Tread-water feeding of Bryde’s whales

Abstract

Many previous studies have shown that rorqual whales (Balaenopteridae), including the blue whale (Balaenoptera musculus), fin whale (B. physalus), sei whale (B. borealis), Bryde's whale (B. edeni), minke whale (B. acutorostrata), and humpback whale (Megaptera novaeangliae), employ a strategy called lunge feeding to capture a large amount of krill and/or fish for nourishment [1]. Lunge feeding entails a high energetic cost due to the drag created by an open mouth at high speeds [1,2]. In the upper Gulf of Thailand, Bryde's whales, which feed on small fish species [3], predominantly anchovies, demonstrated a range of feeding behaviors such as oblique, vertical, and lateral lunging. Moreover, they displayed a novel head-lifting feeding behavior characterized by holding the vertical posture for several seconds with an open mouth at the water surface. This study describes the head-lifting feeding by Bryde's whales, which is distinct from the typical lunge feeding of rorqual whales. Whales showing this behavior were observed on 58 occasions, involving 31 whales and including eight adult-calf pairs. Whales caught their prey using a series of coordinated movements: (i) lifting the head above the water with a closed mouth, (ii) opening the mouth until the lower jaw contacted the sea surface, which created a current of water flowing into the mouth, (iii) holding their position for several seconds, (iv) waiting for the prey to enter the mouth, and (v) closing the mouth and engulfing the prey underwater (Figure 1A-F, Movie S1 in Supplemental Information published with this article online). When a whale kept its upper jaw above the sea surface, many anchovies in the targeted shoal appeared to lose orientation and flowed passively into the mouth of the whale by the current created by the lower mandible breaking the surface of the water. We measured the duration of feeding events when the whales had a wide-open mouth mostly above the sea surface. The mean and maximum feeding durations were 14.5 ± 5.4 (SD; n = 58 events) and 32 s, respectively. Deployment of animal-borne data loggers yielded approximately 44 minutes of recordings from a single whale. The acceleration data showed that stroke rates, including tail beat and whole-body movements during feeding, were faster (approximately 0.7 s cycle) than during a cruising swim (approximately 3 s cycle) (Figure 1G). The swimming speed was lower than that in the stall speed (0.2 m s) of the device during the feeding phase, suggesting that thrust force was used to hold the head up and to stabilize body posture (Figure 1G). Stable positioning using the fluke and flipper was confirmed by video data for both the downward and upward direction of the whale (Figure S1). According to the visual and behavioral data, we named the head-lifting feeding as 'tread-water feeding'. Generally, all species of baleen whale, including rorqual whales, show active chasing and feeding, i.e., skimming, suction, and engulfing with lunging [1]. Tread-water feeding is considered passive feeding as compared wit...

Occurrence of Bryde's whales, Balaenoptera edeni, in the northern Beibu Gulf, China

“…c) that can be easily distinguished from other behaviors and other Balaenoptera species (Iwata et al . ). For the fishermen's recordings, date, time, and GPS location of whale sightings were registered by smart phones.…”

“…, Iwata et al . ). In the northern Beibu Gulf, schooling neritic fish include longjaw thryssa ( Thryssa setirostris ), Japanese gizzard shad ( Nematalosa japonica ), bluestripe herring ( Herklotsichthys quadrimaculatus ), elongate ilisha ( Ilisha elongate ), and white sardine ( Sardinella albella ) (Fishbase, Froese and Pauly ).…”

“…The trap‐feeding, or tread‐water feeding (Iwata et al . ), behavior was specifically targeted because of its characteristic mouth‐open and head‐lifting behavior (Fig. c) that can be easily distinguished from other behaviors and other Balaenoptera species (Iwata et al .…”

“…The trap-feeding, or tread-water feeding (Iwata et al 2017), behavior was specifically targeted because of its characteristic mouth-open and headlifting behavior ( Fig. 1c) that can be easily distinguished from other behaviors and other Balaenoptera species (Iwata et al 2017). For the fishermen's recordings, date, time, and GPS location of whale sightings were registered by smart phones.…”

“…By removing the confounding fishing efforts, (c) and (d) revealed grid-specific activities of whale occurrence and trap-feeding. feeds on schooling fish, likely pelagic-neritic anchovies, shads, and sardines (Tershy 1992, Siciliano et al 2004, Murase et al 2007, Watanabe et al 2012, Iwata et al 2017. In the northern Beibu Gulf, schooling neritic fish include longjaw thryssa (Thryssa setirostris), Japanese gizzard shad (Nematalosa japonica), bluestripe herring (Herklotsichthys quadrimaculatus), elongate ilisha (Ilisha elongate), and white sardine (Sardinella albella) (Fishbase, Froese and Pauly 2018).…”