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

Abstract

Ecosystem dynamics at the northwest Antarctic Peninsula are driven by interactions between physical and biological processes. For example, baleen whale populations are recovering from commercial harvesting against the backdrop of rapid climate change, including reduced sea ice extent and changing ecosystem composition. Concurrently, the commercial harvesting of Antarctic krill is increasing, with the potential to increase the likelihood for competition with and between krill predators and the fishery. However, understanding the ecology, abundance, and spatial distribution of krill predators is often limited, outdated, or at spatial scales that do not match those desired for effective fisheries management. We update current knowledge of predator dependence on krill by integrating telemetry‐based data, at‐sea observational surveys, estimates of predator abundance, and physiological data to estimate the spatial distribution of krill consumption during the austral summer by three species of Pygoscelis penguin, 11 species of flying seabirds, one species of pinniped, and two species of baleen whale. Our models show that the majority of important areas for krill predator foraging are close to penguin breeding colonies in nearshore areas where humpback whales also regularly feed, and along the shelf‐break, though we caution that not all known krill predators are included in these analyses. We show that krill consumption is highly variable across the region, and often concentrated at fine spatial scales, emphasizing the need for the management of the local krill fishery at relevant temporal and spatial scales. We also note that krill consumption by recovering populations of krill predators provides further evidence in support of the krill surplus hypothesis, and highlight that despite less than comprehensive data, cetaceans are likely to consume a significant proportion of the krill consumed by natural predators but are not currently considered directly in the management of the krill fishery. If management of the krill fishery is to be precautionary and operate in a way that minimizes the risks to krill predator populations, it will be necessary in future analyses, to include up‐to‐date and precise abundance and consumption estimates for pack‐ice seals, finfish, squid, and other baleen whale species not currently considered.

Spatiotemporal Overlap of Baleen Whales and Krill Fisheries in the Western Antarctic Peninsula Region

“…In late autumn/ winter, massive, extremely dense aggregations of krill have been documented in the nearshore bays on the western side of the Antarctic Peninsula (Nowacek et al, 2011) coincident with where the krill fishery is known to operate at these times . Baleen whale concentrations (Johnston et al, 2012) mirror these supper aggregations of krill and whales consistently distribute in these places and forage in a manner that optimises energy gain (Friedlaender et al, 2013;Friedlaender et al, 2016;Tyson et al, 2016) Johannessen et al (2022 see also Warwick-Evans et al, 2022) estimate peak potential competition (calculated as humpback whales' per capita consumption multiplied by their abundance through the season, and krill catch through the season) in April, which corresponds with the highest values in our monthly spatial overlap index. Based on the assumption that whales spend 120 days in their Southern Ocean foraging areas (Lockyer, 1981), Johannessen et al (2022) calculated very limited humpback whale-fishery competition by May, but tracking data in this study and shows that some individuals remain in the region as late as July.…”

“…The humpback whale population that uses waters off the Western Antarctic Peninsula is increasing rapidly (Pallin et al, 2018) and unprecedentedly dense 'super-aggregations' have been observed in this region (Nowacek et al, 2011). Spatial abundance models estimate around 19,000 individuals in this region in early summer (Johannessen et al, 2022; see also Warwick-Evans et al, 2022), a 5.1% per annum increase from the estimated 7,000 individuals in the same region in 2000 (Hedley et al, 2001;Johannessen et al, 2022). Their global IUCN Red List classification is Least Concern (Cooke, 2018).…”

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, the fishery does not operate across the entire Subarea, operating almost exclusively over shelf waters around the South Shetland Islands and northern Antarctic Peninsula (see Appendix 2), overlapping with most predator monitoring and krill survey effort (Trathan et al, 2022). Additionally, environmental conditions vary considerably across the latitudinal range of Subarea 48.1 (see Figure S4 in Warwick-Evans et al, 2022b). As such, we have not attempted to project species distributions into areas where there are no, or very few, observational data; extrapolating into geographic and environmental space without empirical data that constrain model projections could lead to biased results and unintended management consequences.…”

“…These distribution rasters were used to estimate spatially explicit krill consumption by each species, by combining estimated energy requirements calculated from Field Metabolic Rates (Shaffer, 2011) with estimates of the proportion of krill in the diet of each species (Croxall et al, 1985) and the energy density of krill (Clarke, 1980). Warwick-Evans et al (2022b) describe the approach to estimate krill consumption by three species of Pygoscelis penguins, fin whales Balaenoptera physalus, humpback whales Megaptera novaeangliae and Antarctic fur seals Arctocephalus gazella (Figure S1). For each cetacean species, sightings data were collected from ship surveys conducted between January and March 2013 -2020 by the Brazilian Antarctic Program (PROANTAR).…”

“…Surveys for flying seabirds are also undertaken only in summer and there is little information on the distribution or abundance of flying seabirds in the study area during winter. In summer, flying seabirds account for approximately 2% of the overall krill consumption in the area (Warwick-Evans et al, 2022b), consequently for the purpose of this risk assessment we have assumed that there is no krill consumption by flying seabirds in the study area during winter, something again that we consider in the discussion.…”