A New Method to Calibrate Attachment Angles of Data Loggers in Swimming Sharks

Hindawi Publishing Corporation EURASIP Journal on Advances in Signal Processing Volume 2010, Article ID 732586, 6 pages doi:10.1155/2010/732586 Research Article A New Method to Calibrate Attachment Angles of Data Loggers in Swimming Sharks Shizuka Kawatsu,1 Katsufumi Sato,2 Yuuki Watanabe,3 Susumu Hyodo,1 Jason P. Breves,4 Bradley K. Fox,4 E. Gordon Grau,4 and Nobuyuki Miyazaki1 1 Ocean Research Institute, The University of Tokyo, 1-15-1 Minamidai, Nakano, Tokyo 164-0014, Japan 2 International Coastal Research Center, Ocean Research Institute, The University of Tokyo, 2-106-1 Akahama, Otsuchi, Iwate 028-1102, Japan 3 National Institute of Polar Research, 10-3 Midorimachi, Tachikawa, Tokyo 190-8518, Japan 4 Hawaii Institute of Marine Biology, University of Hawaii, Kaneohe, HI 96744, USA Correspondence should be addressed to Shizuka Kawatsu, Received 2 May 2009; Accepted 7 August 2009 Academic Editor: João Manuel R. S. Tavares Copyright © 2010 Shizuka Kawatsu et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. Recently, animal-borne accelerometers have been used to record the pitch angle of aquatic animals during swimming. When evaluating pitch angle, it is necessary to consider a discrepancy between the angle of an accelerometer and the long axis of an animal. In this study, we attached accelerometers to 17 free-ranging scalloped hammerhead shark (Sphyrna lewini) pups from Kaneohe Bay, Hawaii. Although there are methods to calibrate attachment angles of accelerometers, we confirmed that previous methods were not applicable for hammerhead pups. According to raw data, some sharks ascended with a negative angle, which differs from tank observations of captive sharks. In turn, we developed a new method to account for this discrepancy in swimming sharks by estimating the attachment angle from the relationship between vertical speed (m/s) and pitch angle obtained by each accelerometer. The new method can be utilized for field observation of a wide range of species. 1. Introduction An accurate determination of pitch angle is critical to gain detailed information about the diving and foraging strategies of aquatic animals. For example, air-breathing aquatic animals that forage underwater control pitch angle and allocate their submerged time. In African penguins, steeper ascent angles presumably occur when they have depleted their oxygen stores and must return to the surface more quickly to breathe [1]. In macaroni penguins, pitch angle is significantly correlated with time spent at the bottom-phase of the dive [2]. A steep pitch angle during ascent indicates that they encountered a prey patch and a shallow pitch angle contributes to movement into a more profitable area in the following dive, due to increasing the horizontal distance [2]. While in fish, Nakaya [3] said that scalloped hammerhead sharks (Sphyrna lewini) have great maneuverability due to a movable large plate on head. Based on the observation of swimming behavior, it is apparent that sharks make a sharp dorsal turn at the bottom, consume food items, and swim away along the bottom [3]. In this sequence, pitch angle is an important indicator of a feeding event. Recent advances in the development of animal-borne accelerometers (data loggers) make it possible for researchers to monitor pitch angle of aquatic animals in situ by attaching an acceleration sensor (accelerometer) along the longitudinal axis of the body. When a data logger is positioned exactly parallel to the longitudinal axis of an animal, the calculated angle of the data logger is the same as the pitch angle of the animal. Nonetheless, it is impossible to align the logger exactly parallel to the longitudinal axis of an animal in field studies. A few methods have been previously described to account for the discrepancy between the pitch angle of data loggers and the longitudinal axis in field studies. In one instance, Watanuki et al. [4] designated the attachment angle to the lower back of seabirds as 0◦ when they were 2 at the water surface, a time in which they were essentially horizontal before release. This methodology is well suited for birds that can be maintained in a horizontal position at the water surface. However, for animals that have a flexible body, it is difficult to keep them positioned horizontally for an extended period of time. In addition, this methodology cannot be applied for obligate swimming fish because of fatal risk for lack of adequate gill ventilation. Another approach to account for the discrepancy between the pitch angle of data loggers and the longitudinal axis was reported by Sato et al. [5] in Weddell seal. In this study, the attachment angle was calculated by using the data logger along with the speed sensor. Sato et al. [5] used the data logger (UWE1000PD2GT: 22 mm diameter, 124 mm length; 80 g in air; Little Leonardo Co., Tokyo, Japan) which contains a propeller and reported that the attachment angle for a specific dive of Weddell seal could be determined using equations including the number of propeller rotations, surging acceleration (m/s2 ), the acceleration of gravity (9.8 m/s2 ), and body angle (degrees). This methodology can only be applied for large animals due to the relatively large size of data loggers that have a propeller. Furthermore, this method is only applicable for diving animals that must come to the surface to breathe. Lastly, in a third study, the attachment angle in flatfish was assigned as 0◦ when they lay on the substrate as reported by Kawabe et al. [6]. This method is only applicable for benthic animals that remain on the bottom. There are currently no reported methods to apply for continuous swimming fish. The previously described methods are specific for particular species and we therefore anticipated that they might not be suitable for use in hammerhead sharks. Scalloped hammerhead shark pups have flexible bodies and are obligatory swimmers [7]. In addition, their small body size allows only for use of the smallest logger available for field studies that can only record depth, temperature and 2-axes accelerations, but not speed. In this study, we attached data loggers to 17 free-ranging pups with the objective to establish a new method for calibrating the attachment angle of loggers in free-ranging sharks. 2. Materials and Methods 2.1. Field Work. Our field studies occurred in Kaneohe Bay, Hawaii (21.26◦ N, 157.47◦ W) in August and October, 2007, and July/August, 2008. Kaneohe Bay is a nursery ground for the scalloped hammerhead shark (Sphyrna lewini) during summer months in which pups spend most of their time near the bottom [8]. In this study, juvenile scalloped hammerhead sharks were collected using hand lines with baited hooks. Upon capture, sharks were immediately transferred to the Hawaii Institute of Marine Biol (...truncated)