Patterns in longline fishery data and catches of bigeye tun, Thunnus obesus

Abstract

Species composition of tunas caught by the large Asian longline fleets has shifred over time. Much of the observed change appears to be market driven, combined with changing economic conditions. as opposed to being driven by resource availabiliry. As part of the strategy for change. the oriental longliners developed the ability to target on certain species with a gear previously thought relatively nonselective with respect to target species. Use of longline catch statistics for resource assessment requires an understanding of the reasons for changes in the data to avoid misuse of the statistics. by removing intermediate floats from a standard longline gear, the mainline sags forming a deeper catenary so that the branch lines fish deeper. Fishermen were able to substantially improve their catch rates for bigeye tuna, Thunnus obesus , a preferred species, while reducing the catch rates for other tuna species (Suzuki et al., 1977). Other such techniques, e.g., night longlining for swordfish (Ueyanagi, 1974) have been described in the literature. In this paper, we review available longline fishery statistics for the Japanese, Korean, and Taiwanese fleets for catch patterns. The patterns are then evaluated with respect to market information, which may have affected tuna fisheries (Suda, 1974; Ueyanagi, 1974; Lee, 1986; Wise and Miyake, 1982). Our emphasis is on the longline catch of bigeye una in the Atlantic, but we also review information on the catch of yellowfin tuna, T. albacares , and albacore, T. alalunga, from other oceans. We do not review, except in a discussion of global events, the catch of bluefin tunas, T. thynnus and T. mccoyi , because they are caught in specialized fisheries that are monopolized by the Japanese fleet.