RESULTS AND DISCUSSION

We recorded a total of 2831 feeding events during 649 foraging dives from tag records. The mean (±s.e.) dive depth and duration for each of two whales was 18±5 m and 1.4±0.4 min, respectively, reaching maximums of 105±1 m and 7.2±3.2 min. Both whales maintained a very high level of feeding activity and performed up to 22±2.82 lunges per dive. At a feeding rate of 102±6 lunges h⁻¹, and an engulfment volume of 1.9 m³, each whale processed ~193±12 m³ of prey-laden water per hour. The number of lunges per dive and dive duration generally increased with depth (Fig. 1A), although a k-means cluster analysis revealed three significantly different foraging dive types: short surface dives (Fig. 1C), long shallow dives (Fig. 1E) and long deep dives (Fig. 1D). Across both whales, shallow dives accounted for 73% of all dives, but only 34% of the lunges occurred in these short dives. Shallow and longer dives accounted for most of the feeding effort, 40% of all lunges (Table 1).

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Fig. 1.

Time–depth profile and feeding performance for an Antarctic minke whale. (A) The entire time depth profile of over 18 h. (B) An example of a shallow, non-feeding dive. (C) A series of short and shallow feeding dives (grey circles indicate feeding lunges). (D) An example of a long, deep feeding dive. (E) A long, shallow feeding dive presumably under sea ice. The dashed blue line indicates the typical depth of sea ice in the region and reveals that in this single dive the whale lunges several times in direct proximity to the underside of the sea ice. (F) A tagged Antarctic minke whale approaching and diving under sea ice (bottom).

The foraging behavior of Antarctic minke whales is characterized by extremely high feeding rates and relatively shallow dives (<100 m) compared with other, larger baleen whales. Our analyses suggest that high lunge frequency, defined by the number of lunges performed per dive, occurs when minke whales target krill under sea ice. This notion was confirmed by our direct observations of whales diving underneath sea ice that link temporally with periods in the tag data that show the occurrence of such dives. These observations highlight the importance of sea ice in structuring the foraging behavior of minke whales because a significant proportion of their diving costs includes extended excursions under sea ice (Fig. 1E). Although this feeding mode has never been described before, this behavior represents a high proportion of foraging effort.

Our results indicate that minke whales take advantage of the sea ice habitat unlike all other baleen whales because their small size provides the increased maneuverability required to navigate in and around ice in search of krill. In addition, the ability to bulk filter feed, compared with other krill predators that are single-item (e.g. penguins) (Watanabe and Takahashi, 2013) or suction/small-batch filter feeders (e.g. leopard and crabeater seals) (Hocking et al., 2013), provides enhanced foraging capacity during extended dives under sea ice. This foraging strategy contrasts with that of larger rorqual whales in the Southern Ocean that are limited to the relatively ice-free continental shelf (humpback and fin whales) or offshore (humpback, fin and blue whales) waters (Friedlaender et al., 2011; Friedlaender et al., 2009; Johnston et al., 2012).

As the smallest rorqual, minke whale feeding performance is limited by its relatively small feeding apparatus (Nakamura et al., 2012), yielding an engulfment capacity that is no more than 70% of body mass. In contrast to large rorquals, which typically execute a small number of extremely large gulps (100–150% of body mass) of water during long dives (Goldbogen et al., 2012), minke whales perform a high number of very small gulps. Minke whales can make over 20 lunges per dive, substantially more than any other rorqual whale (Fig. 2). Hydro-mechanical models of engulfment suggest that minke whales are able to execute such a high number of lunges per dive because the energetic cost of each lunge is very low, with power requirements that are comparable to steady swimming (Potvin et al., 2012). Low feeding costs should help minke whales maximize dive duration, a key advantage when exploiting krill beneath sea ice. Although most minke whale dives were relatively short and shallow (Fig. 1C, Table 1), we observed maximum dive durations of 9.4 min, commensurate with the breath-hold durations observed in some of the largest rorquals during foraging dives (Goldbogen et al., 2012).

The extended dive capacity, high lunge feeding rates and relatively small body size of minke whales facilitate below-ice krill foraging, supporting an energetic niche not explicitly observed in any other sympatric krill predator, including other whales, seals, penguins and seabirds (Friedlaender et al., 2011; Friedlaender et al., 2009). Moreover, the morphological adaptations for lunge feeding at a size exhibited by minke whales enable the exploitation of krill resources that are largely unavailable to other baleen whales (e.g. humpback whales) (Friedlaender et al., 2006; Friedlaender et al., 2009; Friedlaender et al., 2011). Despite being limited to relatively small gulps of prey-laden water, minke whales can feed at high rates at a relatively low energetic cost. This feeding strategy should confer high energetic efficiency for dense, patchily distributed prey, but in a more adaptable way compared with larger rorquals that are committed to larger gulps that take longer to filter and process (Goldbogen et al., 2012). Although our data help clarify the relationship between body size and foraging performance in bulk filter feeders, future studies should focus on adaptable foraging strategies in response to different conditions (prey types and densities) in order to better understand the link between feeding biomechanics and dynamic ecological niches.

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Fig. 2.

Engulfment capacity and maximum number of lunges per dive with respect to body size in four krill-feeding baleen whale species. Antarctic minke whales lunge more frequently and filter less water than any other rorqual whale, a trend that generally reflects ecological niche (inshore to offshore distribution). Data for blue, fin and humpback whales are from Goldbogen et al. (Goldbogen et al., 2012).

Minke whales are found in all major oceans in relatively high abundances, yet little empirical data on their behavior exist for any region. Despite being the most numerous cetacean in the Southern Ocean and the subject of a contentious lethal sampling program, no efforts were made to directly study the underwater behavior of these important predators (Gales et al., 2005). Because of the logistical constraints of working in and around sea ice in a remote environment, the difficulty of approaching whales closely enough for tagging, and the only recent advancement of multi-sensor tag technology, data on Antarctic minke whale behavior remained elusive. While we report on a limited sample size, we believe the long duration of the deployments and the amount of foraging behavior measured combined with the novelty of data for the species is sufficient to make the inferences we have reported.