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Methods <br />Field Collections and Tagging. A sample of 10, relatively large (mean TL 45.3 <br />cm) bonytail was acquired from trammel net collections made on 17 March 2003 <br />and fitted with sonic transmitters (Table 1). A similar collection of nine, smaller <br />(mean TL 28.4 cm) bonytail was made 08 April 2004 and fitted with sonic <br />transmitters (Table 2). Fish were held in a floating live car for a brief time after <br />capture, then measured (total length [TL], nearest mm), weighed (2004 only, <br />nearest 2 gm), examined for sex and general health and condition, scanned for <br />presence of a Passive Integrated Transponder (PIT) tag, tagged if none was <br />present, and fitted with a sonic tag. <br />Sonic tags were purchased from Sonotronics, Inc., Tucson, Arizona. In 2003, <br />Model IBT-96-2 tags with a nominal 60-day life expectancy were attached with <br />black vinyl electrician's tape to a pair of appropriately sized cable ties, and these <br />in turn were affixed around the test fish caudal peduncle (see Mueller et al. <br />2003). Cable ties used on these fish were either solid or incorporated a "fusible <br />link" of metal wire that was designed to corrode in time and allow the cable ties <br />and tag to fall off. <br />Model SMT-01-376 tags with a nominal 14-day life expectancy were used on <br />smaller fish in 2004. Tags were affixed directly to the caudal peduncle using <br />black vinyl electrician's tape. One of 10 tags malfunctioned when it was initially <br />activated so only nine total fish were used. All fish were released near the south <br />end of the Cibola HLP and allowed to disperse without being disturbed. <br />Tracking. Tracking was done using one or a pair of DH-2 directional <br />hydrophones, an omni-directional hydrophone (DH-2 with shield removed), USR- <br />5W ultrasonic receiver, and either headphones or external speakers. Additional <br />shielding comprised of 6.4-mm neoprene sheeting was affixed to the directional <br />hydrophones to reduced interterence from extraneous signals and increase <br />directional specificity. Hydrophones were mounted on 2-m long x 12.7-mm <br />diameter PVC poles, and directional ones were fitted with a horizontally mounted <br />Suunto A-30L magnetic compass that was aligned with directional hydrophone <br />such that compass bearing of an incoming signal could be approximated within <br />about 30 degrees. <br />Tracking was performed utilizing three different protocols. First, to obtain <br />directional data, listening stations were established at two fixed sites, one at the <br />north end of the high levee, the other on a small spit at the northern end of the <br />river levee (see Fig 1). Nominally, simultaneous readings of fish number (pulse <br />code) and compass bearing were to be taken at 15-minute intervals at each site <br />beginning approximately at sunset and extending until dawn. Additional <br />directional readings were taken from each listening station but these were not <br />temporally synchronized so as to be simultaneous. Second, to acquire "point <br />2 <br />