AMLR 2003 Weekly Report No. 8

2 March 2003

1. Our current position is approximately one-third of the distance across Drakes Passage in route from the South Shetland Islands to Punta Arenas, Chile. Operations have been completed for the second leg of the AMLR 2003 cruise. During the three-days in port, there will be a major personnel exchange, provisions and fuel will be brought aboard, and the ship will again sail on 8 March. Leg III will consist of a benthic trawl survey and oceanographic measurements, around the South Orkney Islands and Antarctica Peninsula.

2. Earlier in the week a deep depression brought heavy gale conditions to the South Shetland Island archipelago. The ship was required to heave to for 48 hours with east winds gusting to 50 knots. Calls were to be made at the Admiralty Bay and Cape Shirreff field camps after the finishing of the large area survey to recover personnel (R. Holt, D. Krause, M. Antolos, L. Shill, J. Lyons, W. Trivelpiece, I. Saxer, S. Buckelew, L. Morse), equipment and garbage. The large swells associated with the heavy weather made zodiac operation unsafe and unmanageable at both camps during the gale. When the conditions changed for the better operations were conducted and all personnel and equipment were recovered safe and sound effectively closing both camps for the winter.

3. Krill, salps and other zooplankton. Survey D and 2003 field season overview

SURVEY D: Overall krill abundance in the large survey area during February was slightly lower than during the previous month (mean and median values 152 and 9 per 1000 m³ vs. 193 and 14 per 1000 m³). Greatest concentrations were located in the South (i.e., Bransfield Strait) vs. Elephant Island area the previous month. Krill lengths ranged from 15-54 mm and centered around a 33 mm median. Larger individuals predominated in offshore waters of the West and Elephant Island areas which, respectively, had 43 mm and 35 mm median lengths; smaller forms (< 35 mm) comprised the majority collected in South and Joinville Island areas. Accordingly, mature stages dominated in the West (64%) while juvenile and immature stages dominated in the South and Joinville Island areas where each represented between 42-48% of total krill. Krill size frequency distribution and maturity stage composition in the Elephant Island area differed from the previous month by having greater proportions of large, mature individuals (3+ year old krill) and fewer juveniles (1 year old krill). Juvenile, immature and mature stages, respectively, comprised 21%, 52% and 27% compared to 42%, 39% and 19% during January. These results reflect the seasonal southward migration of age/maturity categories. Apparent regression of secondary sex characteristics indicated the impending end of the seasonal reproductive period.

As during January, few reproductively active krill were collected. Greatest proportions of mature females in advanced stages (i.e., gravid or spent) were in the West (31%) and Elephant Island (5%) areas. The mature krill appeared gaunt and the few gravid females did not appear to have invested much in egg production suggesting poor feeding conditions. However, larval krill were found in albeit low abundance. Greatest concentrations (mean 6.1 per 1000 m³) were in the Elephant Island. While the majority of these were early Calyptopis stages (1 and 2), the presence of Furcilia stage 1 and 2 larvae (14% of total) indicates a prolonged, possibly pulsed, spawning season.

Copepods again numerically dominated the zooplankton catches and exhibited an overall seasonal abundance increase (mean and median values, respectively, 1535 and 610 per 1000 m³ vs. 610 and 180 per 1000 m³ in January). Even greater proportions were represented by the coastal species Metridia gerlachei which alone contributed 45% of total mean zooplankton abundance. Oceanic copepod species Calanoides acutus and Calanus propinquus were comparatively rare (together ca. 7%). The euphausiids Thysanoessa macrura and post larval krill followed M. gerlachei in overall mean abundance (respectively, 13% and 7% of total).

Salpa thompsoni continued to be in relatively low abundance, with overall mean and median values (78 and 10 per 1000 m³) unchanged from the January survey. Greatest salp concentrations were primarily limited to oceanic portions of the West area. Aggregates made up 93% and over wintering solitary stages 7% of the total salp catch. Median aggregate length (33 mm) reflected growth with minor new production (10% < 20 mm) from the previous month. Substantially increased abundance of small solitaries (80% < 45 mm), in conjunction with reduced aggregate production, indicate the impending end of the summer production period.

