Seasonal movements and diving behaviour of black marlin (Istiompax indica) in the northwestern Pacific Ocean

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Black marlin (Istiompax indica) is a highly migratory apex predator distributed widely throughout the Pacific and Indian Oceans, with seasonal densities occurring in the East and South China Seas and near Taiwan. The peak seasonal presence of black marlin in eastern Taiwan (September to November) has led to the development of a commercially important harpoon fishery that specifically targets this aggregation. The harpoon fishery provides relatively easy access to black marlin for tagging research to monitor their movement patterns with pop-up satellite archival tags (PSATs). Nine black marlin were tagged from February 2011 to March 2012 with PSATs that acquired depth, temperature and ambient light-level data. PSATs remained affixed for a median 95 days (average 132 days±38 SE, range: 10–360 days) which are the highest documented PSAT retention statistics for any istiophorid billfish. No post-release mortalities occurred in these data and the harpoon method of PSAT application, as well as an improved anchor, may promote longer retention times by tagging fish that are not subjected to stress or injury associated with prolonged fishing bouts or handling. Linear displacements ranged from 567 to 1226km from deployment to pop-up locations with speeds of 3.1–122.6kmday−1 (average: 6.4kmday−1, 0.3kmh−1). Most probable tracks calculated by the state-space Kalman filter, augmented with sea surface temperature (SST), suggested that black marlin undergo dichotomous seasonal movement patterns: fishes tagged in spring and summer moved in a northerly direction to the East China Sea and those tagged in winter moved in a southerly direction to the South China Sea. Estimated track models during March–April for tagged fish in southwestern Taiwan coincided with known spawning activity in this vicinity though the tracks and vertical data were not sufficiently detailed to discriminate this putative behaviour. Diving depths ranged from the surface to ∼258m and ambient water temperatures occupied ranged from 30.3°C to 14.5°C. The distribution of time spent at depth and temperature was significantly different between daytime and nighttime diving activity. Tagged black marlin spent the majority of daytime in the surface mixed-layer to ∼50m and exhibited occasional basking behaviour; and at nighttime, they were confined almost exclusively to the surface to ∼20m. Depth distribution appeared to be limited by an 8°C change in water temperature relative to the warmest water (ΔSST). In the context of regional fisheries management and stock assessments, our study reports the first tagging data of black marlin in Southeast Asia and it will be especially important to compare diving behaviour patterns with other specimens tagged using other fishing methods to discern differences. Like many previous electronic tagging studies on istiophorid billfish in the Pacific, the data suggest that the majority of habitat for black marlin in Taiwan may be adequately described by two dimensions (this species spends most of their time in the surface mixed-layer).