Aerial surveys for Antarctic minke whales (Balaenoptera bonaerensis) reveal sea ice dependent distribution patterns

Abstract

This study investigates the distribution of Antarctic minke whales (AMW) in relation to sea ice concentration and variations therein. Information on AMW densities in the sea ice‐covered parts of the Southern Ocean is required to contextualize abundance estimates obtained from circumpolar shipboard surveys in open waters, suggesting a 30% decline in AMW abundance. Conventional line‐transect shipboard surveys for density estimation are impossible in ice‐covered regions, therefore we used icebreaker‐supported helicopter surveys to obtain information on AMW densities along gradients of 0%–100% of ice concentration. We conducted five helicopter surveys in the Southern Ocean, between 2006 and 2013. Distance sampling data, satellite‐derived sea‐ice data, and bathymetric parameters were used in generalized additive models (GAMs) to produce predictions on how the density of AMWs varied over space and time, and with environmental covariates. Ice concentration, distance to the ice edge and distance from the shelf break were found to describe the distribution of AMWs. Highest densities were predicted at the ice edge and through to medium ice concentrations. Medium densities were found up to 500 km into the ice edge in all concentrations of ice. Very low numbers of AMWs were found in the ice‐free waters of the West Antarctic Peninsula (WAP). A consistent relationship between AMW distribution and sea ice concentration weakens the support for the hypothesis that varying numbers of AMWs in ice‐covered waters were responsible for observed changes in estimated abundance. The potential decline in AMW abundance stresses the need for conservation measures and further studies into the AMW population status. Very low numbers of AMWs recorded in the ice‐free waters along the WAP support the hypothesis that this species is strongly dependent on sea ice and that forecasted sea ice changes have the potential of heavily impacting AMWs.

Common and Antarctic Minke Whales: Conservation Status and Future Research Directions

“…Antarctic minke whales are the largest ice-dependent krill predators in the Southern Ocean. As their distribution and ecology are directly tied to sea ice (Herr et al, 2019) and foraging on krill, any changes that affect the quantity and quality of their habitat and food availability could be significant. Currently, Antarctic minke whales and humpback whales are thought to partition prey resources in this region by feeding in different habitats (sea ice versus open water) but when the two species do overlap in time and space, they partition prey vertically (Friedlaender et al, 2008).…”

“…There is also disagreement as to whether current abundance estimates are similar to pre-whaling abundance figures. One of the difficulties in generating abundance estimates for this species is due to its affinity for sea ice, as has been shown by recent aerial surveys (Williams et al, 2014;Herr et al, 2019). Despite these logistical challenges and ongoing discussions, current population estimates and their trajectory give reason for concern and accordingly have resulted in the recent classification of Antarctic minke whales as Near Threatened under the IUCN Red List and under Appendix I of CITES (Cooke et al, 2018b).…”

“…Antarctic minke whales have a circumpolar distribution south of 60 • S during summer (Figure 1), with high densities reported in parts of the Weddell and Ross Seas (Kasamatsu et al, 1995). The species is highly associated with sea ice and is generally less abundant in ice-free waters (Herr et al, 2019). Recent acoustic data show detections of the species in association with sea ice for most of the year (Dominello and Široviae, 2016).…”

Managing the Effects of Noise From Ship Traffic, Seismic Surveying and Construction on Marine Mammals in Antarctica

“…These and most other visual surveys have been generally confined to ice-free areas and undertaken during the brief austral summer. Information on migrations, spatial distribution, and abundance in ice-covered areas (e.g., Herr et al, 2019) or during other times of the year is limited though growing-for example, as a result of autonomous passive acoustic monitoring, which can collect information on acoustic presence year-round (e.g., Van Opzeeland et al, 2008;Van Parijs et al, 2009). Field research in the Antarctic is expensive and limited in space and time, resulting in numerous data gaps ( Table 2).…”

“…Passive acoustic observations have shown that Antarctic blue, Antarctic minke, and humpback whale distributions are, however, not limited by ice (van Opzeeland et al, 2013;Dominello and Širović, 2016;Thomisch et al, 2016). Observations of Antarctic minke whales show this species predominantly occurs in areas with dense ice cover (Williams et al, 2014b;Herr et al, 2019). Fin whales are acoustically present year-round in some areas (E. Burkhardt pers.…”