AMLR 2003 in Perspective:

Mean and median krill abundance and carbon biomass values in the Elephant Island area during January 2003 were the highest noted over the 1992-2003 period. These exceedingly large values were directly due to large, widespread concentrations of 1 year old juvenile and 2 year old immature stages and reflect good recruitment success of the 2001/2002 and 2000/2001 year classes.

Reduced, but still relatively large abundance and biomass values in February resulted from typical seasonal southward migration of young/small krill out of, and older/large mature krill into, the Elephant Island area. A substantial proportion of carbon biomass is attributed to abundant small krill.

Extremely small proportions of actively spawning adult krill during both 2003 surveys is in marked contrast to the past four years and the 1995-1997 period when ca. 60-93% of mature females were in advanced reproductive stages. However, low spawning activity also characterized the 1993 and 1998 “salp” years. Low larval krill abundance during 2003 was similar to that in summer 1996 (which also was preceded by extensive winter sea ice development) but modestly higher than during the 1998 salp year. These observations support the hypothesis that extremely abundant salps may negatively impact larval krill abundance through competition and/or predation; they do not support the hypothesis that extensive sea ice favors strong reproductive success that following summer.

Salp abundance values during 2003 were similar in magnitude to the long-term lows of 1995 and 1996 and are also attributed to extensive sea ice development the previous winter, theoretically through reduced abundance of over wintering solitary “seed” populations and delayed spring chain production. Likewise, Salp: Krill biomass values (0.2-0.03) match the lows during those years and reflect the inverse relations between krill recruitment success and salp abundance with winter sea ice extent.

Copepod abundance values, while generally 2-10× lower than during the 2000-2002 summers, were similar in magnitude to those in 1996, 1997 and 1999, and an order of magnitude larger than in the 1993 and 1998 salp years. The extreme dominance by Metridia gerlachei during February 2003 is unprecedented within the AMLR data set but equals that observed during the February-March 1984 salp year.

In summary, krill and salp conditions during the 2003 field season, support hypothesized associations with preceding winter sea ice conditions. Indications are that the 2003 summer was temporally limited and that another early, cold and icy winter is quite possible. Krill recruitment success from 2002/2003 is likely to be low and resulting from the pulse of early seasonal spawning. However, substantial increases in spawning stock size resulting from the past two years of strong recruitment success should help to buffer the impact of a modest 2002/2003 year class size. Coincidentally, the short summer 2003 season should once again limit the over wintering salp “seed” stock size and bloom potential during spring/summer 2004.

4. Krill biomass and dispersion. A total of nine transects were completed over the Joinville Island and South Areas during the week ending 02 March 2003. Ice conditions prevented the completion of all but 2 partial transects in the Joinville Island area and necessitated the truncation of four of the seven transects in the South Area. Highest densities of krill biomass were mapped along the 500 meter isobath on the westernmost transect of the Joinville area. This trend of elevated biomass along the 500 meter isobath continues in the South area with additional high biomass areas occurring over the 2000 m isobaths. Highest krill biomass in the South area was mapped to the south of Deception Island, to the south of Greenwich and Robert Islands, and the transect to the south of Admiralty Bay, King George Island. Krill densities for each transect in the Joinville Island transects ranged from 0.89 g/m² to 15.07 g/m². Mean krill density for the Joinville area is estimated to be 11.8 g/m^-2. Ice conditions prevented a survey for this area during Leg I of the 2003 survey. In 2002, krill biomass was estimated to be 7.44 and 4.3 g/m² (leg I and II respectively) for this area.

Krill densities in the South area transects ranged from 1.99 g/m² to 27.35 g/m². Mean krill density for the South area is estimated to be 13.63 g/m^-2. This estimate is similar to last months estimate of 11.34 g/m² and represents an decrease over 2002 estimates for Leg I (18.44 g/m²) and an increase over Leg II (4.81 g/m²).

Myctophid biomass estimates in the South and Joinville Areas were low, 8.11 g/m² and 9.95 g/m², respectively.

A successful calibration of the EK500 echo sounder occurred this morning at Ezcurra Inlet, Admiralty Bay, and King George Island. This ends the 2003 AMLR acoustic survey of krill biomass and dispersion in the South Shetland Island Area.

