Larval billfish abundance in the Western Indian Ocean and future research endeavors

Stock assessment of highly migratory species such as tunas and billfishes from fisheries data
alone is challenging. Using fisheries-independent larval data may be useful as a supplement
to those models. As of date, billfish spawning is only reported in the Pacific and eastern
Indian Ocean and no scientific record of billfish spawning in the western Indian Ocean has
been reported. Opportunistic larval fish surveys were conducted on January 2022 in Tromelin
and Reunion EEZ and on April 2023 in the Mozambique Channel within the French EEZ.
Larval billfishes were collected from surface and subsurface tows. Maximum density was
19.77 istiophorids / 1000 m2 from the Mozambique Channel. Mean sea surface temperature
was 28.79°C (± 0.69 SE). Istiophorids collected in 2022 were genetically identified to be blue
marlin (Makaira nigricans, n = 25) and 2023 collection identified morphologically by
pigment pattern and morphometrics relationships were blue marlin (n = 6) and Indo-Pacific
sailfish (Istiophorus platypterus, n = 15). Monthly or bimonthly systematical sampling is
recommended to reveal spatio-temporal spawning of the billfish species in the WIO and to
better understand the role of the Mozambique Channel and adjacent water as spawning
ground and nursery site for the billfish species.