NOTE Klimley et al.: Occurrence of Thunnus albacares in the Gulf of California 



685 



115° 



110° 



Espiritu Santo Seamount (ES) 



Figure 1 



Bathymetric contour map of seamount Espiritu Santo (ES). The circles with 

 cross-hatching indicate the range of the tag-detecting monitor from the sea- 

 mount. The insert shows the geographic location of the seamount (ES) in the 

 Gulf of Cahfomia. 



We determined the maximum range of signal-detection 

 of one monitor by lowering a transmitter to 10 m under 

 a small boat and lowering the monitor to a similar depth 

 under a larger support vessel. We recorded the separation 

 distance between the two boats using radar because the 

 small boat and transmitter drifted away from the support 

 vessel that was anchored in place at the highest point on 

 the seamount. The VROl monitor (Vemco Ltd., Shad Bay, 

 Nova Scotia, Canada) detected tags at a distance of 150 m 

 in seas with waves <0.5 m high (see circles. Fig. 1). Later 

 models (Vemco Ltd., VR02) used in the study have a pub- 

 lished reception range of aSOO m in calm seas (see http:// 

 www. vemco.com). The range of tag detection by the moni- 

 tors decreases with rising sea state because of the increase 

 in wave-generated ambient noise. 



The tuna were caught by rod and reel and lifted aboard 1- 

 30 minutes after being hooked. Smaller individuals (^15 kg) 

 were weighed with a scale with a hook that fit into the oper- 



culum; intermediate sized fish (>15 and s25 kg) were 

 weighed in the net and the net's mass subtracted from the 

 cumulative value; and the masses of largest tuna (>25 kg) 

 were estimated on the basis of their length by using the re- 

 gression equation, >'=0.216ac + 2.981 given in Moore (1951). 

 The tags were inserted into the peritoneum of the tuna while 

 salt water was flushed over their gills by using the technique 

 described in Klimley and Holloway (1999). The tuna were 

 retained on board for tag implantation less than a minute. 

 The transmitters (Vemco Ltd., V16-6L) were cylindrical 

 and had a diameter of 16 mm, length of 106 mm, and net 

 mass in water of 16 g. They emitted individually coded 

 tone bursts of 70 kHz separated by 60-90 s intervals. The 

 amplitude of the pulses was 147 dB (re: 1 ^P) at a distance 

 of 1 m. The theoretical operating life of a transmitter was 

 476 days. Each tag was distinguished on the basis of a 

 unique pulse burst by an automated tag-detecting moni- 

 tor attached to the ESS and ESN detection stations. Water 



