Use of historic fisheries data to determine trends in relative abundance and body size of sailfish, Istiophorus platypterus, off northwestern Australia

Sailfish (Istiophorus platypterus) appear annually off northwestern Australia in large numbers, supporting substantial recreational and charter fisheries in which almost all fish are released. The main centres for this activity are the towns of Broome, Dampier and Exmouth. Examination of historic Japanese longline catch data from northwestern Australia indicated that relatively few sailfish were caught off this area between 1979 and 1998, although little fishing effort occurred during the peak sailfish 'season'. Long term recreational fishery data, consisting of tag and release and charter boat diary data, were used to investigate locations of captures, seasonality of the sailfish aggregations and trends in body size and relative abundance of sailfish through time. No trends in annual catch rates were discernible from the mid 1990s to the present. Sailfish may be caught in most months off Broome, with the average peak period being June through September. Sailfish caught off Broome, as determined from estimated weights at tag-and-release, are slightly, but significantly smaller than those caught off Dampier while fish caught near the Rowley Shoals, and in the Exmouth Gulf, are larger than those caught off Broome and Dampier.