SEASONAL DYNAMICS OF FLATFISH ASSEMBLAGE IN COASTAL WATERS OFF NORTHEASTERN TAIWAN: WITH IMPLICATIONS FOR SHIFT IN SPECIES DOMINANCE OF THE ECOSYSTEM

摘要

The seasonal dynamics of flatfish assemblage in the coastal waters off northeastern Taiwan was examined based on samples collected between March 2004 and March 2005 taken by bottom trawlers and landed at Da-Shi fishing port, Yilan county. A total of 28 species from 5 families and 18 genera were found. The Bothidae was the most dominant family, accounting for 81% of the total catch, while the most dominant species was the Pseudorhombus pentophthalmus. Species richness and diversity were high between the end of fall and the end of spring, but low in summer. Multivariate analyses based on cluster and nonparametric multi-dimensional scaling identified three seasonal assemblages as follows: late-spring to summer, fall to mid-winter, and late-winter to mid-spring. The two dominant species in each assemblage were the same, namely P. pentophthalmus and Cynoglossus kopsii. Rare or less abundant species were found to account for most of the differences among seasonal assemblages. The overwhelming dominance of one or two species suggests that monitoring of ecosystem change may be possible using these species. In addition, in this region a striking increase has been observed in both flatfish catch (3.5 folds from 2001 to 2007) and catch percentage (2.5 folds from 2002 to 2008), during the period when total landings of trawl fisheries either remained stable (2002-2007), or declined (after 2007). These findings may be due to the increase of fishing pressure and may reflect changes in the community structure of demersal fish in the region.