47 Sy Piecco & Briones / Phil J of Nat Sci 15 (2010): 101-108 Cruz et al. / Phil J of Nat Sci 18 (2013): 47-54 Hydrodynamic Drag Characteristics of an Anchored Bamboo Fish Aggregating Device (Payao) Based on Model Experiments Ramon S. Cruz 1 , Munechika Ishizaki 2* and Ricardo P. Babaran 1 1 Institute of Marine Fisheries and Oceanology 2 Faculty of Fisheries College of Fisheries and Ocean Sciences Kagoshima University University of the Philippines Visayas Shimoarata 4-50-20 Miagao, Iloilo, 5023 Philippines Kagoshima, Japan AbstrAct. Fish aggregating devices (FADs) are widely used as accessory ishing gears in pelagic capture ishery. Preliminary experiments in a lume tank were conducted to examine the hydrodynamic characteristics of a model raft that was made of 22 bundled small bamboo poles, each with a mean diameter of 22.8 mm and a length of 1 m. The model raft represents a 10:1 scaled-down replica of the bamboo FADs, locally called payao, in the Philippines. Two experimental set-ups were performed: (1) the raft was not anchored to the tank loor but tied horizontally to a strain-gauge sensor to determine the relationship between current velocity and raft drag force, and (2) the raft was anchored to the tank loor with a rope, with both mooring rope and raft attached to a strain-gauge sensor, and the relationship between rope length and drag force was determined at different current velocities. Results indicate that the drag force (D) is a power function of the current velocity (V), with the exponent of V equal to 2.62, which suggests a signiicant deviation from the theoretically quadratic relationship between D and V. The coeficient of drag (C D ) in the irst experiment (unmoored raft) increased as both current velocity and drag force increased, but the trend tends to reverse in the second experiment (moored raft). A longer mooring line increases the critical velocity and leads to decreasing C D , which improves the stability of the raft. The signiicance of the present study was discussed, especially noting the implications of the length of the mooring rope to achieve a greater stability of payaos and the issue of lost payaos. Keywords: Fishing technology, ish aggregating devices (FADs), payao, hydrodynamic drag, lost payao Introduction Floating fish aggregating devices (FADs) are used as effective auxiliary gears in pelagic capture ishery (WCPFC 2013). FADs function in concentrating large quantities of pelagic ish, such as scads, mackerels, and various tuna species, within ishable depths to facilitate their capture by various ishing gears (Freon and Dagorn 2000, Castro et al. 2002). Structurally, FADs consist of a loating *Corresponding author: Kagoshima University Shimoarata 4-50-20, Kagoshima, Japan Tel. No.: Tel: +81-99-286-4111 Email Address: ishizaki@ish.kagoshima-u.ac.jp component that is anchored to the seabed with a synthetic rope (Ishizaki et al. 2013). In the Philippines, FADs (or payao in the vernacular) have played a vital role in boosting the Philippine pelagic isheries production since the 1970s (Floyd 1986; Babaran and Ishizaki 2009). Payaos are widely used as an important accessory ishing device in the tuna industry in the country (Aprieto 1990). Many payaos consist of bundled bamboo poles (rafts) which serve as a floating module moored to the seabed with a synthetic rope typically tied to a cemented drum (anchoring device) (Ishizaki et al. 2013). The raft also probably helps to concentrate large quantities of pelagic ish, most of which exhibit a schooling habit, in coastal and offshore ishing grounds; however, this function is not yet fully explored (Babaran 2009). In some payaos, Original Article