Mozambique · Since 2022
Mozambique Marlin Research Project
Our research combines satellite tagging, genetics, and collaboration with the Bazaruto Seascape's sport-fishing community to investigate the movements and population structure of black marlin in the western Indian Ocean.
Black marlin (Istiompax indica) are highly migratory apex predators and one of the world’s largest bony fishes. Fishing pressure in the Indian Ocean is increasing, yet management is limited, in part because so little is known about the species’ movements and population structure.
The Bazaruto Seascape, in southern Mozambique, is one of the few places in the world where very large adult black marlin are caught. The sport-fishing community that catches and releases them believes the area may be a spawning ground. Since October 2022, our research here has combined satellite tagging, genetics, and the skills of the region’s most experienced billfish fishers to investigate how black marlin use the Mozambican coast and how they connect with the wider western Indian Ocean. The project is led by MMF’s Dr. Clare Prebble with Dr. Chris Rohner, who led our earlier marlin tracking study in Kenya.
Objectives
- Clarify the connectivity of marlin populations at regional and ocean-basin scales, combining satellite tagging, stable isotope analysis, and genetics.
- Identify spawning habitats for black marlin and other billfish, and evaluate where they overlap with fisheries.
- Deploy pop-up satellite tags to assess short-term movements in relation to biological and environmental conditions.
- Collect tissue samples from billfish and their prey to establish their ecological role in the open ocean.
- Provide data on connectivity and genetic structure to the Indian Ocean Tuna Commission, Mozambican institutions, and the IUCN Tuna and Billfish Specialist Group.
Sponsor a satellite tag
Each pop-up satellite tag costs $5,000 and can record one marlin's movements across the western Indian Ocean for weeks or months. Smaller gifts fund essential project expenses such as boat hire, fuel, and scientific analysis.
Our work on marlin in Mozambique
Fieldwork and publications
- Completed four field seasons in the Bazaruto Seascape since launching the project in October 2022, working with four expert boat captains.
- Fitted 12 black marlin, weighing roughly 140 to 410 kg (300 to 900 lb), with pop-up satellite tags.
- Collected tissue samples for genetic analysis alongside the tagging.
- Refined tag rigging and placement season by season, with the aim of keeping tags attached for longer to these fast, powerful fish.
- Published three papers from our earlier Kenya studies, the first large-scale satellite-tracking research on black and striped marlin in the Indian Ocean.
Research priorities
- Deploy the three remaining satellite tags in the October to December peak, targeting deployments of four to six months.
- Complete the analysis of the Mozambique tracks and publish a peer-reviewed paper on black marlin movement ecology in the Mozambique Channel.
- Integrate oceanographic context, including the Agulhas Current and its eddies, into the analysis of black marlin movement patterns.
- Investigate whether the Bazaruto Seascape is a spawning ground, using larval tows (plankton-net sampling for marlin larvae) and examination of landed fish alongside the tracking.
Food and fishing
Black marlin are one of the largest bony fishes, with females weighing over 700 kg (1,500 lb). They are highly mobile apex predators and an important part of the structure and functioning of healthy marine systems. However, the movement ecology of black marlin in the Indian Ocean is poorly understood, and few management measures of any kind have been implemented as a result.
Black marlin contribute to livelihoods and food security through artisanal fisheries, and their large size and fighting qualities also make them one of the most prized targets for recreational fishers. Recreational sport fisheries in Mozambique are primarily catch-and-release, make significant contributions to the tourism industry, and are an important source of billfish behavioral data. Increased billfish landings in the commercial sector over the last decade have raised concerns, however, and deep-freeze longliners off the coast of Mozambique have been highlighted as a developing fishing industry to watch. Knowing how black marlin use and move around the Mozambican coastline is therefore essential to a sustainable future for the species, and for all those who depend on it.
Black marlin in Mozambique
The Bazaruto Seascape sees large, potentially gravid (egg-bearing) female black marlin almost exclusively from October to December. Their seasonal density is high enough that experienced fishers catch them regularly, and sport fishers travel internationally to this hotspot in season. Resident billfish fishers, as well as global experts, believe this area could be a spawning ground for the species. If confirmed, it would be the first identified spawning ground for black marlin in the western Indian Ocean, which makes the region a unique opportunity to study a data-deficient species in what is likely to be a highly sensitive habitat for the wider population.
Our earlier satellite tagging in Kenya, together with preliminary genetic results, suggests a single, highly connected population of black marlin across the western Indian Ocean. What remained unclear was how the marlin use the coastline in Mozambique, and how best to protect them while they are here. Since 2022, we have worked with the Bazaruto Seascape’s expert captains and sport fishers, who lead the tagging and tissue-sampling fieldwork, to answer those questions directly. The first analysis of the Mozambique tracks is in preparation, and its results will be added here once they are published.
Findings from Kenya
Between 2015 and 2019, experienced skippers and trained crew on recreational fishing boats in Kenya deployed 98 pop-up satellite tags on black and striped marlin (Kajikia audax) for our Kenya studies, led by Dr. Chris Rohner. Striped marlin tagged off Kenya traveled up to 9,187 km (5,709 miles), with tracks lasting up to 183 days. After March, their activity hotspot shifted from the East African coast toward the Horn of Africa and Socotra; none of the tagged fish swam south of East Africa (Rohner et al. 2020). Black marlin, in contrast, dispersed in several directions, including south into the Mozambique Channel, with a mean track distance of more than 1,800 km (1,100 miles) over a mean of 38 days. Their rapid, extensive dispersal from Kenya suggests a single stock (a population managed as one unit) in the western Indian Ocean, ranging across areas of high commercial, artisanal, and recreational catches (Rohner et al. 2021). Both species reached similar maximum depths of 460 to 470 m (1,500 to 1,540 feet), and their depth use showed how susceptible they are to capture in the region’s drift gillnet and longline fisheries (Rohner et al. 2022). The Mozambique project extends this research along the Mozambican coast.
Satellite-tag technology
Each pop-up satellite tag is a small computer, built to withstand pressure at depths of 2 km (1.2 miles), that records light level (used to calculate location), temperature, and depth every few seconds while attached to the fish. The data are summarized and stored on the tag until it releases after a set period, floats to the surface, and transmits its summaries to Argos satellites; if we recover the tag itself, we can download far more detailed records. Tagging a fast, powerful predator at sea is difficult, and not every tag reports back, so we refine our rigging and tag placement each season. Tissue samples collected at the same time will support genetic analyses of how closely Mozambique’s marlin are related to those elsewhere in the Indian Ocean.
Dr. Clare Prebble
Dr. Chris Rohner
Dr. Simon Pierce
Kenya Marlin Project
Bazaruto Seascape Marine Conservation
Mozambique Bull Shark Research Project