Ontogenetic Niche Shift in the Spotted Scat, Scatophagus argus, in Pak Phanang Estuary, Nakhon Si Thammarat Province, Thailand

Abstract

The estuarine fish, Scatophagus argus, or spotted cat, utilized the mangrove forests and estuarine waterways in Pak Phanang Estuary, Nakhon Si Thammarat Province, Thailand, as habitat and feeding grounds. At this location, spotted scats revealed flexibility in their feeding ecology being omnivores and opportunistic feeders. Subject to gape-mediated prey size limitations, their diets changed in accordance with the availability or profitability of potential prey items. Morphologically, the subterminal mouth supported that spotted scats were mainly benthic feeders. Optimal food sizes, with respect to the mouth gape, were chosen, so as to maximize food consumed per unit capture. In large fish, smaller and more numerous teeth increasingly appeared on the jaws. Short gill rakers and a U-shaped stomach with pyloric caeca appeared to be the important sites for absorption and increased in number with fish developmental age, as did the ratio of the intestinal to standard fish length, within the range of 0.59 – 4.29. Spotted scats switch from one type of food to another as the relative abundance of the food types changes, whilst the relationship between feeding structure morphology, the diversity of food items and feeding preferences support that spotted scats have an ontogenetic niche shift. Spotted scat larvae predominantly fed on microphytoplankton in the surface water and in the water column. Juvenile fish are a transitional stage feeding both in the water column and the mangrove floor on resuspended benthic diatoms, zooplankton, benthos and detritus. Adults, feed mainly in the midwater level and the mangrove floor, with the most diversified diets ranging from microphytoplankton, protozoan, zooplankton, benthos and detritus. The latter two food items are greatly increased in adult diets.