AMLR 2003 Weekly Report No. 8
Our current position is approximately one-third of the distance across Drakes
Passage in route from the
Earlier in the week a deep depression brought heavy gale conditions to the
3. Krill, salps and other zooplankton. Survey D and 2003 field season overview
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
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
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:
and median krill abundance and carbon biomass values in the
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
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.
biomass and dispersion. A total of nine transects were completed over the
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.
successful calibration of the EK500 echo sounder occurred this morning at Ezcurra Inlet,
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
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.
CTD stations were occupied in the
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.
and marine mammal observations. This report includes information regarding
A. Jenkins sends