Resource utilization of terapontid fishes terapon jarbua and pelates quadrilineatus in coastal area, Trang province

Abstract

Terapon jarbua and Pelates quadrilineatus are found coexisted in the coastal area of Trang Province for spawning ground, nursing ground and feeding ground. It is interesting to understand how similar species coexisted despite the pressure of competition. Resource partitioning between species or between size classes is an adaptation to reduce the competition in shared resources. The ontogenetic niche shift in term of diet and habitat is one important process in resource partitioning. Ecomorphology in fishes oftens described the patterns of association between morphology and resource use. Therefore this study had focus on (1) the development pattern and definding length at juvenile of two terapontid fishes (2) the distribution patterns of fishes in different stages and reproductive ecology of adult fish of two terapontid fishes (3) the feeding structure development and stomach content analysis in these two terapontid fishes. The understanding on morphological development and distribution as well as reproductive biology and feeding ecology of both species can help to explain the resource utilization and partitioning between the coexisting species in the coastal area of Trang Province. Terapon jarbua and Pelates quadrilineatus can be divided into 6 development stages: (1) preflexion larvae, (2) flexion larvae, (3) postflexion larvae, (4) transforming larvae, (5) juvenile (6) adult. The length-at-juvenile of T. jarbua and P. quadrilineatus were 23.16 and 18.24 mm, respectively. The size at first maturity of female and male of T. jarbua are 156.92 mm and 153.77 mm, respectively while the size at first maturity of female and male of P. quadrilineatus are 120.31 mm and 124.02 mm, respectively. Terapon jarbua and Pelates quadrilineatus showed the clear habitat utilization between developmental stages which planktonic larvae stage (preflexion, flexion and postflexion) and adult mostly inhabit in offshore. This indicated that the offshore area is the spawning ground of adult fish. However, they showed the clear differences in spawning period. Terapon jarbua had two peak of spawning periods during April to June and September to October. Pelates quadrilineatus had only a short discrete period for spawning from November to December. The differences in spawning period lead to temporal resource partitioning in planktonic larvae stage of both species. This also help to reduce food competition between their offspring. Transforming larvae and juveniles moved inshore and distributed in the coastal habitats. Transforming larvae and juveniles of T. jarbua distributed in the coastal swamps while transformation larvae and juveniles of P. quadrilineatus distributed in the seagrass beds. The complexity of microhabitat structure in the coastal swamps and seagrass beds are suitable for nursery area and refuge from predators. Terapon jarbua and Pelates quadrilineatus demonstrated the ontogenetic dietary shift. Larval stage of T. jarbua consumed predominantly on calanoid copepods and shifted to feed on small benthos harpacticoid copepods, large benthos, fishes and fish scales in juvenile stage. Adult stage feed mainly on fish and fish scales. The major food items of larval stage of P. quadrilineatus was also calanoid copepod and shifted to feed on small benthos harpacticoid copepods in juvenile stage. Adult stage feed mainly on large benthic animal such as, bivalves, polychaetes and fish scales. The ontogenetic differences in feeding habits have resulted from acquiring high energy diets for growth and development of fish. The study on morphology and feeding structure development indicated that body shape, eye, mouth, jaw, teeth, gill raker, intestinal length and fins were important characters which showed relationship between the morphological development and ontogenetic niche shifts. The morphological change in body depth and fin ray support mobility in the habitat shifts. The development of mouth, jaw, teeth, gill raker and intestinal length were important in the ontogenetic dietary shifts. The developments of eye and pigment pattern enhanced the feeding performance and predator avoidance. The differences in habitat and food utilization in the two terapontid species in each stage as well as the reproductive patterns and spawning periods are the important strategies to reduce competition between the two coexisting species in coastal area of Trang Province.