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.

The evolution of 'thunniform' body shapes in several different groups of vertebrates, including whales, ichthyosaurs and several species of large pelagic fishes supports the view that physical and hydromechanical demands provided important selection pressures to optimize body design for locomotion during vertebrate evolution. Recognition of morphological similarities between lamnid sharks (the most well known being the great white and the mako) and tunas has led to a general expectation that they also have converged in their functional design; however, no quantitative data exist on the mechanical performance of the locomotor system in lamnid sharks. Here we examine the swimming kinematics, in vivo muscle dynamics and functional morphology of the force-transmission system in a lamnid shark, and show that the evolutionary convergence in body shape and mechanical design between the distantly related lamnids and tunas is much more than skin deep; it extends to the depths of the myotendinous architecture and the mechanical basis for propulsive movements. We demonstrate that not only have lamnids and tunas converged to a much greater extent than previously known, but they have also developed morphological and functional adaptations in their locomotor systems that are unlike virtually all other fishes.

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This study reports on the fine-scale movements of swordfish (Xiphias gladius) outfitted with pop-off satellite archival transmitters (PSATs) in the Southern California Bight (SCB). PSATs were deployed on basking swordfish using traditional harpoon methods from 2004 to 2006. Transmitters were programmed for short-term deployment (2-90 days) and re-acquired using a signal direction finder. High-resolution (min )1 ) depth and temperature data from nine swordfish (approximately 45-120 kg) were collected (>193 days). All swordfish displayed diurnal vertical movements similar to those reported for other geographic locations. The dominant diurnal movement pattern entailed swordfish remaining below the thermocline (>68 ± 15 m) during the day and near the surface, within the upper-mixed layer, at night. Collectively, the average daytime depth (±SE) was 273 ± 11 m and the average night depth 31 ± 5 m. Three distinct vertical behaviors were recorded: 35% of the records following a strict diurnal pattern, with the entire day below the thermocline and the entire night near the surface; 52% of the records revealed routine surface-basking events during the day, with an otherwise similar distribution at night; and 13% of the records exhibited surface-oriented activity during the day and night. Surface basking (<3 m during the day) was recorded for eight individuals and occurred on 131 of the 193 days (68% of the dataset). Collectively, surface basking accounted for 8% of the total daytime records. The relevance of these vertical behaviors to SCB fisheries is discussed.

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This comparative study of the gill morphometrics in scombrids (tunas, bonitos, and mackerels) and billfishes (marlins, swordfish) examines features of gill design related to high rates of gas transfer and the high-pressure branchial flow associated with fast, continuous swimming. Tunas have the largest relative gill surface areas of any fish group, and although the gill areas of non-tuna scombrids and billfishes are smaller than those of tunas, they are also disproportionally larger than those of most other teleosts. The morphometric features contributing to the large gill surface areas of these high-energy demand teleosts include: 1) a relative increase in the number and length of gill filaments that have, 2) a high lamellar frequency (i.e., the number of lamellae per length of filament), and 3) lamellae that are long and low in profile (height), which allows a greater number of filaments to be tightly packed into the branchial cavity. Augmentation of gill area through these morphometric changes represents a departure from the general mechanism of area enhancement utilized by most teleosts, which lengthen filaments and increase the size of the lamellae. The gill design of scombrids and billfishes reflects the combined requirements for ram ventilation and elevated energetic demands. The high lamellar frequencies and long lamellae increase branchial resistance to water flow which slows and streamlines the ram ventilatory stream. In general, scombrid and billfish gill surface areas correlate with metabolic requirements and this character may serve to predict the energetic demands of fish species for which direct measurement is not possible. The branching of the gill filaments documented for the swordfish in this study appears to increase its gill surface area above that of other billfishes and may allow it to penetrate oxygen-poor waters at depth.

SUMMARYTunas (family Scombridae) and sharks in the family Lamnidae are highly convergent for features commonly related to efficient and high-performance(i.e. sustained, aerobic) swimming. High-performance swimming by fishes requires adaptations augmenting the delivery, transfer and utilization of O2 by the red myotomal muscle (RM), which powers continuous swimming. Tuna swimming performance is enhanced by a unique anterior and centrally positioned RM (i.e. closer to the vertebral column) and by structural features (relatively small fiber diameter, high capillary density and greater myoglobin concentration) increasing O2 flux from RM capillaries to the mitochondria. A study of the structural and biochemical features of the mako shark (Isurus oxyrinchus) RM was undertaken to enable performance-capacity comparisons of tuna and lamnid RM. Similar to tunas, mako RM is positioned centrally and more anterior in the body. Another lamnid, the salmon shark (Lamna ditropis), also has this RM distribution, as does the closely related common thresher shark (Alopias vulpinus; family Alopiidae). However, in both the leopard shark(Triakis semifasciata) and the blue shark (Prionace glauca),RM occupies the position where it is typically found in most fishes; more posterior and along the lateral edge of the body. Comparisons among sharks in this study revealed no differences in the total RM quantity (approximately 2–3% of body mass) and, irrespective of position within the body, RM scaling is isometric in all species. Sharks thus have less RM than do tunas(4–13% of body mass). Relative to published data on other shark species,mako RM appears to have a higher capillary density, a greater capillary-to-fiber ratio and a higher myoglobin concentration. However, mako RM fiber size does not differ from that reported for other shark species and the total volume of mitochondria in mako RM is similar to that reported for other sharks and for tunas. Lamnid RM properties thus suggest a higher O2 flux capacity than in other sharks; however, lamnid RM aerobic capacity appears to be less than that of tuna RM.

The common thresher shark, Alopias vulpinus, is the basis of the largest commercial shark fishery in California waters. We used acoustic telemetry to determine the diel movement patterns and habitat preferences of this species in the Southern California Bight (SCB), where commercial fishing for the common thresher shark is concentrated. Eight common threshers (fork length: 122–203 cm) were tagged with temperature and depth-sensing acoustic transmitters and tracked for periods ranging from 22 to 49 h. Tracked sharks preferentially utilized deep offshore waters, and avoided shallower waters over the continental shelf. Mean rate of movement (ROM ± s.d.) was 2.15 ± 0.46 km h−1. ROM and angular concentration (r, a measure of relative linearity) both showed a strong daytime pattern, with highest values at dawn that decreased throughout the day, whereas nocturnal ROM and r were less variable. Daytime vertical movements consisted of either vertical excursions below the thermocline or relatively level swimming within the upper portion of the thermocline. Nocturnally, all sharks remained within the mixed layer. These findings suggest that the common thresher shark is primarily a daytime predator, and have relevance for estimating how the alteration of the set depth of fishing-gear could affect catch rates of this species in the SCB.