SCIENTIFIC COMMITTEE THIRD REGULAR SESSION 13-24 August 2007 Honolulu, United States of America PRELIMINARY ESTIMATES OF HISTORICAL VARIATIONS IN THE FISHING POWER AND CATCHABILITY OF PELAGIC LONGLINE FISHING GEAR WCPFC-SC3-ME SWG/WP-7

I quantify the effects of 11 variables on the catchability of pelagic longlines, which are used to catch tunas and billfishes in the open ocean. Extension of the depth range and the duration of longline operations have reduced the catchability of several epipelagic species, such as mako sharks (Isurus spp.), since industrial longlining commenced in the tropical Pacific Ocean in the early 1950s. Reductions in the body size of many species also resulted in reduced searching for food and fewer encounters with longline hooks. By contrast, the catchability of commercially valuable species, like bigeye tuna (Thunnus obesus), increased substantially as a result of the extension of the depth range of longlines and the longer duration of fishing operations. Stronger and less visible line materials, new bait species, and skipper experience also contributed to increased catchability. Dedicated surveys should be used to obtain fishery independent estimates of abundance. Another option is to use surveys to measure variations in the relationship between longline catch rates and animal abundance.