Ontogenetic variation in diet and habitat of Irrawaddy dolphins (Orcaella brevirostris) in the Gulf of Thailand and the Andaman Sea

Abstract

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.

Genetic structure of the endangered Irrawaddy dolphin (Orcaella brevirostris) in the Gulf of Thailand

“…Overall, genetic differentiation among geographic regions was statistically significant based on mtDNA and microsatellite analyses. Our results are consistent with statistical differences in average stable isotope values reported from O. brevirostris teeth in the same regions by Jackson-Ricketts et al (2018 ), indicating distinct geographic groups (Eastern Gulf, Northern Gulf, Western Gulf, and Andaman Sea) may exist in Thailand’s waters. However, no significant difference was detected for average tooth stable isotope values between northern and western Gulf regions.…”

“…Geographic coordinates for all except one individual from Phetchaburi were recorded at the time of collection. Samples were grouped into four general regions as described in Jackson-Ricketts et al (2018 ), each region encompassing specific sites: (1) Mueang Trat, representing the eastern Gulf of Thailand (EG, n = 11), (2) Chonburi, Chachengsao, Samut Sakorn, and Phetchaburi, making up the northern Gulf (NG, n = 12), (3) Chumporn and Surattani, making up the western Gulf (WG, n = 9), and (4) Trang and Satun, representing Andaman Sea sites (AS, n = 5) ( Figure 1 ). Genomic DNA from minced tissue samples was extracted using DNeasy blood and tissue extraction kits (QIAGEN, Valencia, USA) according to the manufacturer instructions.…”

“…Recently, field observations and acoustic studies targeting O. brevirostris were conducted at locations such as Trat Bay, Bangpakong Estuary, and Donsak, in the eastern, northern, and western Gulf of Thailand, respectively ( Tongnunui et al, 2011 ; Hines et al ., 2015 ; Jutapruet et al, 2017 ; Niu et al, 2019 ; Jackson-Ricketts et al, 2020 ). Additionally, significantly different isotope values in O. brevirostris teeth, indicating strong geographic variation and potential subpopulation structure in the Gulf of Thailand and Andaman Sea coast, have been reported ( Jackson-Ricketts et al ., 2018 ). Despite this, little is known about individual movements and genetic exchange of O. brevirostris among regions in Thailand.…”

“…Different habitat preferences are observed between the O. brevirostris populations in fresh waters and coastal waters. In rivers, O. brevirostris occur in deeper waters (10-50 m) at the confluences of two rivers, or above and below rapids, while in estuaries and coastal waters they frequent shallower depths (generally < 10 m) within a few kilometers of shore ( Dolar et al, 2002 ; Smith and Hobbs, 2002 ; Smith et al ., 2006 ; Minton et al, 2011 ; 2013 ; Jackson-Ricketts et al, 2018 ). Because of their patchy and fragmented distribution in rivers and coastal waters, Irrawaddy dolphins are particularly vulnerable to disturbance ( Minton et al ., 2017 ).…”

Distribution and habitat characteristics of three sympatric cetacean species in the coastal waters of Matang, Perak, Peninsular Malaysia

“…Most of the dietary studies of Orcaella dolphins have been limited to either the freshwater populations of the Irrawaddy dolphin (Adulyanukosol, ; Baird & Mounsouphom, ; Marsh, Lloze, Heinsohn, & Kasuya, ; Smith, Tun, Chit, Win, & Moe, ) or the Australian snubfin dolphin, Orcaella heinsohni (Parra, ; Parra & Jedensjö, ); however, Ponnampalam et al () observed Irrawaddy dolphins in the eastern Gulf of Thailand foraging for cephalopods, as indicated by the presence of squid ink and detached squid tentacles at the water surface. Results from a stable isotope study by Jackson‐Ricketts et al () indicated that Irrawaddy dolphins in the eastern Gulf of Thailand primarily consumed ponyfish ( Nuchequula sp. ), mackerels ( Rastrelliger spp.…”