5. Phytoplankton. Chlorophyll concentrations in the South Area have remained near the same since Leg I. For 18 stations surveyed, 5 m chlorophyll was 0.43 +/- 0.18 mg m^-3 and integrated (to 100 m) was 23.13 +/- 6.92 mg chl m^-2. These values compared to 0.4 +/- 0.2 mg chl m^-3 and 22 +/- 8 mg chl m^-2 for 5 m and integrated values, respectively, measured for Leg I. Leg II values are lower than the February average (1995 - 2002) of 2.1 +/- 2.3 mg chl m^-3 for 5 m and 137.4 +/- 150.4 mg chl m^-2 for integrated (to 100 m) chlorophyll concentrations (39 stations). Highest chlorophyll concentrations were observed near shelf and shelf-break waters (bottom depth less than 1000 m), with 5 meter samples having 0.8 mg chl m^-3 (Stations 10-10 and 16-12). The Joinville Island area (4 stations) had 0.37 +/- 0.1 mg chl m^-3 and 22.66 +/- 5.57 mg chl m^-2 for 5 m and integrated (to 100 m) chlorophyll concentrations. Too few data have been obtained to give any historical comparisons. No SeaWiFS satellite data describing the surface chlorophyll distribution in the western Antarctic Peninsula were obtained. Due to failing ship to shore communications, no drifter buoy data have been obtained this week.

6. Oceanography and meteorology. Two major bad weather events occurred during the week, with corresponding drops in air pressure from 996 mbar to 976 mbar and 988 mbar to 978 mbar respectively. These lows were related to changes in wind direction from the north-west to east and accompanied by strong wind speeds averaging 35 knots, and gusting to over 40 knots. Air temperature showed a steady decrease throughout the week, dropping from 5°C to 2°C towards the end of the week. Most days were overcast and light snow occurred on the last three days of the week.

22 CTD stations were occupied in the Joinville Island and the South areas. 5 CTD stations in the northern Joinville Island area and 4 CTD stations in the South Area had to be abandoned due to ice. As per the Water Zone classification tables, Type IV (Bransfield Strait) water was found to occur throughout the region surveyed, with only 2 stations on the southern part of the grid found to be Type V (Weddell Sea) water and one station on the extreme western side of the South area, on the shelf, being Type II (Transition) water.

7. Predator diet studies. To the extreme relief of anyone with a slight sense of smell, this week the remaining scat samples were finished, in addition to the milk lipid extractions. This season a total of 60 scats and one enema were processed. There was strong evidence of an early to late season shift in the diet of the fur seals. The first three weeks, krill were the predominant prey type seen, with few examples of fish and squid. The last three weeks of scats showed a strong shift towards fish being the main prey, with krill appearing with much less frequency. There was also an increase in the number of squid beaks discovered. In total, 4,598 otoliths were collected, 13% in the first three weeks, and 87% in the last three. Twelve squid beaks were saved in ethanol for later species identification: four from leg one and 8 from leg two. In addition, 1,432 whole krill carapaces were recovered from the scats, and were measured for length and width. Between legs one and two, 102 fur seal milk samples underwent the Folch method of lipid extraction, and were frozen for transport back to the lab. Finally, to assist in the ground-truthing of a krill back-calculating equation, 269 fresh krill (134 females and 135 males) were gendered, measured for total length, and their carapaces removed and measured for length and width.

8. Bird and marine mammal observations. This report includes information regarding the Joinville Island, and Bransfield Strait surveys, and one transect in the Drake Passage. Systematic observations of vertebrate abundance and behavior were conducted during daylight hours in transit between stations. A total of 17 transits were sampled and 22 species of seabirds were recorded. Highlights included Snow petrel, Soft-plumaged petrel, and Parasitic Jaeger. Numerous aggregations of Black-browed albatrosses and Wilsons storm petrels were observed engaged in feeding activities in the Bransfield Strait, especially on transect line S4. In many instances, Fur seals were observed in the middle of such aggregations. These feeding assemblages may be associated with dense krill swarms, which were detected by the echo sounder on line S4. Humpback whales were abundant in the study area and were frequently observed feeding and breaching. Two Southern Right Whales were observed on transit to station 13-13, and were in the vicinity of dense ice floes. Other marine mammal observations included: Fur seal, Weddell seal, Elephant seal, Fin whale, Minke whale, Southern bottlenose whale, and Hourglass dolphin. In total, there were 66 transits conducted and 32 species of seabirds recorded during leg 2 of the 2003 AMLR survey.

A. Jenkins sends