Modeling swordfish daytime vertical habitat in the North Pacific Ocean from pop-up archival tags

Abstract

The daytime foraging depth of swordfish in the North Pacific was investigated with data from an 8 yr tagging program, using 28 Wildlife Computer pop-up archival tags deployed on swordfish in the North Pacific. The tags transmitted data from 1°S to 44°N latitude and from 206 to 249°E longitude. Five tags were recovered, providing a full archival record that showed that when swordfish did not engage in daytime basking behavior, they remained within a narrow range of light level during both day and night, suggesting swordfish stay within a soundscattering layer (SSL) to feed during both day and night. Daytime mean depth of non-basking swordfish ranged from 32 to 760 m. Seventy-seven percent of the daytime mean depth could be explained with a generalized additive model that used 3 environmental indices: satellite-derived surface chlorophyll as a proxy for light at depth, oxygen at 400 m obtained from the World Ocean Atlas, and temperature at 400 m inferred from the tag data. This model, when used in a predictive mode, generated a basin-wide map of swordfish daytime mean depth that showed depths exceeding 600 m to the north of Hawaii, shoaling to 300 m off the coast of California. This information could improve daytime swordfish catch by longliners and potentially allow them to switch from shallow night sets that result in interactions with sea turtles. This approach in effect defines the habitat of swordfish prey, giving us insight into the vertical behavior of those mid-trophic level organisms inhabiting the SSL. Our model could be easily applied to other deep-foraging species. KEY WORDS: Xiphias gladius · Swordfish · Satellite tracking · Generalized additive model · Habitat modellingResale or republication not permitted without written consent of the publisher

Satellite telemetry reveals physical processes driving billfish behavior

“…As obligate ram ventilators with high metabolic demand and, by extension, high oxygen consumption [128], it is unlikely that billfish can remain in hypoxic waters for an extended period before they incur oxygen debt and require greater DO levels. This may drive avoidance of waters with low DO [129]; however, Dewar et al [54] report that all 31 Pacific swordfish in their study experienced oxygen levels below 2.15 mL L −1 , and Abecassis et al [70] demonstrate that swordfish can endure severe hypoxia (<0.2 mL L −1 ). The apparent ability of swordfish to tolerate low DO compared to istiophorids may be at least partially due to differences in gill surface area.…”

“…Specifically, maximum daytime depth is significantly correlated with light penetration, and nighttime depth has been shown to increase during the full moon, suggesting that swordfish are feeding on deep scattering layers at depth during the day and vertical migrators in the upper water column at night [52,54,55]. Their large eyes likely provide significant benefits for prey capture in low light conditions [79], and, as such, this species is generally exposed to very limited solar illumination except when basking during the day [54,70].…”

“…Swordfish vertical movement is strongly related to light penetration in the Atlantic [52,54,124], Pacific [54,68,70], and Mediterranean Sea [125]. Specifically, maximum daytime depth is significantly correlated with light penetration, and nighttime depth has been shown to increase during the full moon, suggesting that swordfish are feeding on deep scattering layers at depth during the day and vertical migrators in the upper water column at night [52,54,55].…”

“…[55]. This use of a cooler, deep layer may contribute to the occasional and brief daytime basking behavior commonly observed in swordfish tagging studies [54,70].…”

“…Selective heating of the neural infrastructure of the brain and eyes in billfishes and tunas also contribute to their ability to conduct sustained forays into cold water [78,89,122] and provide superior vision for deep foraging [79]. Additionally, surface basking may be a behavioral mechanism for thermoregulation [54,70].…”