pups when compared to previous surveys. The exceptions have been North Cove, Seal Island and Cape Lindsey, Elephant Island, both of which had significant reductions in total pups. However, two other satellite colonies at Seal Island had modest increases.