VALERIE LOEB, Moss Landing Marine Laboratories, Moss Landing, California 95039
Krill ( Euphausia superba ) is the keystone prey species in the antarctic seasonal sea-ice zone. Demographic information for krill includes length, sex, reproductive condition, and maturity stage composition. Length information is important for the acoustical estimation of krill biomass. Information on length, maturity stage composition, and reproductive condition is essential to assess between-year differences in krill spawning success and recruitment (i.e., the supply of juveniles spawned the previous summer). This assessment has relevance to the availability of krill to their predators, which include penguins, other seabirds, and seals monitored by the AMLR program.
Krill were obtained from a 1.8-meter (6-foot) Isaacs-Kidd Midwater Trawl (IKMT) fitted with a 505-micrometer mesh plankton net. Flow volumes were measured using a calibrated General Oceanics flow meter mounted in front of the net mouth. Samples were collected at 105 survey stations during Leg I and 16 stations during Leg II ( see Martin, Hewitt, and Holt, Antarctic Journal , in this issue). The station locations during Leg II permitted results representative of the large-area survey despite the small sample size (Loeb unpublished manuscript). All tows were fished obliquely from a depth of 170 meters or about 10 meters above bottom in shallower waters. Tow speeds were about 2 knots. Fresh or freshly frozen specimens were processed onboard. All krill were analyzed from samples containing less than 150 individuals. For larger samples, 150 to 200 individuals were measured, sexed, and staged. Measurements were made of total length (millimeters); stages were based on the classification scheme of Makarov and Denys (1981). Density is expressed as numbers per 1,000 cubic meters water filtered.
A total of 13,560 krill were collected in 98 (93 percent) of the Survey A stations during the period from 26 January to 9 February; mean and median abundances were 40.4 and 5.6 per 1,000 cubic meters, respectively (table). Six of the largest catches (145.5-569.0 per 1,000 cubic meters) occurred in Bransfield Strait, and three (115.8-483.2 per 1,000 cubic meters) occurred in Drake Passage waters adjacent to Livingston and Elephant Islands (figure 1). Krill lengths ranged from 15 to 58 millimeters (figure 2), and three distinct groups were centered around 26-27 millimeters, 34-39 millimeters, and 49-50 millimeters; these conform to the 1+, 2+, and 3+ age groups. The 30-42-millimeter length category was most abundant (54 percent of the total) and reflects successful recruitment of the 1994-1995 year class. Smaller and larger krill, respectively, made up 20 and 28 percent of the total. Mature stages dominated (37.2 percent). Juveniles constituted 29.9 percent, reflecting substantially lower recruitment of the 1995-1996 year class compared to that of 1994-1995. Most of the mature females exhibited ovarian development (3c stage), were gravid (3d), or spent (3e), indicating that peak spawning probably would occur 1-3 weeks later (i.e., late February to early March; Harrington and Ikeda 1986). Overall, 77.4 percent of mature females were in these advanced stages. Predominantly calyptopis stage krill larvae in our samples (approximately 5-7 weeks old) resulted from late December to early January spawning, whereas the few furcilia stages came from earlier (i.e., mid-December) spawning (Ross, Quetin, and Kirsh 1988)
A total of 1,327 krill were collected at 11 of 16 IKMT stations (69 percent) during Survey D, 16-23 March. Largest (204.2 and 134.4 per 1,000 cubic meters) and moderate-sized catches (22.8 to 65.2 per 1,000 cubic meters) occurred north and southwest of Elephant Island, whereas small or no catches generally occurred west and south of the island (figure 1). Mean and median krill abundance estimates were 30.4 and 4.6 per 1,000 cubic meters, respectively. These values are 18-25 percent lower than during Survey A (table). Lengths ranged from 24 to 57 millimeters and demonstrated three size groups centered around 29-30 millimeters (1+ age group), 37-41 millimeters (2+), and 49-52 millimeters (3+) (figure 2). The two smaller modes are 3 millimeters longer than those observed during Survey A; the larger mode is unchanged. This finding suggests summertime growth rates over a 45-day period of approximately 0.07 millimeters per day for the 1+ and 2+ age classes; this growth rate has also been reported for 2+ krill but is nearly half of that reported for the 1+ age group (Siegel and Kalinowski 1994). Limited sampling of juvenile krill (the 1+ age group) during Survey D could explain this underestimate. In contrast to Survey A, large krill 44 to 57 millimeters were most abundant (62.2 percent); intermediate 35-to 43-millimeter-sized krill made up 31.9 percent; and smaller krill made up 6.5 percent. These changes in composition could result from seasonal migrations of the different maturity stages (Siegel 1988). In accordance with the size distribution, most of the krill were mature (72.3 percent), whereas immature stages made up 19.7 percent and juveniles 8.0 percent. The mature females were primarily gravid (3d stage, 76.0 percent) and spent (3e, 19.0 percent), indicating a mid-March to late March spawning peak. The female maturity stage composition observed during Survey A suggested peak spawning in mid-February to early March. Either that spawning peak was delayed or a second spawning peak was initiated 1 month later.
Krill abundance during the 1997 surveys was similar to that observed during 1992 and 1993 (table). The marked abundance decrease relative to 1996 values was due to lower recruitment success of the 1995-1996 year class relative to that of 1994-1995. Exceedingly good recruitment success of the 1994-1995 year class was associated with above average sea-ice conditions during winter 1994 and 1995. The regional sea-ice index values for those years were 4.86 and 5.00, respectively, compared to an 18-year average value of 4.04. This good recruitment success probably results from early seasonal spawning activity and larval survival associated with extensive winter sea-ice conditions (Siegel and Loeb 1995; Loeb et al. 1997). Winter 1996 was characterized by average sea-ice conditions (sea-ice index of 4.02). Lower recruitment success of the 1995-1996 year class suggests that winter sea-ice extent greatly affects larval survival and recruitment even when krill spawning is relatively early.
This work was supported by National Oceanic and Atmospheric Administration contract number 50ABNF600014.
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