Movements and behaviors of swordfish in the Atlantic and Pacific Oceans examined using pop-up satellite archival tags

Abstract

Swordfish are highly specialized top-level predators that have been challenging to study. In this paper, data from 31 pop-up satellite archival tags attached to swordfish from (i) the eastern Pacific, (ii) central Pacific, and (iii) western North Atlantic-Caribbean were analyzed. Common across locations was a pronounced diel vertical pattern with daytime hours spent primarily below the thermocline and nighttime hours spent in warmer waters, close to the surface. One exception to this pattern was periodic daytime basking events which were most common in cooler waters off California. Maximum daytime depths were significantly correlated with light penetration as measured by the diffuse attenuation coefficient at 490 nm. Temperature did not appear to influence daytime depths, and swordfish tolerated both extremely low temperatures (4°C) and rapid and dramatic temperature changes (>20°C). Temperature did appear to influence the nighttime depths in the Pacific where fish typically remained in the surface mixed layer. In contrast, in the warm tropical Atlantic this was not the case, and nighttime depths were much deeper. In all areas, nighttime depth increased around the full moon. Given the parallels between the vertical movement patterns of swordfish and those of the deep sound scattering layer we suggest that swordfish vertical distribution patterns, especially during daytime, are influenced largely by resource availability. At night, when swordfish are typically targeted by fisheries, both ambient light and temperature influence movements. Understanding vertical movement patterns of swordfish can help evaluate gear vulnerability, improve population assessments, and potentially reduce fisheries bycatch.

Satellite telemetry reveals physical processes driving billfish behavior

“…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].…”

“…[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].…”

“…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].…”

“…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].…”

Recreational Swordfish (Xiphias gladius) Fishery: Angler Practices in South Florida (USA)

“…Highly migratory in nature, swordfish undertake large-scale horizontal migrations [3,4] and demonstrate diel vertical behavior characterized by ascending to the surface and shallow depths during the night and descending to deeper depths during the day [5,6]. Dewar et al speculated these diel behavioral patterns are associated with the deep scattering layer and the availability of prey [5].…”