Seabird and seal responses to the physical environment and to spatio-temporal variation in the distribution and abundance of Antarctic krill at South Georgia, with implications for local fisheries management

Abstract

We used 22 years of seasonally and spatially consistent monitoring data to explore marine predator–prey numerical response relationships. Specifically, we tested whether indices of offspring performance (e.g. offspring mass near the time of their independence and/or growth rate) from three Antarctic krill-dependent predators, showed positive relationships with estimates of krill density, determined using fishery-independent acoustic surveys undertaken towards the middle of the predator breeding season. Results showed that indices of predator reproductive performance had little relationship with krill density. In most years, average krill densities were higher on-shelf than off-shelf, potentially providing ecological buffering for predators provisioning offspring. Interestingly, positive response relationships were evident between predator offspring mass and the spatial distribution of krill, measured using indices that represent levels of inequality (patchiness) in krill distribution. These relationships were strongest using indices that reflected the off-shelf krill spatial distribution. We found that krill density and predator offspring mass were also both negatively influenced by sea surface temperature and the Southern Annular Mode, indicating that the environment exerts strong control over ecosystem processes. Finally, we consider the relevance of our results to the ecological framework used by managers responsible for setting catch limits for the regional fishery for krill.

A dynamic framework for assessing and managing risks to ecosystems from fisheries: demonstration for conserving the krill-based food web in Antarctica

“…Krafft et al (2021) reported that while variation in the distribution of krill may occur between years, there is no evidence of a general regional decline in the last 20 years. Smaller, regional acoustic surveys of krill biomass reflect high levels of intra-and inter-annual variability in stock density and biomass, but reveal no trend in abundance (Reiss et al, 2008;Reid et al, 2010;Fielding et al, 2014;Trathan et al, 2022a). However, these surveys do show differences in distribution and biomass between summer and winter (Saunders et al, 2007;Reiss et al, 2017), including with biomass occurring deeper vertically in the water column during winter (Lascara et al, 1999).…”

“…The mesoscale distribution of krill is thought to be related to sea ice, shelf bathymetry, and ocean currents, particularly the fronts in the Antarctic Circumpolar Current (ACC) (Johnston et al, 2022) or further south (Trathan et al, 2022a). Although there are no consistent environmental relationships apparent across regions in space and time (Silk et al, 2016), the ACC plays an important role in transporting krill across the region, beginning in the west Antarctic Peninsula and southern Weddell Sea (Hofmann and Murphy, 2004;Thorpe and Murphy, 2022), indicating that the 'stock' of krill extends across the south Atlantic, at least to CCAMLR Subareas 48.1,48.2,48.3,and 48.4, but probably more extensively.…”

Interactions between krill and its predators in the western Ross Sea

“…Weddell seals are less dependent on krill as a food source and remain connected to the stable ice and to the preys that can be found there (Wege et al, 2021). Trathan et al (2022) found a positive response relationship between predator offspring mass (Antarctic fur seals, gentoo penguins and macaroni penguins) and the spatial distribution of krill, measuring the patchiness in krill distribution, while they found little relation between predator performance and krill density levels at South Georgia. This result evidence the importance of the information derived from krill spatial distribution when studying krill predators' diet and breeding success.…”