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Phenomenon on Fishing Down Marine Food Web in East Coast Gulf of Thailand

Several studies presented the occurrence of fishing down the marine food web in different part of the world including Thailand. Some data from Gulf of Thailand were conducted from 1966-1982 and 1977-1997 which showed a decline in the mean trophic level but did not determined the subsequence trends. Therefore, this study was focus on the analyzing of trend in trophic level and fishing-in-balance index to determine the effect of fishing and other factors. The data of trawl survey from 1988-2007 and the data of fish landing from 1990-2007 which used in this study derived from the department of fisheries, Thailand. Base on number of species or groups composition, hierachical cluster was analyzed using 10 trawl stations resulted in two main clusters covered province of Rayong, Chanthaburi and Trad. The data indicated that from 1988 to 2007, mean trophic level decreased from 3.28 to 3.10 (decline 0.09/decade) in cluster1 and 3.27 to 3.13 (decline 0.07/decade) in cluster 2. The fishing-in-balance index was above zero in the early 1990s but later on values were mostly below zero. The significantly decrease of mean trophic level, catch per unit effort (CPUE) and total landing in cluster 1 supported the fishing down marine food web phenomenon in the East Coast Gulf of Thailand. Data analyzed indicated that it was fishing down marine food web in the East Coast Gulf of Thailand consistent with the trend has been reported. By the concept of fishing down marine food web, 4 phases were classified; phase 1 was high catch of species with high mean trophic level (TLm), phase 2 was low catch of specieswith high TLm, phase 3 was high catch of species with low TLm and phase 4 was low catch of species with low TLm. The analyzed data were evaluated that fishery in the East Coast Gulf of Thailand was classified in phase 3 and eventually reach phase 4 in the near future. Overfishing, particularly of species high level in food chain was the main cause for significantly decreased in observed mean trophic levels. Keywords — Fishing down; Gulf of Thailand; Trawl survey