Abundance, size and maturity of krill (Euphausia superba) in the krill fishing ground of Subarea 48.1 during the 1990/91 austral summer
Acoustic and net sampling surveys for krill were conducted in the krill fishing ground 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 area 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), intermediate in the frontal (37.3 g/m2) and neritic (28.1 g/m2) zones, and extremely high in the nearshore zone (134.7 g/m2). The last zone corresponds to the shelf break: or the shelf area where topographic eddies were generated, suggesting that hydrodynamic convergence might be responsible for accumulation of krill in this zone. The total biomass over the survey area was estimated to be 1.59±0.45 million tonnes (95% confidence limit), of which 1.22±0.42 million tonnes was concentrated in the fishing ground (frontal + neritic + nearshore zones). 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 survey described, were found restricted mainly to the nearshore zone. Gravid females were exceedingly abundant in the slope frontal zone, having a mean density of 23.9 g/m2 (411 000 tonnes), as contrasted with a low 3.7 g/m2 (163000 tonnes) in the oceanic zone. Gravid females were nearly absent in the neritic and nearshore zones. This indicates that slope frontal features may be important for the formation of favourable conditions for krill spawning.