Estimating the average distribution of Antarctic krill Euphausia superba at the northern Antarctic Peninsula during austral summer and winter

Abstract

This study was performed to aid the management of the fishery for Antarctic krill Euphausia superba. Krill are an important component of the Antarctic marine ecosystem, providing a key food source for many marine predators. Additionally, krill are the target of the largest commercial fishery in the Southern Ocean, for which annual catches have been increasing and concentrating in recent years. The krill fishery is managed by the Commission for the Conservation of Antarctic Marine Living Resources (CCAMLR), which has endorsed a new management framework that requires information about the spatial distribution and biomass of krill. Here, we use krill density estimates from acoustic surveys and a GAMM framework to model habitat properties associated with high krill biomass during summer and winter in the northern Antarctic Peninsula region, an area important to the commercial fishery. Our models show elevated krill density associated with the shelf break, increased sea surface temperature, moderate chlorophyll-a concentration and increased salinity. During winter, our models show associations with shallow waters (< 1500 m) with low sea-ice concentration, medium sea-level anomaly and medium current speed. Our models predict temporal averages of the distribution and density of krill, which can be used to aid CCAMLR’s revised ecosystem approach to fisheries management. Our models have the potential to help in the spatial and temporal design of future acoustic surveys that would preclude the need for modelled extrapolations. We highlight that the ecosystem approach to fisheries management of krill critically depends upon such field observations at relevant spatial and temporal scales.

Using seabird and whale distribution models to estimate spatial consumption of krill to inform fishery management

“…In the Antarctic Peninsula and South Shetland Islands region, larger krill are found mainly in the open ocean and along the shelf‐break during spring and summer and juvenile krill occupy the inner shelf waters (Atkinson et al, 2008; Reiss et al, 2008; Siegel et al, 2013). Additionally, adult krill perform seasonal migrations from offshore waters in summer to on‐shelf habitats, often under sea ice or in the marginal ice zone, in winter (Lascara et al, 1999; Marschall, 1988; Nicol, 2006; Reiss et al, 2017; Siegel, 1988; Warwick‐Evans et al., 2022).…”

Using a risk assessment framework to spatially and temporally spread the fishery catch limit for Antarctic krill in the west Antarctic Peninsula: A template for krill fisheries elsewhere

“…However, for other ecosystem components, considerable inter-annual variation exists; for example, the maximum acoustic density of krill sampled along the U.S. AMLR survey transects ranged from 827 to 6944 g m -2 but with no long-term trend observed. Indeed, estimates of summer krill biomass were lower than the 13 year average in seven of the sampled years (Warwick-Evans et al, 2022a), potentially resulting in an underestimate of risk in those years. Fortunately, and as noted previously, our results are largely consistent with those from dynamic ecosystem models (e.g., Plagańyi and Butterworth, 2012;Watters et al, 2013) that do attempt to quantify risks while the underlying abundances of species change.…”

“…The distribution of krill across the study area was estimated using generalised additive mixed models (GAMMs) as described by Warwick-Evans et al (2022a). Analyses were conducted independently for summer and winter.…”

“…We also note that the U.S. AMLR krill survey area no longer overlaps with much of the area used by the krill fishery, as the fishery has become more concentrated in nearshore shallow waters at depths of <1000 m (Warwick-Evans et al, 2022a). We emphasise that it is important to collect new krill acoustic density data, including in areas representative of those areas where the fishery currently operates, to validate our models.…”

“…In recent years the fishery has concentrated in the west Antarctic Peninsula region, particularly within the Bransfield and Gerlache Straits (Trathan et al, 2018;Krüger, 2019), and increased periods of reduced sea-ice have allowed the fishery to operate further south than previously (Silk et al, 2014). The majority of krill is currently caught in autumn when krill are more concentrated on the continental shelf (Trathan et al, 2022;Warwick-Evans et al, 2022a), though some are caught north of the South Shetland Islands during summer (Appendix 2). The timing of krill catch will have implications for the risk to the ecosystem, due to the variability in requirements and distributions of both krill and predators.…”