When it was shown launching out of the sea to snatch birds from the air in the first episode of the BBC’s Blue Planet II, the Seychelles giant trevally, or ‘karang ledan’ as it is called in Creole, became world-famous. Typically eating fish, not birds, this reef predator is critical for maintaining healthy balanced ecosystems. ‘The giant trevally is a popular, sought-after prize fish in Seychelles, particularly in the big-game sportfishing industry. Seychelles has a reputation for being one of the best trevally destinations in the world. The species is also caught in the handline fishery,’ explains Helena Sims, the Save Our Seas Foundation’s Seychelles ambassador. Sims, with more than a decade of conservation experience, is part of the initiative determining which 30% of Seychelles waters should become marine protected areas (MPAs).

‘Although the giant trevally is very popular and sought after, not much was known about its population dynamics in Seychelles until recently. For Seychelles, it is important to study the range of targeted species to better understand how to manage and conserve them effectively,’ Sims explains. New research by the Save Our Seas Foundation D’Arros Research Centre (SOSF-DRC) has revealed that to protect giant trevally throughout their lifespan, the nursery areas of this iconic predator, which are critical habitats for the survival of the next generation, should be conserved. In addition, the larger areas these fish move through and frequently use as adults should be taken into account when conservation planning is undertaken.

To find out where giant trevally went, and when and why, individuals were tagged with acoustic tags in 2019 and tracked across the Amirantes, a group of outer islands in Seychelles. Some were tracked for more than three years. This revealed that juveniles stayed around St Joseph Atoll, but that the extent of the area they frequently used quickly increased as they grew bigger. By the time they were adults, they were using the whole Amirantes Bank. Although small and large adults utilised similar areas, large adults occasionally travelled longer distances, probably driven by their need to find more food to eat as they grew, in order to meet their increasing energy requirements. Both the reduced risk of predation on account of their increased size and the onset of sexual maturity appear to be linked to them using a larger area and a wider diversity of habitats. This study showed that the area giant trevally frequently used was larger than had been reported from other tropical islands and atolls around the world.

This is part of the Marine Spatial Plan Initiative by the Seychelles government, which announced a series of 13 new MPAs in March 2020.  Dr Ryan Daly recommends that St Joseph Atoll’s Zone 2 status should not allow fishing of giant trevally because the area protects the next generation. If fishing is to be allowed at St Joseph Atoll, he advises that it should only be catch-and-release. He suggests the areas that the adults commonly use and occasionally move through, as revealed by this study, should also be considered when further conservation planning is carried out in the region. Daly explains that it will be important to find a balance between harvesting giant trevally as a food source and high-value giant trevally catch-and-release fishing. He encourages high-value catch-and-release fishing of giant trevally, but stresses that regardless of the balance chosen, it is important to protect nursery areas like St Joseph Atoll so that giant trevally fishing is sustainable.

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