Movement ecology and stenothermy of satellite-tagged shortbill spearfish (Tetrapturus angustirostris)

Abstract

The shortbill spearfish (Tetrapturus angustirostris) is an understudied, istiophorid billfish primarily encountered as bycatch in pelagic commercial fisheries of the Indo-Pacific. The species is listed as data-deficient, and little is known of its biology, ecology, and population structure or status. We assessed the species’ movement ecology and thermal niche with telemetry data from the first shortbill spearfishes ever outfitted with pop-up satellite archival transmitting tags (n = 3 with successfully transmitted data). Short (4–15 day) deployments offshore of the Island of Hawai’i revealed that spearfish primarily occupied the mixed layer, spending >90% of each 24-hr period between the surface and 100 m in water temperatures between 24–26 °C. These individuals consistently exhibited vertical activity at night regardless of the prevailing lunar phase. Nocturnal movements throughout the mixed layer may enable shortbill spearfish to forage on mesopelagic species undergoing diel vertical migration and reduce trophic niche overlap with primarily diurnal, pelagic species. The narrow thermal distribution of shortbill spearfish in this study, almost exclusively within 2 °C of sea surface temperature, suggests that they are more stenothermal than extra-generic istiophorid species.