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The objective of this work was to provide information on the demographic structure of antarctic krill stocks (Euphausia superba) in the Antarctic Marine Living Resources (AMLR) study area. Demographic information for krill includes length, sex, reproductive condition, and maturity stage composition. The length information is important to the AMLR acoustics program to estimate 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). The size of this supply has relevance to the availability of krill to their predators.
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 on the frame in front of the net mouth opening. Samples were collected at the 91 large-area survey stations during each cruise leg (see Martin, Hewitt, and Holt, Antarctic Journal, in this issue). All tows were fished obliquely from a depth of 170 meters (m) or about 10 m above bottom in shallower waters. Tow depths were derived from a real-time depth recorder mounted on the trawl bridle and monitored in the acoustics lab. Tow speeds were about 2 knots. Samples were processed onboard using fresh or freshly frozen specimens. All krill were analyzed from samples containing fewer than 150 individuals. For larger samples, 100-200 individuals were measured, sexed, and staged. Measurements were made of total length [in millimeters (mm)]; stages were based on the classification scheme of Makarov and Denys (1981). Density estimates are expressed as numbers per square meter sea surface area. Data are presented for the large-area surveys and for the more restricted "Elephant Island Area" (a box around Elephant Island) to allow comparison with previous AMLR cruises ( figure 1).
Krill were present at all but three stations during Survey A (23 January to 4 February), yielding an estimated total of 46,831 individuals. Relatively high krill densities occurred in waters over and adjacent to the northern and southeastern shelves of King George and Elephant Islands (figure 1). The relatively high mean abundance of 18.9 per square meter (m-2) (±70.3) was mostly due to three large catches (233.4 to 579.6 m-2); the median abundance was 2.0 m-2. The overall length frequency distribution was bimodal ( figure 2) and numerically dominated (71.3 percent) by small krill 22 to 33 mm long; large krill of 44- to 53-mm lengths constituted the second mode (18.6 percent). The small length mode was due to large numbers of juveniles (63.2 percent) and immature males (2a stage; table) and represents a relatively abundant 1-year-old age class (i.e., the 1994-1995 year class). The large-size mode included primarily mature stages older than 4 years of age. Cluster analysis indicated that the small krill were broadly distributed across the southern portion of the survey area whereas large mature forms were primarily within the Drake Passage. Relatively small proportions of intermediate sized krill (34 to 43 mm, 7.5 percent) result from poor recruitment of the 1992-1993 and 1993-1994 year classes. Almost all of the mature females were in advanced stages: 24.6 percent had developing gonads (3c stage); 7.4 percent were gravid (3d); and 66.0 percent were spent (3e). The common occurrence of spent forms that still had numerous eggs remaining in the ovaries suggests that a substantial proportion of the females were undergoing batch spawning during the survey period.
Approximately 33,795 krill were collected at 79 Survey D stations (24 February to 8 March). The larger samples generally occurred in the area extending from northeast of King George Island to south and east of Elephant Island. The largest catch, estimated to be 26,650 krill (1,255.5 m-2), was at Station D80 northeast of Clarence Island (figure 1). This sample represented 79 percent of the krill collected during Survey D. The extremely large krill catch overwhelms those from other survey stations and is generally omitted here to describe conditions characteristic of the broader survey area. For example, the mean krill abundance for all Survey D stations was 16.9 m-2 (±130.7), whereas that after excluding Station D80 was 3.1 m-2 (±7.0). The median abundance estimates with and without D80 were similar, 0.6 and 0.5 m-2, respectively, reflecting relatively low abundance of krill across the survey area during this time.
The overall krill length-frequency distribution from the survey area (excluding D80) was again bimodal, and most of the krill were within 25- to 33-mm and 45- to 53-mm length categories (figure 2). In contrast to Survey A, large mature forms numerically dominated (71 percent), whereas juveniles and young immature males smaller than 34 mm constituted only 23 percent of the total (table). The distribution of these smaller krill was limited to a few stations adjacent to and south of Elephant Island. As during Survey A, most of the mature females were in advanced maturity stages: 57.6 percent were gravid (3d) and 22.1 percent spent (3e).
Mean and median krill abundance values in the Elephant Island area during Survey A were the highest recorded in the past 5 years (table). This high abundance was primarily due to large proportions of 1-year-old krill (juveniles and stage 2a males). The substantial decrease in Survey D krill abundance estimates relative to January values was also observed in February and March 1994 and 1995 and most likely represents seasonal migration out of the area. Decreased proportions of juveniles in February and March compared to January sampling periods have been observed each year and probably result from different distribution and seasonal migration patterns of the various maturity stages (Siegel 1988, pp. 219-230).
Most of the female krill were in advanced maturity stages during the 1995 and 1996 surveys (table), but smaller proportions of spent females and low numbers of krill larvae collected in February and March 1996 compared to 1995 (Loeb and Outram, Antarctic Journal, in this issue) suggest a later peak spawning period in 1996. The early spawning seasons of 1994-1995 and 1995-1996 differ substantially from the previous 3 years when most females were in early stages of gonadal development during the period from January to March.
The krill recruitment index (R1) based on the relative proportion of total krill constituted by the 1-year-old age group during Survey A was 0.622. This value was one of the highest recorded over the past 18 years and indicates strong recruitment of the 1994-1995 year class. Strong recruitment results from good spawning success and survival of early stages spawned the previous year. These two conditions were associated with large proportions of advanced female maturity stages during the period from January to March 1995 (table) and extensive sea-ice conditions in the Antarctic Peninsula region during winter 1995. These results support the hypothesized relations between krill recruitment success, krill spawning seasonality, and winter sea-ice conditions presented in Siegel and Loeb (1995). In accordance with the apparent relationship between early spawning and good recruitment, strong year class success may result from the relatively large proportions of advanced female stages during 1996.
This work was supported by National Oceanic and Atmospheric Administration Contract Number 50ABNF600014.
References
Loeb, V., and D. Outram. 1996. AMLR program: Salps and other macrozooplankton sampled, January to March 1996. Antarctic Journal of the U.S., 31(2).
Makarov, R.R., and C.J.I. Denys. 1981. Stages of sexual maturity of Euphausia superba. In BIOMASS Handbook 11.
Martin, J.E., R.P. Hewitt, and R.S. Holt. 1996. The U.S. Antarctic Marine Living Resources (AMLR) program: 1995-1996 field season activities. Antarctic Journal of the U.S., 31(2).
Siegel, V. 1988. A concept of seasonal variation of krill (Euphausia superba) distribution and abundance west of the Antarctic Peninsula. In D. Sahrhage (Ed.), Antarctic ocean and resources variability. Springer-Verlag Berlin.
Siegel, V., and V. Loeb. 1995. Recruitment of antarctic krill Euphausia superba and possible causes for its variability. Marine Ecology Progress Series, 123, 45-56.