The differences in behavioral responses to a net obstacle between day and night in captive manatees; does entanglement happen at night?


Entanglement in fishing gear occurs in endangered manatees and may result in serious injury or death. Such incidents may happen more frequently at night when the animal’s visual sense is limited. In this study, we examined the differences in behavioral response of captive manatees to a net obstacle during light (day) and dark (night) periods. We used a plastic net as the obstacle, and video-recorded the manatees’ behavior. The experiments showed that captive manatees avoided the obstacle during the day more frequently than at night, which suggests that the manatees can perceive the obstacle more readily during light periods. However, there was no difference in the frequency of bumping or actively touching the obstacle between light and dark periods. The results suggest that the manatees can recognize the net obstacle even at night by purposely touching it, but they avoid it less frequently, and that entanglement during light periods may occur during accidental bumping, rather than from a failure to recognize it altogether.

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We would like to thank all of the staff at the Okinawa Churaumi Aquarium for their assistance in conducting these experiments. We would like to thank K. Asahina at Nihon University, H. Kato at Tokyo University of Marine Science and Technology, L. D. Olivera-Gomez at the Autonomous University Juarez of Tabasco, and D. Gonzalez-Socoloske at Duke University for their discussions and insightful comments on previous drafts. We also thank J. A. Mobley and D. Gonzalez-Socoloske for correcting the English manuscript. The present study was supported by the Fisheries Research Agency, National Research Institute of Far Seas Fisheries.

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Correspondence to Mumi Kikuchi.

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Kikuchi, M., Suzuki, M., Ueda, K. et al. The differences in behavioral responses to a net obstacle between day and night in captive manatees; does entanglement happen at night?. Fish Sci 77, 795 (2011).

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  • Obstacle
  • Behavior
  • Trichechus manatus