Examining on the Bycatch and Discard Catch of the Shrimp Beam Trawl Fishery in the Coastal Waters off Chiayi, Southwest Taiwan

A total of 80 hauls from December 1996 to December 2000 including 14 hauls for survival experiment of discarded catch were conducted on the monthly basis in the coastal waters off Chiayi, southwest Taiwan to examine the catch composition, estimate the ratio of landed catch and discarded catch, and survival rate of discarded catch from shrimp beam trawlers. A total of 28, 107 and 6 species of crustaceans, finfishes and cephalopods were identified for all the catches. The catch is divided into two categories namely landed catch and discarded catch and the latter consists of 52.2% and 31.9% in number and weight, respectively. Shrimps are the dominant animals both in number (43.8%) and weight (43.2%) of the landed catch. On the other hand, fishes are dominant in weight (42.5%) and shrimps are dominant in number (51.2%) of discarded catch. Monthly catch rate (g/(haul×hour)) indicated that higher landed catch and discarded catch occur in December to next February. The major target species is Parapenaeopsis cornuta (51.2% of landed shrimps), followed by P. hardwickii (18.9%). Among the discarded catch, P. hardwickii is the dominant species of shrimps (37.5%), Portunus sanguinolentus is the dominant species of crabs, Trachinocephalus myops is the dominant species of fishes, and Sepia esculenta is the dominant cephalopods. The landing-selectivity of shrimps increased with their economical values based on the length frequency distribution of dominant species on landed catch and a logistic curve was fitted. No significantly temporal variations on length were found for discarded catch. The survival rates of discarded catch were estimated to be 56.3%, 62%, 34.8%, and 8.4% for shrimps, crabs, cephalopods, and fishes, respectively.