Quantifying and understanding the largest aggregation of giant trevally on record

By: Ryan Daly

Giant trevally (locally known as giant kingfish) are large (up to 80 kg) keystone predators that are prized by recreational anglers for their size and strong fighting abilities. Giant trevally also form predictable spawning aggregations during which time they can be especially vulnerable to overexploitation. However, very little is known about their aggregations and there are very few records of giant trevally aggregations in the western Indian Ocean. Thus, understanding the location and timing of giant trevally aggregation events are critical for the effective conservation and management of the species. In 2018 we described the largest known aggregation of giant trevally on record in the Ponta do Ouro Partial Marine Reserve in southern Mozambique.

Unbelievably, the aggregation was made up of almost 2500 fish with an estimated biomass of ~30 tonnes. Realising the importance of the aggregation, we initiated an acoustic tagging project to understand more about the aggregation timing and where all the fish taking part in the aggregation came from. Incredibly, we found that fish taking part in the aggregation were travelling up to 1200 km between aggregation events (i.e. Ponto do Ouro to Port St Johns and back), which is considerably further than has been recorded for this species elsewhere in the world. Interestingly, the majority of fish taking part in the aggregation in Mozambique crossed the border to South Africa. We also found that the aggregation timing was associated with the lunar phase and was seasonally highly predictable. These results highlighted the fact that the aggregation could be extremely vulnerable to exploitation and that specific management plans are required to protect the aggregation. Furthermore, our results suggested that the conservation of the giant trevally aggregation in Mozambique will be key to ensuring the future health of the giant trevally population in South Africa, highlighting the importance of transboundary conservation. Going forward we will continue to monitor the giant trevally aggregation and work to ensure that this unique event of one of our most charismatic game fishes is protected.

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