US - Researchers have used satellite tagging to track the movements of marlins, revealing their preferred habitats.
Using the largest tagging data set to date, Chi Hin "Tim" Lam of the University of Massachusetts and colleagues showed that across the Pacific Ocean, the vertical habitat of striped marlin is defined by the light-penetrated, uppermost part of the ocean.
This is known as the epipelagic layer, and is within 8°C of sea surface temperature.
Overfishing is already occurring for striped marlin in certain parts of the Pacific Ocean.
The findings could help to avoid or reduce undesired catch of this species, for example by removing hooks shallower than about 390 feet (120 metres).
It is important to characterise the marlins' water column activity because if striped marlin occupy a vertical habitat different from commercially more important species like tuna, fishers can exploit this separation to target higher-value species and avoid catching striped marlin.
For this work, the researchers deployed about 250 smart tags, or popup satellite archival tags, valued at roughly $4,000 per unit, over five years across eight field locations from Australia to Hawaii and Ecuador to Southern California, providing one of the largest data sets for any species in the open ocean.
Tags returned depth, temperature and light-based positional data for up to eight months.
The results showed that striped marlin demonstrated "pronounced use" of the mixed layer, with fish spending 54 to 84 percent of their time at the top 40 feet of the ocean surface (10 metres) across all regions.
The mixed-layer is between the surface and a depth of roughly 80 to 656 feet (25 to 200m), where salt and temperature levels are about the same as at the surface. Individuals remained near the surface in the night and were located near the bottom of the mixed layer during the day.
Newly observed behaviours reveal striped marlin have short-term cold and low oxygen tolerance. Temperature and depth data from the tagged fish show a broader range of habitat than scientists had noted before.
Fish exploited cold water, lower than about 60°F (15°C) or low-oxygen environments for short durations of 10 to 40 minutes, presumably for hunting and avoiding predators.
Mr Lam said the study allows "common patterns of striped marlin behaviour to be identified throughout the Pacific Ocean."
Direct field observations and common habitat patterns provide information to formulate clear management measures to rebuild striped marlin stocks, and may help to identify realistic bycatch reduction measures.
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TheFishSite News Desk
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