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The black marlin Istiompax indica is an apex marine predator and is susceptible to
overfishing. The movement ecology of the species remains poorly known, particularly
within the Indian Ocean, which has hampered assessment of their conservation status
and fisheries management requirements. Here, we used pop-up archival satellite
tags to track I. indica movement and examine their dispersal. Forty-nine tags were
deployed off Kenya during both the north-east (November–April) and south-west
(August–September) monsoon seasons, providing locations from every month of the
year. Individual I. indica were highly mobile and track distance correlated with the
duration of tag attachment. Mean track duration was 38 days and mean track distance
was >1800 km. Individuals dispersed in several directions: north-east into
Somalian waters and up to northern Oman, east towards the Seychelles, and south
into the Mozambique Channel. Their core habitat shifted seasonally and overlapped
with areas of high productivity off Kenya, Somalia and Oman during the first half of
the year. A second annual aggregation off the Kenyan coast, during August and
September, did not coincide with high chlorophyll-a (chl-a) concentrations or thermal
fronts, and the drivers of the species' presence and movement from this second
aggregation was unclear. We tested their habitat preferences by comparing environmental
conditions at track locations to the conditions at locations along simulated
tracks based on the empirical data. Observed I. indica preferred cooler water with
higher chl-a concentrations and stayed closer to the coast than simulated tracks. The
rapid and extensive dispersal of I. indica from Kenya suggests that there is likely a single
stock in the Western Indian Ocean, with individuals swimming between areas of
high commercial catches off northern Somalia and Oman, and artisanal and recreational
fisheries catches throughout East Africa and Mozambique.
