Shipboard tracking study for krill-eating predators (penguins and female fur seals) at Seal Island were conducted to identify and evaluate their foraging areas during early January 1990/91. Penguin foraging areas were formed in inshore region, where krill frequently occurred but higher density areas of krill (≥250g/m2) were rather limited. In contrast, fur sea! foraging areas were formed in offshore region, where krill occurred only occasionally but they tended to form extensive aggregations (ca. 2-3 km in horizontal distance) of higher densities (≥250g/m2). In the inshore foraging areas krill showed a diurnal vertical migration, tending to be at deeper depth range (50-100 m deep) in the day time while at shallower layer (20-50 m) at night. In the offshore foraging areas krill showed no diurnal vertical migration, staying close to the surface throughout the day. As for body size and maturity of krill, in the inshore foraging areas middle-sized krill (modal length 43 mm), which consisted mainly of nongravid krill, were dominant with occasional occurrences of juveniles (modal length 21 mm). In contrast, in the offshore foraging areas large-sized krill (modal length 47 mm), of which the majority were gravid females, dominated. Thus horizontal and vertical distributions and population structure of krill were totally different between the two foraging areas. The reasons why the fur seal chose offshore foraging areas over inshore foraging areas were discussed.
Abstract:
Acoustic and net sampling surveys for krill were conducted in the krill fishing area north of the South Shetland Islands from 18 January to 3 February 1991. Distinct offshore-inshore heterogeneities in abundance and maturity of krill were observed. The survey region was divided into four zones; oceanic, slope frontal, neritic and nearshore zones. The mean density of krill was low in the oceanic zone (8.5 g/m2), while, intermediate values in the frontal (37.3 g/m2) and neritic zone (28.1 g/m2), and extremely high in the nearshore zone (134.7 g/m2). The last zone corresponds to the shelf break or on the shelf where topographic eddies were generated, suggesting the hydrodynamic convergence might accumulate krill into this zone. The total biomass over the survey region was estimated to be 1.59 ± 0.45 million t (95% confidence limit), of which 1.22 ± 0.42 million t was concentrated in the fishing grounds ("frontal" + "neritic" + "nearshore"). Information from other studies indicated that krill biomass in this region had been lower than expected until early February 1991. As for maturity stages of krill, spawning krill (modal body length 49 mm) were dominant in the oceanic and frontal zones, whereas less mature krill (modal length 45 mm) dominated in the neritic and nearshore zones. Juveniles, which were scarce in the present stock, were restricted mainly to the nearshore zone. Gravid females were exceedingly abundant in the slope frontal zone with the mean density of 23.9 g/m2 (411,000t), as contrasted with the lower value of 3.7 g/m2 (163,000t) in the oceanic zone and almost absence from the neritic and nearshore zones. This indicates that slope frontal features may be important for the formation of favorable spawning grounds for krill.
