Effects of mangrove deforestation on fish assemblage at Pak Phanang Bay, South Thailand

Study Number:

35

Author:

Tatsuya Shinnaka, Mitsuhiko Sano, Kou Ikejima, Prasert Tongnunui, Masahiro Horinouchi & Hisashi Kurokura

Abstract:

Daytime sampling using a seine net was conducted at Pak Phanang Bay (Nakhon Si Thammarat Province, Thailand) in February and July 2006, to determine differences in fish assemblage structures between a mangrove site and an adjacent site completely cleared of mangroves. The overall numbers of fish species and individuals were significantly higher at the mangrove site than the cleared site in both months. Although benthic crustacean feeders showed more species and individual numbers at the mangrove site in both months, the opposite was found for zooplankton feeders. A cluster analysis, based on the abundance of each species, demonstrated that the fish assemblage structures were distinctly different between the two sites. In addition, significant differences in length frequency distributions for each of the four most abundant species were found between the sites in February and/or July. Small individuals of Scatophagus argus, Ambassis nalua, and Tetraodon nigroviridis were more common at the mangrove site, and of Chelon subviridis  at the cleared site. These results suggest that mangrove deforestation exerts marked effects on fish assemblages.

Main Results and Conclusions:

• A variety of fish species were studied to determine the impact of mangrove habitat loss: “To ascertain changes in the trophic structure of the fish assemblage following mangrove deforestation, fishes collected were assigned to one of seven trophic categories on the basis of dietary data… The categories included benthic crustacean feeders (feeding mostly on shrimps and amphipods), zooplankton feeders (mainly calanoid copepods), detritus feeders, hard-shelled mollusc feeders (mainly gastropods), fish-scale feeders, polychaete feeders, and terrestrial insect feeders (mainly ants)”(863-864).
• The results from the study showed significant differences among fish species between forested and deforested mangrove habitat. Abiotic factors were also compared between the two sites:
  • “The total catches of fishes were 425 and 182 individuals, comprising 22 and 19 species, at the mangrove and cleared sites in February, respectively. In July, catches were 244 and 153 individuals, comprising 18 and 14 species, respectively. A two-way anova revealed that the mean numbers of species and individuals per tow were significantly greater at the mangrove site than at the cleared site (Table 3, Fig. 2)”(864).
  • “The two-way anova results demonstrated that a greater number of benthic crustacean feeding species was collected from the mangrove site, where the opposite was found for zooplankton feeders (species and individual numbers) (i.e. mangrove site < cleared site), and the three variables showed no significant site ¥  month interactions (Table 4, Fig. 3)”(865). 
  • Only salinity differed between the two sites, where “salinity tended to be higher at the mangrove site than at the cleared site in both months”(864). However, this change in salinity showed little effect on fish assemblage because the degree of similarity between fish species during low and high salinity levels was high (~70%) (865). 
• In conclusion, because fish densities changed due to the lack of mangrove habitat, it can be assumed that mangrove habitat is crucial for many fish species “The results of this study suggest that mangrove deforestation generated corresponding changes in the fish assemblage structure. In each of the two study months, species richness, and abundance of fishes decreased significantly at the cleared site, the similarity index indicating differences in species composition of the assemblages at the mangrove and cleared sites”(865).