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...
Irrawaddy dolphins (Orcaella brevirostris) are threatened, little‐known cetaceans primarily jeopardized by fishing gear entanglement. Information regarding life history, foraging ecology, and movement are crucial to determine management units and conservation regions. This is the first analysis of the ontogenetic changes in diet and habitat of Irrawaddy dolphins from the Gulf of Thailand and the Andaman Sea using stable isotopes from skin and teeth. Results suggest limited age and sex structure indicated by weak ontogenetic and sexual variability in δ15N and δ13C in teeth. Significant variation in δ13C and δ15N values were observed between three Gulf regions and the Andaman Sea, indicating distinct regional groups. Isotope mixing models run on soft tissues from the eastern Gulf revealed a diet based primarily on fishes and secondarily on crustaceans, suggesting overlap with fishery targets. Conservation strategies that focus on reducing regional competition with humans and bycatch mortality are proposed. Fishing gear improvements and monitoring of fishing operations are a potential immediate response to reduce anthropogenic impact.
On 5 September 2022, a dead baleen whale was found stranded at Laem Phak Bia, Phetchaburi, the Gulf of Thailand, Thailand but could not be identified because it was in an advanced stage of decomposition. It was first suspected to be Omura’s whale (Balaenoptera Omurai), as that is a common species in the Gulf of Thailand. However, the cranium morphology was different from B. omurai and more similar to the common minke whale (Balaenoptera acutorostrata) from the North Pacific Ocean, which has never been reported in Thai territorial waters. The mitochondrial DNA control region (D-loop) was then used to identify the species through the Basic Local Alignment Search Tool (BLAST) available at the National Center for Biotechnology Information (NCBI) GenBank, which resulted in a high percent identity, 96.49 to 98.84, with B. acutorostrata. A Bayesian phylogenetic tree was further used to confirm the species, which grouped with B. acutorostrata from the North Pacific Ocean. This study provides evidence of the first stranding event of B. acutorostrata in the Gulf of Thailand. It is new information that extends previous knowledge on the distribution of the common minke whale and raises the need for more active surveys of cetaceans in the South China Sea going forward.
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