Monitoring the behaviour of longline gears and the depth and time of fish capture in the Australian Eastern Tuna and Billfish Fishery

We describe the results of a detailed monitoring program undertaken to understand the fishing practices and operational characteristics of longline gears deployed in the Australian Eastern Tuna and Billfish Fishery. Gear monitors were used to ascertain both the time-at-depth-profiles of the deployed longline hooks and the time- and depth-profiles of fish capture. Operational details and environmental conditions recorded by observers were also used to gain an understanding of factors influencing the depths attained by hooks and the shape of the longline. The results indicate that while hooks fish a range of depths down to around 400 m the most common depths fished are relatively shallow, with 34 percent of all hooks deployed during the project period being between 40 and 60 m. The depths to which hooks were observed to sink were found to be highly variable for a given longline configuration and hook-number, though the mean depths attained across sets were found to be reasonably well predicted by the longline catenary equation. Current-speed, hook-position and wind-speed were found to be the main factors influencing variation from the predicted depth. While the depth profiles fished by sets deploying less than 10 hooks-per-float were found to be similar, for sets deploying 10 or more hooks-per-float a near linear relationship was found in the mean depth fished by all hooks and the number of hooks-per-float. Finally, a procedure is outlined to estimate the availability-by-depth of the species caught and the resulting profiles were found to be similar to profiles obtained from studies using archival tags. The results of this study support the need to improve the information used to account for factors influencing the fishing effectiveness of longline gears and the use of set-based analyses to standardise catch rates.

► Depth profiles of hooks and fish capture were monitored on pelagic longline gears.
► Hooks fished to 400 m and depth was highly variable for a given line configuration.
► Hook-position, current and wind speed influenced predicted hook depth.
► Species availability-by-depth and time profiles of fish capture were determined.
► Results indicate spatial and temporal stratification of species availability.