The Pelagic Habitat of Swordfish (Xiphias gladius) in the Changing Environment of the North Indian Ocean

Swordfish (Xiphias gladius) are a highly migratory keystone species, found in tropical and temperate seas that are influenced by environmental parameters. In the Bay of Bengal, the Arabian Sea, and the ocean region around Sri Lanka, the environment is gradually changing as a result of climate change. In this study, we identified the preferable environmental conditions for swordfish using satellite-derived environmental data and in-situ fish catch data. We modeled the relationships between fish distribution and the environment changes using Boosted Regression Trees (BRT) and Generalized Additive Model (GAM) methods. The monthly mean fishing effort is comparatively high from October to March and the fish catch rates are high from September to November. Chlorophyll-a concentration has a positive relationship with catch rates while sea surface temperature (SST), sea salt surface mass concentration (SSS), and effort show negative relationships. Approximately 0.3-0.4 mgm-3 of chlorophyll-a, 28-28.5 °C SST, and (3-5)10-8 kgm-3 of SSS were significantly correlated with high swordfish catch rates. According to the optimum environmental conditions identified using the above models, the suitable environmental spatial and temporal distribution was mapped. The results show that the optimum conditions for swordfish are in the eastern region of Sri Lanka, around Thailand and Myanmar, from June to August, and around Bangladesh, Myanmar, Pakistan, the west coast of Sri Lanka, and the east coast of India during September to November.