Feeding ecology of spotted scat scatophagus argus, linnaeus in mangrove forests Pak Phanang estuary, Nakhon Si Thammarat Province

Abstract

Feeding ecology of spotted scat Scatophagus argus, Linnaeus in mangrove forests Pak Phanang Estuary, Nakhon Si Thammarat Province is used as the assessment on the role of mangrove forest as food source for fish. Feeding strategies in different stages in relation to feeding structure morphology, development of alimentary tracts and associated digestive organs and assimilation efficiency are determined. The trophic role of Scatophagus argus in mangrove food webs was demonstrated. Niche partitioning in spotted scat, Scatophagus argus and adult tade mullet, Liza planiceps sharing the same food resources was also demonstrated. The role of mangrove plantations in Pak Phanang Estuary supported the availability of food source, nursery ground and habitat for spotted scat as true resident species. The study showed the increase in organic detritus in term of forest biomass as the mangrove plantations aged. The gastropods, nematodes and insect larvae also found as dominant benthos. Diatom and cyanobacteria were most dominant microphytoplankton. Zooplankton diversity dominated by copepod, mollusk larvae, cirripedia nauplii, zoea of brachyuran, rotifers and shrimp larvae. These provided rich food sources for spotted scat. Diatoms and cyanobacteria were the most common dietary components in each stage. Spotted scat showed broad diets, being omnivore and opportunistic feeder. Spotted scat showed ontogenetic niche shift with larval stage predominantly fed in the surface water and in water column. Juveniles were the transition stages feeding in the water column and bottom substrates. Adults were adapted to feed on bottom substrates. Protozoa, benthos and detritus showed different ratio in diet of each stage. Due to the flexibility in spotted scat feeding, this fish was classified as secondary consumers in the Pak Phanang mangrove food webs sharing the same trophic levels with zooplankton feeders and benthic feeders. Histological study of alimentary system of spotted scat revealed the variations according to ages and feeding habits. In large fish, smaller and numerous teeth were found. The fundic portion has gastric glands increasing markedly with ages which secrete gastric juices for digestion. The pyloric caeca appeared to be the important sites for lipid digestion allowing optimized absorption. The villi on the intestinal walls increased markedly in the duodenum as the main absorptive site. The bile duct from the liver and gall bladder together with enzymes from the pancreas are all sent into the duodenum portion. Morphological and histological studies of the alimentary system correlated to the relative importance of components in diets of each stage and ontogenetic niche shift. Spotted scat in each stage showed the feeding ecology following optimal foraging theory. Optimal prey size suitable to mouth gape that maximizes food consumed per unit capture, were choosen. Acquisition of diets also based on the nutritional quality and energy as revealed from the assimilation efficiency study. Niche partitioning between spotted scat and tade mullet were shown by differences in feeding morphology and behavior, alimentary system and seasonal shifts in relative importance of different components in diets. The study confirmed tade mullet were herbivore. Niche overlap in food items, mainly microphytoplankton and detritus, were evidenced in spotted scat and tade mullet diets. From this study, the role of mangrove reforestation in Pak Phanang Estuary is important in providing food sources and habitats for spotted scat. This fish showed flexibility in term of feeding and physiological tolerances which allow survivals in estuarine environment in Pak Phanang Estuary. This fish is the good indicator for monitoring changes in mangrove and environmental condition. Thus the spotted scat culture should be promoted as supplementary income for coastal communities. Releasing spotted scat fry into the estuary along as a part of the mangrove reforestation program are recommended to sustain the fishery in the area.