156 



Fishery Bulletin 100(2) 



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Figure 1 



(A> Map of the east coast of the United States. The rectangle shows the area enlarged in panel B. 

 (B) Movements of five juvenile bluefin tuna tThunnun thynnui^). The limit of the continental shelf is 

 shown by the 50-, 100-, and 200-m isobath lines. The topographic features considered by local fisher- 

 men to aggregate juvenile bluefin tuna are shown by the shaded areas. Place names are taken from 

 local fishing charts. The rectangles show the areas enlarged in Figures 3 and 4. 



of the United States, from North Carohna to Rhode Island, 

 usually during June and July (Rivas, 1978; Sakagawa, 

 1975; koffer, 1987; Lucy et al.," 1990; Mather et al, 1995 1. 

 Their presence provides an opportunity for direct popu- 

 lation assessments with aerial surveys similar to those 

 conducted on adult Atlantic bluefin tuna (Lutcavage and 

 Ki'aus, 1995; Lutcavage et al., 1997), southern bluefin tu- 

 na tTluininis maccoyii).^ and other fish species (e.g. Lo et 

 al., 1992). Assessments of juvenile bluefin tuna abundance 

 are considered particularly crucial for effective stock man- 

 agement because these will allow the forecasting of re- 

 cruitment and long-term population trends iPolacheck et 

 al., 1996; Sissenwine et al., 1998). There is, however, a 

 need to establish the probability of detecting schools and 

 estimating school size before aerial survey data can pro- 

 vide robust population assessments. This need is present 

 regardless of whether the census techniques are simple 

 photography (Lutcavage and Kraus, 1995; Lutcavage et 

 al., 1997) or new laser-based digital remote sensing tech- 

 niques (Oliver et al., 1994; Lo et al., 1999). As with tra- 

 ditional CPUE-based abundance estimates, knowledge of 

 the effects of oceanographic conditions on depth distribu- 

 tion, surfacing frequency, travel speeds, and residence pat- 

 terns is critical because these conditions will affect vul- 

 nerability to "capture," either on photographic film or as 

 digital data. 



To obtain the necessary data, we undertook a study of 

 the horizontal and vertical movements of juvenile Atlan- 



Cowling, A., C. Millar, and T. Polacheck. 1996. Data analysis 

 of the aerial surveys (1991-19971 for juvenile southern bluefin 

 tuna in the Great Australian Bight. Rep. RMWS/96/4, 87 p. 

 Recruitment Monitoring Program, CSIRO Division of Marine 

 Research, GPO Box 1538, Hobart 7001, Australia. 



tic bluefin tuna using depth sensitive ultrasonic telemetry 

 devices. LHtrasonic telemetry is a proven technique for ac- 

 quiring the required precise and detailed data on the be- 

 haviors of pelagic fishes in relation to oceanographic condi- 

 tions (e.g. Holland et al., 1990; Dagorn et al., 1999, 2000a; 

 Lutcavage et al., 2000). Besides being useful for improv- 

 ing stock assessments (Brill and Lutcavage, 2001), the re- 

 sultant data can also help clarify basic ecological relation- 

 ships and provide inferences on physiological abilities and 

 species-specific behaviors (Carey, 1983; Brill. 1994; Brill et 

 al., 1999). 



Materials and methods 



Fishing operations were conducted from a 16-m commer- 

 cial fishing boat (FV Gruryipy) in the western North Atlan- 

 tic off the eastern shore of Virginia (Fig. lA) during June 

 and July 1998. Bluefin tuna were captured with standard 

 recreational trolling gear. The fish were brought aboard 

 with a plastic sling and detached from hooks. Straight 

 line fork length was measured, and a Vemco (Halifax, 

 Nova Scotia, Canada) ultrasonic transmitter (model V32) 

 was attached near the second dorsal fin with nylon straps 

 as described by Holland et al. ( 1986, 1990). The transmit- 

 ted signal was detected with a Vemco VR-60 ultrasonic 

 receiver connected to a directional hydrophone mounted 

 on the end of an aluminum pipe. The pipe was clamped 

 to the side of the vessel with a custom designed alumi- 

 num bracket that allowed the hydrophone to be rotated 

 to find the relative bearing to the transmitter Fish depth, 

 encoded by the interval of the transmitters pulsed signal, 

 was decoded by the receiver and the resultant digital data 

 recorded by an attached laptop computer. Geographic 



