Objectives: This study was performed to test the hypothesis that
the common thresher shark uses its elongate caudal fin for both thrust
production and predation.
Baited lures were
slow-trolled from the research vessel within the field of view of an
underwater video camera. All video clips containing thresher shark feeding
events were selected for frame-by-frame analysis to evaluate the specific
body posture and caudal fin orientation associated with each feeding
behavior. Video footage was recorded for 33 thresher sharks, 42% of which
made active attempts to strike the baited lure with their caudal fin. A
total of 29 caudal fin strikes were observed from 14 individuals with a 65%
strike success ratio. All feeding events were initiated with the upper lobe
of the caudal fin. One individual was observed to bite at the lure after
first striking with the tail.
Two distinct caudal-based
feeding behaviors were documented during this study. The most prevalent
feeding strategy was initiated by a rapid forward undulation of the anterior
body which resulted in a posterior-traveling sinusoidal wave that
consequently advanced to the uppermost tip of the caudal fin (Video clip #1
link). The second predominant feeding behavior involved a lateral strike of
the caudal fin while the shark was positioned directly adjacent to the
baited lure (Video clip #2 link).
This study confirms that the
common thresher shark uses its elongate caudal fin to immobilize prey prior
to consumption and provides insight into the evolution of this unique
feeding strategy among Alopiid sharks. Additional work is necessary to more
precisely understand the kinematics of the disproportionately long caudal
fin during swimming and feeding.
This material is based on
work supported by the National Science Foundation under grants (IOS-0617384
& IOS-0617403) any opinions, findings or conclusions expressed in this
material are those of the authors and do not necessarily reflect the views
of NSF. Additional support was provided by the George T. Pfleger Foundation
and the Bycatch Reduction and Engineering program through NOAA.
