Kenya · Since 2015
Kenya Marlin Project
Sport-fishing crews in Kenya fitted 98 satellite tags to black and striped marlin, and the tracks showed fish ranging thousands of kilometers across the western Indian Ocean.
Black marlin (Istiompax indica) and striped marlin (Kajikia audax) are fast, wide-ranging predators of the open ocean, yet little was known about their movements in the Indian Ocean. Working with Kenya’s sport fishers, we conducted the first satellite-tracking study of either species in the region.
The project began when marlin sport fishers in Kenya, concerned that so little was known about these fish, approached MMF and funded satellite tags for the marlin they caught and released. Dr. Chris Rohner led the study, which he designed with Dr. Simon Pierce, and the fieldwork was organized and run by Roy Bealey of Pelagic Fisheries Consulting and Dr. Bernerd Fulanda of Pwani University in Kilifi. Dr. Clare Prebble co-authored the analysis of diving behavior. Skippers and crews tagged the fish off Watamu, Shimoni, and Mombasa, and we analyzed the tracks and published the results in three papers. (Top photo: Julia Bahlsen.)
What we've done
- Between November 2015 and March 2019, worked with Kenyan sport-fishing crews to fit 98 pop-up satellite tags: 49 on striped marlin and 49 on black marlin, all caught and released.
- Tracked the movements of 40 striped marlin and 34 black marlin, and compared their tracks with ocean temperature and productivity.
- Analyzed the diving behavior of 34 black marlin and 39 striped marlin, and how their depth use overlaps with gillnet and longline fisheries.
- Published the results in the Journal of Fish Biology in 2020 (striped marlin) and 2021 (black marlin), and in Marine Ecology Progress Series in 2022 (diving behavior of both species).
Support our marlin research
This study is complete. Your gift supports our current black marlin research in Mozambique, which builds on what we learned in Kenya.
What the study found
Tagging with Kenya’s sport-fishing fleet
Kenya’s coast supports more than 30 recreational billfish boats in the peak season, and sport fishing for marlin is typically catch-and-release. Experienced skippers, trained crew, and scientists fitted the tags while each fish remained in the water, then released it. Striped marlin were tagged mostly off Watamu, 25–45 km (16–28 miles) offshore, with others off Shimoni on the south coast and Mombasa. Black marlin, in contrast, were all tagged off Watamu, most within 40 km (25 miles) of shore, both during the northeast monsoon season, from November to April, and in a shorter August–September season. Each tag was programmed to detach after up to six months, then float to the surface and transmit its data by satellite. (Photo: Roy Bealey.)
Seasonal movements of striped marlin
Striped marlin were highly mobile, covering about 48 km (30 miles) a day, and one fish swam 9,187 km (5,709 miles) in six months. They moved north and east, never south of Kenya and Tanzania: to the Gulf of Aden and the Arabian Sea, around the island of Socotra, past the Seychelles, and, in one case, to waters near the Maldives. The tagged fish shifted between two seasonal hotspots, off Kenya and southern Somalia from December to March, then around the Horn of Africa and Socotra from April to August; satellite maps of sea temperature and chlorophyll suggest that this shift may follow changes in prey availability. When near the surface, they spent three-quarters of that time in water of 28–30 °C (82–86 °F). (Photo: Henley Spiers.)
Black marlin movements and survival
Black marlin occur off Kenya twice a year, and fish tagged in the two seasons showed no clear differences in dispersal, with their core habitat shifting north to Somalia, the Horn of Africa, and Oman from May. The tagged fish traveled about 57 km (35 miles) a day; one reached the Gulf of Oman, 8,200 km (5,100 miles) over 184 days, while another swam south to the northwest coast of Madagascar and into the Mozambique Channel. Compared with simulated tracks, the tagged fish favored cooler, more productive water (with higher concentrations of chlorophyll, a measure of plankton growth) closer to the coast. Their locations spanned more than 5,500 km (3,400 miles), which suggests that black marlin throughout the western Indian Ocean could belong to a single stock, the population unit used in fisheries management. The tags also suggest that black marlin are more vulnerable to capture and release than striped marlin: about a third of tagged black marlin appear to have died soon after release, compared with about 4% of striped marlin, although none of the 11 fish of 100 kg (220 lb) or more died.
Diving behavior
Both species spent much of their time near the surface, but dived to more than 460 m (1,510 ft), into water as cold as 10.2 °C (50.4 °F). Striped marlin spent half of their time in the top 5 m (16 ft), compared with a third for black marlin, yet dived deep more often, and most dived deeper by day. About a third of black marlin showed a crepuscular pattern, diving particularly deep at dawn and dusk. In addition to about 50 km (30 miles) of horizontal travel each day, a marlin could accumulate up to 14 km (9 miles) of vertical movement in a day through repeated dives. The tags also recorded two marlin leaping clear of the water and dislodging their tags, the first recorded instances of this tag-shedding behavior in billfish.
Implications for fisheries
Kenyan marlin regularly reached the commercial fishing grounds of the northwest Indian Ocean; as a result, catches there can affect the fish available to Kenya’s artisanal and recreational fishers. Marlin also spent more than half of their time in depths fished by drift gillnets, and almost all of their time within reach of longlines. The papers therefore recommend assessing and managing western Indian Ocean striped marlin as a distinct stock, setting species-specific catch limits, using circle hooks, considering seasonal closures to surface longlining in the marlin hotspots off the Horn of Africa and Kenya, and limiting the growth of fishing capacity.
Dr. Chris Rohner
Dr. Simon Pierce
Dr. Clare Prebble
Mozambique Marlin Research Project