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Fishery Bulletin 105(3) 



enable this species to slow the cooling rate of the body. 

 Thermal inertia allows skipjack tuna to undertake 

 brief dives into cooler waters below the thermocline 

 to exploit deep prey resources or to escape predators 

 (Neill et al., 1976; Stevens and Neill, 1978). 



The objectives of this investigation were to eluci- 

 date vertical movement patterns of and habitat use by 

 skipjack tuna in the equatorial EPO. By examining 

 the behavioral and physiological constraints and envi- 

 ronmental variables that define skipjack tuna vertical 

 habitat, we can improve our ecological understanding 

 and provide useful data for inclusion in stock assess- 

 ments. 



Materials and methods 



Tag releases 



Thirty-three skipjack tuna were captured, tagged, and 

 released during 9-10 April 2004 in close proximity to 

 a Tropical Atmosphere-Ocean (TAO) mooring at 1°59'N 

 95°19"W in the equatorial EPO. The estimated depth of 

 the seafloor at the TAO mooring was 3091 m. Tagging 

 was conducted on the chartered FV Her Grace, a 17.7- 

 m pole-and-line fishing vessel. The tagged fish (tagged 

 from an aggregation estimated at 10 t of skipjack tuna), 

 remained associated with the tagging vessel as it drifted 

 away from the TAO mooring at 0520 h on 11 April 2004 

 until 0940 h and about 15 km west of the mooring, when 

 the aggregation dispersed as the vessel departed at 

 about at a cruising speed of 8 kt. 



The archival tags (ATs) used in this study were model 

 LTD_1100 (Lotek Wireless Inc., St. John's, Newfound- 

 land, Canada). The tag is a rectangular solid, measur- 

 ing 8 mm X 16 mmx27 mm, and weighing 5 g in air. 

 Information on how to report the recovery of the tag 

 and how to claim the associated reward (US$50) was 

 printed in Spanish on a label encased in the epoxy of 

 the tag. 



Depth and temperature data were stored in the mem- 

 ory of the ATs at 28-s intervals. At this sampling rate, 

 the memory of each AT (64 KB) was capable of storing 

 10.7 days of data. The maximum depth sensing was 

 1000 m (with a resolution of 0.4%, and an accuracy of 

 ±1%). The temperature sensing range was from -5° 

 to 35°C, with a resolution of 0.2°C and an accuracy of 

 better than 0.3°C. 



Skipjack tuna were captured with fishing rods and 

 reels (« = 18) and handline gear (72=9) equipped with 

 chrome jigs and barbless hooks during the night, and 

 with pole-and-line gear (/? = 6) and by chumming live 

 bait during the day. Each fish was lifted directly into 

 a padded aluminum cradle that was covered with wet 

 smooth vinyl. The fish were placed ventral side up, their 

 eyes were immediately covered with a wet synthetic 

 chamois, the hook was removed, and the condition of the 

 fish was determined. If the fish was in excellent condi- 

 tion (i.e., no damage to the eyes or gills and no signifi- 

 cant bleeding), the surgery required for implanting an 



AT was initiated. An incision about 2 cm long was made 

 with a sterile surgical scalpel blade in the abdominal 

 wall about 1/3 of the distance between the anus and 

 the base of the pelvic fins and about 2 cm to the left 

 of the centerline of the fish. Special care was taken to 

 cut through the dermis and only partially through the 

 muscle, but not into the peritoneal cavity. A gloved fin- 

 ger was inserted into the incision and forced through 

 the muscle into the peritoneal cavity. The tag, sterilized 

 in Betadine solution, was inserted through the incision 

 into the peritoneal cavity, and a 13-cm length of 64-kg 

 spectra line was tied to the tag that protruded outside. 

 The incision was closed with three staples, by using a 

 surgical staple gun. 



After implantation of the AT, each fish was also 

 tagged with one numbered 12.5-cm green plastic dart 

 tag (Hallprint, Victor Harbor, Australia), by using tubu- 

 lar stainless steel applicators. Tags were inserted into 

 the dorsal musculature with the barbed heads passing 

 between the pterygiophores below the base of the second 

 dorsal fin, from either side of the fish. 



The 33 skipjack tuna released with ATs were mea- 

 sured to the nearest centimeter (mean = 66.9 cm fork 

 length (FL), range: 53-73 cm). The fish were then 

 picked up by hand from the cradle and released back 

 into the ocean. The total time that fish were out of the 

 water was less than 1 min. All fish released with ATs 

 were observed to swim away from the vessel after re- 

 lease, and all appeared to be in good condition. 



Tag recoveries 



Six of 33 skipjack tuna released in 2004 were recaptured 

 (18.2%), but just five of the archival tags were recovered. 

 Data on the fish lengths, release and recapture dates 

 and locations, numbers of days at liberty, and linear 

 displacements of the six fish are given in Table 1. Recap- 

 ture locations ranged from 341 km to 2548 km and 123° 

 (southeast) to 260° (west) of the release location. Of the 

 six fish recaptured, four were initially caught by using 

 fi.shing rods and reels, one by using handline gear, and 

 one by using pole-and-line gear. 



Data processing 



Data were downloaded from the tags, and initial explor- 

 atory data analyses were conducted with software pro- 

 vided by the tag manufacturer. For the five skipjack 

 tuna, the behavior for each day at liberty was classified 

 as associated or unassociated with floating objects, on 

 the basis of behavioral characteristics from ultrasonic 

 telemetry observations of skipjack tuna associated with 

 floating objects (reported by Schaefer and Fuller [2005]). 

 For each day at liberty, surface-oriented and deep diving 

 behavior were also classified. Behavior associated with 

 floating objects was characterized by tunas remaining 

 primarily at depths of less than 50 m during the day. 

 Behavior unassociated with floating objects was defined 

 as the behavior of fish that made 10 or more dives to 

 depths greater than 150 m during the daytime within 



