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



689 



Tuna arrivals at Espiritu Santo North 



24 00 

 22 00 



02 00 



- 02 00 



-r? 00:00 



T — f M' ' 1 **■ I  " r— r-» r**- r"'^* h oooo 



:£ 15-Sep-98 16-Sep-98 17-Sef)-98 18-Sep-98 19-Sep-98 20-Sep-98 21-Sep-98 22-Sep-98 23-Sep-98 24-Sep-98 



V) 



E 



r (Order of ten symbols from left to right, indicate tunas witti tags 5, 8, 9, 16, 17, 19, 20,21,22 and 23) 



23-Sep-98 24-Sep-98 



25-Sep-98 



26-Sep-98 27-Sep-98 28-Sep-98 29-Sep-98 



30-Sep-98 



1-Oct-98 



Figure 3 



Twenty-four hour chronology of visits by 10 tagged tuna to the monitoring station ESN during 15 days from 16 to 30 September 

 1998. A unique symbol indicates the presence of a particular individual within the range of the monitor during a 15-min position. 

 Note the predominance of daytime visits during the first nine 24-h periods and then a progressive shift to an equal number of visits 

 during daytime and nighttime (see 28-30 Sept. 1998). 



Tuna repeatedly moved in and out of the monitor range 

 over many days or left for the duration of the study. Sixty 

 percent of all absences at ESN and 65 % of the absences 

 from ESS were for less than 1 hour. If these tunas were to 

 swim at a sustained rate of 0.5 m/s (see Magnuson, 1978), 

 they would not move more than 900 m out the reception 

 range of the monitors (60 min x 60 s x 0.5 m/s/2).This close 

 attachment to the seamount contrasts with the behavior of 

 tuna at FADs offshore of Hawaii. Tunas visited the FADs 

 there rarely and spent little time within the range of the 

 monitor before departing for a period of several weeks 

 (Klimley and Holloway, 1999). The present study suggests 

 that the Espiritu Santo Seamount is a substantial feeding 

 ground that can support a year-round resident population 

 of yellowfin tunas. However, other tunas may stay only 



briefly at the seamount, using it as a landmark, before 

 continuing on their nomadic migrations. 



Seamoimts have dipole magnetic fields associated with 

 them because of the antiparallel polarity of magnetite within 

 volcanic magma extruded during periods when the earth's 

 polarity was reversed (Parker et al., 1987). Furthermore, 

 maxima (ridges) and minima (valleys) in the magnetic field 

 often lead outward from seamounts due to the extrusion of 

 magma. Klimley (1993) proposed that hammerhead sharks 

 use these for guidance during their nocturnal migrations into 

 the surrounding water to forage. This physical property of the 

 sea floor, originating fair below where the fishes swim, could 

 provide a fixed reference (or waypoint) for yellowfin during 

 their migrations. This species of tuna has been shown to 

 sense distinct patterns in a magnetic field (Walker, 1984). 



