The tri-axial accelerometer tag was either a TDR10-XB-340 (56 × 38 × 24 mm 69 g Wildlife Computers., Redmond, WA) or a TDR10-Daily Diary-278 (74 × 57 × 36 mm, 117 g Wildlife Computers., Redmond, WA). To measure body orientation and swimming behavior in scalloped hammerhead sharks, we used an instrument package consisting of a tri-axial accelerometer tag combined with a depth and temperature archiving tag housed in a syntactic foam float (2000 m depth rating) equipped with a timed-release mechanism and Argos satellite-linked telemetry tag to facilitate recovery. Measurement of body orientation and swimming behavior
Our objectives were to determine whether scalloped hammerhead sharks exhibit rolling behavior and if so, whether there are any patterns in that behavior and any interplay between rolling behavior and other aspects of swimming performance. We deployed multi-sensor accelerometer biologging packages on free-swimming adult scalloped hammerhead sharks to directly measure swimming depth, body orientation and swimming performance. Scalloped hammerhead sharks ( Sphyrna lewini) have a similar body plan to great hammerhead sharks, including a tall first dorsal fin that may be longer than their pectoral fins and thus, in theory, could also exhibit side swimming behavior to reduce their transport costs.
Hydrodynamic modeling using empirical data from a rigid model of a great hammerhead shark in a wind tunnel demonstrated that this side swimming behavior could reduce drag relative to lift generation, thus reducing the cost of transport (defined as energy expenditure per distance swum) by about 10% compared to conventional upright swimming. Great hammerhead sharks are unusual among sharks in having a dorsal fin longer than their pectoral fins, and it was hypothesized that they use this tall first dorsal fin as a lift-generating surface during side swimming, thus increasing the effective span of the lifting surfaces. using multi-sensor accelerometer instrument packages observed great hammerhead sharks ( Sphyrna mokarran) spend up to 90% of their time swimming on their sides at a roll angle of between 50 and 75°. Most shark species swim in an upright posture with lateral body oscillations, utilizing the dorsal fin for lateral stability and pectoral fins for horizontal stability as well as anterior lift generators that counteract the posterior lift generated by the caudal fin. These changes are possibly due to interplay between reducing cost of transport and social interactions with conspecifics. The diel changes in roll angle and periodicity were accompanied by other changes in swimming behavior. We observed rolling behavior in scalloped hammerhead sharks similar to that observed in great hammerhead sharks. In addition to an increase in degree of roll and roll duration, overall dynamic body acceleration (ODBA) also increased at night, and tailbeat frequency was more regular and consistent than during daytime.
At night, the sharks spent an average 82% of their time at an angle > 30°, with an average roll angle of 60° and rolling periodicity of around 13 min. During daytime, the sharks spent an average of 48% of the time swimming at a roll angle > 30°, with an average roll angle of 41° and rolling periodicity of around 4 min. The roll angle magnitude and periodicity of rolling showed a clear diel pattern. All sharks exhibited rolling behavior throughout the entire period of observation. We obtained 196.7 total days (4720 h) of data from 9 free-swimming adult scalloped hammerhead sharks equipped with multi-instrument biologgers with deployment durations ranging from 7 to 29 days.
Specific objectives were to (1) determine whether scalloped hammerhead sharks exhibit side swimming and rolling behavior, (2) characterize the patterns of these behaviors, and (3) evaluate the purpose of these behaviors. This was assessed by deploying multi-sensor accelerometer instrument packages on free-swimming adult scalloped hammerhead sharks to directly measure swimming depth, body orientation and swimming performance. Scalloped hammerhead sharks ( Sphyrna lewini) also have proportionally taller dorsal fins compared to pectoral fins than most shark species and similar to that of great hammerhead sharks, and thus might exhibit similar rolling behavior. A previous study used wind tunnel tests with a rigid model hammerhead shark to demonstrate that the rolling behavior could improve swimming efficiency using the tall first dorsal fin as a lift-generating surface. Great hammerhead sharks ( Sphyrna mokarran) routinely swim on their sides and periodically roll from side to side.
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