Euphausia superba (Antarctic krill) is a key component of the South Georgia (SG) ecosystem, supporting important populations of higher predators including seabirds, seals and whales. It is also the focus of a commercial fishery, which operates exclusively during winter. However, until recently, little has been known about the distribution of krill or their predators during this period. To address this, six winter ecosystem surveys were carried out along the northern shelf of South Georgia over two years (2022 and 2023), with timings set to correspond to the start, middle and end of the typical krill fishing season (Thorpe et al., this meeting). These surveys included the acoustic assessment of krill abundance and distribution (with transects repeated during day and night), cetacean, pinniped and seabird observations following JNCC Seabird at Sea Team (SAST) methods, and more detailed distance sampling and passive acoustic monitoring of marine mammals on selected surveys. Simultaneously, the movements of gentoo penguins from two locations on the northern shelf of SG were tracked with PTT and GPS tracking devices. Results from the first year of surveys (2022) were presented in Liszka et al. (2023a). Updated results pertaining to both full years of surveys (2022 and 2023) are presented here.
Krill biomass in the area (5,927 km2) normally used by the krill fishery (Eastern Core Box (ECB)), using combined day and night transect data in 2022, was estimated as 268,571 tonnes in May, 245,816 tonnes in July and 75,676 tonnes in September, whilst in 2023 it was 574,249 in April, 63,389 in July and 8,641 in September. Distinct diurnal variability in estimated biomass was observed, with significantly higher biomass estimated at night in July and September 2022 and higher, although non-significant, night-time biomasses in 2023 in all surveys. Krill swarms were also deeper in July of each year compared to the earlier and later surveys, and during the day compared to the night in all 2023 surveys.
In terms of predators, Antarctic fur seals were frequently sighted except on the May 2022 survey. The most frequently sighted whale species were humpback and southern right whales. Blue and diving petrels were the most commonly observed seabird species in both years, with higher abundances generally observed in 2022. In 2022, gentoo penguins remained almost entirely over the continental shelf, demonstrating little evidence of direct overlap with the area occupied by the fishery. In 2023, gentoos tagged at Maiviken spent more time offshore and hence overlapped more with the fishery.
Our results indicate a seasonal component to the distribution and behaviour of krill, with biomass decreasing in the ECB throughout the winter season. Our results also suggested that both overall biomass, and swarms, were deeper in July than other survey months. However, superimposed on this is distinct inter- and intra-annual variability, the drivers of which will require further work to better understand. Predator distributions exhibited patterns concurrent with the variable prey field, and further work will examine associations between predator distributions and krill.