Abundance, size and maturity of krill (Euphausia superba) in the krill fishing ground of Subarea 48.1 during 1990/91 austral summer

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.