684 



The occurrence of yellowfin tuna iThunnus albacares) 

 at Espiritu Santo Seamount in the Gulf of California 



A. Peter Klimley 



Salvador J. Jorgensen 



Bodega Marine Laboratory 



University of California, Davis 



Westside Road 



Bodega Bay, California 94923 



Present address (for A. P. Klimley): Department of Wildlife, Fisfi, and Conservation Biology 



University of California Davis 



Davis, California 95616 

 E-mail address (for A. P. Klimley): apklimley@ucdavis.edu, 



Arturo Muhlia-Melo 



Centre de Investigaciones Biologicas del Baja Norte 

 Apartado Postal 128 

 La Paz, Mexico 



later identifying them from these tags. 

 This method results in the removal of 

 individuals from the population and 

 yields a percentage of individuals 

 that have either left the area or have 

 been captured (Holland et al., 1999). 

 Detecting coded ultrasonic tags by an 

 automated monitor provides additional 

 information because marked individu- 

 als can be detected repeatedly over a 

 period of time. However, fewer tags can 

 be deployed because of their greater 

 cost. We used this method to reveal 

 synchronicity among visits of yellowfin 

 tuna, time of visits, and duration of 

 visits at the Espiritu Santo Seamount 

 in the Gulf of California. 



Methods 



Sallie C. Beavers 



Bodega Marine Laboratory 

 University of California, Davis 

 Westside Road 

 Bodega Bay, California 94923 



Pelagic fishes are not evenly dispersed 

 in the oceans, but aggregate at dis- 

 tinct locations in this vast and open 

 environment. Nomadic species such 

 as mackerels, tunas, and sharks form 

 assemblages at seamounts (Klimley 

 and Butler, 1988; Fontenau, 1991). 

 Fishermen have recognized this 

 behavior and have placed moorings 

 with surface buoys in deep waters to 

 provide artificial landmarks, around 

 which fish concentrate and are more 

 easily captured. These fish aggregating 

 devices (termed FADs) are common in 

 the tropical oceans (see review, Hol- 

 land, 1996). In a sense, it may only be 

 the larger size that separates a sea- 

 mount from a man-made FAD. 



Fish may aggregate at seamounts for 

 very different reasons. The opportunity 

 to feed is greater because biomass at all 

 trophic levels, from primary producer 

 to apex consumer, is greater than in 

 the open ocean (Boehlert and (Jenin, 

 1987). The disturbance of flow by the 

 seamount creates eddies downstream 

 that retain nutrients critical to the 

 growth of phytoplankton, and this 

 enrichment supports a greater abun- 



dance of consumers from zooplankton 

 to apex predators. The dipole nature of 

 seamount magnetic fields and the out- 

 ward radiating valleys and ridges of 

 magnetic minimums and maximums 

 might provide landmarks in oceanic 

 landscape that fish use as a reference to 

 guide migration (see discussion of mag- 

 netic "topotaxis" in Klimley, 1993). Yel- 

 lowfin (Thunnus albacares) and bigeye 

 (Thiinniis obesus) tunas do not reside 

 long at the Cross Seamount near Ha- 

 waii, an observation inconsistent with 

 the theory that tunas feed on prey that 

 remain aggregated at the site; rather 

 their rapid passage suggests that the 

 site is a landmark used to guide migra- 

 tions (Holland et al., 1999). Adult yel- 

 lowfin tuna also stay briefly (<5 min) at 

 FADs off Kaena Point, Oahu (Klimley 

 and Holloway 1999). 



Describing the degree of residency of 

 pelagic fishes at different geographic 

 locations helps ascertain whether the 

 affinity to seamounts and FADs is com- 

 mon throughout the oceans. Holland 

 et al. (1999) determined the rates of 

 dispersion of tuna by attaching unique 

 tags to individuals, releasing them, and 



We tagged 23 yellowfin tunas with 

 coded ultrasonic beacons during a 

 five-month period between 11 April 

 and 12 September 1998. They were 

 tagged <150 m from two monitoring 

 stations: Espiritu Santo North (ESN) 

 and South (ESS), separated by 500 

 m at the Espiritu Santo Seamount 

 (24°42'N; 110°18'W) in the south- 

 ern Gulf of California (Fig. 1). The 

 seamount rose to within 18 m of the 

 surface and extended 700 m along a 

 northwesterly-southwesterly axis. 

 Monitoring station ESN was situated 

 at the northwest end of the seamount 

 ridge at a depth of 47 m; station ESS 

 was at 37 m on the southwest end. 



The monitors were deployed for 30 

 months, during which they recorded 

 when the tagged tuna swam within 

 the 150-m range of reception of the 

 monitors. Using SCUBA, we removed 

 the monitors from the moorings at 

 four-month intervals, downloaded 

 the records of tuna presence near the 

 seamounts to a laptop computer, and 

 replaced the monitors during the same 

 day We located a station by the rosette 

 of buoys, which floated at a depth of 

 <10 m and which was visible from 

 the surface, by towing a diver at the 

 surface near the GPS coordinates for 

 the mooring. 



Manuscript approved for publication 

 30 January 2003 by Scientific Editor. 



Manuscript received 4 ApriL 2003 at NMFS 

 Sciontifu- Publications OfTicc. 



Fish. Bull. 101:684-692 (2003). 



