Vertical habitat use of Atlantic blue marlin Makaira nigricans: interaction with pelagic longline gear

Abstract

We characterized Atlantic blue marlin Makaira nigricans temperature-depth vertical habitat utilization from data collected using 51 electronic pop-up satellite archival tags (PSATs) at- tached to fish released by recreational and commercial fishers. Most source data were in the form of 3 or 6 h, temperature- and depth-frequency histograms transmitted by the tags to the ARGOS satellite system. However, high resolution time series of temperatures, depths (30 or 60 s resolution), and light intensity were obtained from 6 tags that were physically recovered. The distributions of times at depth were significantly different between day and night. During daylight hours, the fish were typically be- low the near-surface layer, often at 40 to >100 m, sometimes remaining below the near-surface layer at depth throughout the daylight hours, but often returning briefly to the surface. At night, the fish spent most of their time at or very close to the surface. This pattern of behavior also meant that the distribu- tions of time at temperature were significantly different between day and night, with the fish occupy- ing warmer strata during darkness. We evaluated the fractions of time spent by each fish within each degree of water temperature relative to the temperature of the surface mixed layer to assess assump- tions used to model population abundance trends from pelagic longline catch per unit effort (CPUE) data. Frequency distributions were determined for periods of darkness, daylight and, where possible, twilight. Results were highly variable within the time series for individual fish and among individuals. Assumptions about habitat usage in previous CPUE analyses are clearly inappropriate and may lead to serious errors that can propagate through the fisheries management system.