Songklanakarin J. Sci. Technol. 41 (2), 276-284, Mar. – Apr. 2019 Original Article Food partitioning of two co-occurring Terapontid fishes, Terapon jarbua and Pelates quadrilineatus, in coastal areas of Trang Province, Southern Thailand Nuengruetai Yoknoi 1 , Nittharatana Paphavasit 1* , Jes Kettratad 1 , and Prasert Tongnunui 2 1 Department of Marine Sciences, Faculty of Science, Chulalongkorn University, Pathum Wan, Bangkok, 10330 Thailand 2 Department of Marine Sciences, Faculty of Science and Fisheries Technology, Rajamangala University of Technology Srivijaya, Trang Campus, Sikao, Trang, 92150 Thailand Received: 18 February 2017; Revised: 11 September 2017; Accepted: 8 November 2017 Abstract This study clearly demonstrates that Terapon jarbua and Pelates quadrilineatus showed obvious food partitioning by their dietary ontogenetic changes. The larvae of both species fed mainly on the calanoid copepods. Juveniles of T. jarbua shifted to feed mainly on harpacticoid copepods, crabs and fish scales. Juveniles of P. quadrilineatus shifted to fed mainly on harpacticoid copepods and Lucifer spp. Adult fish shifted to fed on larger prey size. AdultT. jarbua fed mainly on fish and fish scales while adult of P. quadrilineatus fed mainly on large benthic animals, amphipods, bivalves, hermit crabs and polychaetes. The study on morphology and feeding structure development indicated that, the development of mouth, jaw, teeth, gill raker and intestinal length were important ontogeneticchange in the diet of fish. The ontogenetic change in the diet was the important strategies to reduce competition between these two coexisting species in coastal area of Trang Province. Keywords: food partitioning, ontogenetic dietary shift, Pelates quadrilineatus, Terapon jarbua, Trang Province 1. Introduction Several studies demonstrated the importance of mangrove and seagrass beds in the tropical coastal ecosystem as feeding grounds for fishes (Hajisamae, Chou, & Ibrahim, 2003; Hajisamae & Ibrahim, 2008; Horinouchi, 2007; Horinouchi et al., 2009, 2012; Ikejima, Thongnunui, Medej, & Taniuchi, 2003; Laegdsgaard & Johnson, 2001;). Habitat type is also one of the important factors influencing the feeding strategy of a species by determining foraging opportunities. A fish may have to choose between a habitat that provides more abundant and diverse prey, but in which prey is harder to capture, and a habitat which has less prey, but better capture opportunity. A fish may also choose among different habitats that provide different prey diversity and abundance to reduce food competition (Crowder & Cooper, 1982). Terapontid fishes, Terapon jarbua and Pelates quadrilineatus, were found to coexist in the coastal area of Trang Province, southern Thailand. They are carnivorous fish feeding on the wide range of food items from both the water column and benthic substrates such as insect larvae, zoo- plankton, fish, zoobenthos and organic matter (Davis et al., 2011; Davis, Pusey, & Pearson, 2012a; Hajisamae et al., 2003; Hajisamae & Ibrahim, 2008; Horinouchi et al., 2012; Kanou, Sano, & Kohno, 2004; Kulbickiet al., 2005; Lugendo, Nagelkerken, Kruitwagen, Velde, & Mgaya, 2007; Nakane, Suda, & Sano, 2011; Rao & Prasad, 2002). Resource partitioning in fish distinguished three resource dimensions; the trophic, spatial and temporal dimension, of which trophic dimension was found to be the most important dimension for *Corresponding author Email address: nyoknoi@gmail.com