Mar Biol (2007) 152:503–525 DOI 10.1007/s00227-007-0689-x RESEARCH ARTICLE Movements, behavior, and habitat utilization of yellowfin tuna (Thunnus albacares) in the northeastern Pacific Ocean, ascertained through archival tag data Kurt M. Schaefer Æ Daniel W. Fuller Æ Barbara A. Block Received: 23 August 2006 / Accepted: 28 March 2007 / Published online: 21 June 2007 Ó Springer-Verlag 2007 Abstract Sixty-eight yellowfin tuna, Thunnus albacares, resulted in discrimination of four distinct behaviors. When (60-135 cm fork length) were caught and released with exhibiting type-1 diving behavior (78.1% of all days at implanted archival tags offshore off Baja California, liberty) the fish remained at depths less than 50 m at night Mexico, during October 2002 and October 2003. Thirty-six and did not dive to depths greater than about 100 m during fish (53%) were recaptured and the data were downloaded the day. Type-2 diving behavior (21.2% of all days at from all 36 recovered tags. Time at liberty ranged from 9 to liberty) was characterized by ten or more dives in excess of 1,161 days, and the data were analyzed for the 20 fish that 150 m during the day. Type-2 diving behavior is apparently were at liberty for 154 or more days. The accuracy in the a foraging strategy for fish targeting prey organisms of the position estimates, derived from light-level longitude data deep-scattering layer during the day, following nighttime and sea-surface temperatures (SSTs) based latitude, is foraging within the mixed layer on the same prey. Yel- about 0.41° in longitude and 0.82° in latitude, in this lowfin tuna exhibited occasional deep-diving behavior, and region. The movement paths, derived from position esti- some dives exceeded 1,000 m, where ambient tempera- mates, for the 20 yellowfin indicated that 19 (95%) re- tures were less than 5°C. Surface-oriented behavior, de- mained within 1,445 km of their release locations. The fined as the time fish remained at depths less than 10 m for estimated mean velocity along movement paths was more than 10 min, were evaluated. The mean number and 77 km/day. The southern and northern seasonal movement duration of surface-oriented events per day for all fish was paths observed for yellowfin off Baja California are influ- 14.3 and 28.5 min, respectively. Habitat utilization of enced by the seasonal movements of the 18°C SST iso- yellowfin, presented as monthly composite horizontal and therm. Cyclical movements to and from suitable spawning vertical distributions, indicates confined geographical dis- habitat (‡24°C SST) was observed only for mature fish. For tributions, apparently resulting from an affinity to an area the 12 fish that demonstrated site fidelity, the mean 95 and of high prey availability. The vertical distributions indicate 50% utilization distributions were 258,730 km2 and 41,260 greater daytime depths in relation to a seasonally deeper km2, respectively. Evaluations of the timed depth records mixed layer and a greater proportion of daytime at shal- lower depths in relation to a seasonally shallower mixed layer. Communicated by P.W. Sammarco. K. M. Schaefer (&) Á D. W. Fuller Inter-American Tropical Tuna Commission, Introduction 8604 La Jolla Shores Drive, La Jolla, CA 92037-1508, USA e-mail: [email protected] Yellowfin tuna, Thunnus albacares, a large epipelagic oceanic species, occurs worldwide in subtropical and B. A. Block tropical seas (Collette and Nauen 1983). Yellowfin is the Tuna Research and Conservation Center, Hopkins Marine Station, Stanford University, principal target species of a large high-seas international 120 Oceanview Boulevard, Pacific Grove, CA 93950, USA fishery in the eastern Pacific Ocean (EPO), from which the 123 504 Mar Biol (2007) 152:503–525 annual catch in 2000 was nearly one-third the world Block 2005). Tagging studies using plastic dart tags or catches of yellowfin (Bayliff 2002). The annual catch of acoustic tags, and studies of longline catch records have yellowfin by all types of gear combined in the EPO has previously provided valuable information on these topics. averaged 297,000 mt (range 226,000–440,000 mt) from ATs record swimming depths, ambient and internal tem- 1985 to 2004 (Anonymous 2005). A recent stock assess- peratures, and ambient light levels. The light data can be ment for yellowfin in the EPO (Hoyle and Maunder 2006) processed using astronomical algorithms to provide daily indicates that the biomass decreased during 2002–2004 and estimates of longitudes and latitudes (Hill 1994; Hill and that the stock is likely below the level corresponding to the Braun 2001; Ekstrom 2004), and daily sea-surface tem- average maximum sustainable yield (AMSY), with recent peratures (SSTs) recorded by the tags matched to SSTs fishing mortality rates about 20% above those corre- from remote sensing can be used to significantly improve sponding to the AMSY. estimates of latitude (Teo et al. 2004; Clear et al. 2005; Tagging studies on yellowfin in the EPO have indicated Domeier et al. 2005; Nielsen et al. 2006). Current-gener- movements to be mostly restricted to less than 1,852 km ation ATs are capable of storing data for up to 5 years, (Fink and Bayliff 1970; Bayliff and Rothschild 1974; providing a unique opportunity to evaluate the influence of Bayliff 1979, 1984). Geographic variation observed in seasonal and annual environmental variability and onto- morphometrics and gill raker counts of yellowfin in the genetic changes on tuna movement patterns, behavior, and EPO results from restricted movements, limited mixing, habitat utilization. and environmental variation (Schaefer 1992). Life history The objectives of this investigation are to elucidate characteristics for yellowfin in the EPO, including age at movement patterns, behavior, and habitat utilization of size, growth, and reproductive biology, also indicate yellowfin in the northern region of the EPO, and to eval- geographic variation (Schaefer 1998; Schaefer 2001). In uate the influence of oceanographic factors on those char- addition, a recent genomic study utilizing microsatellite acteristics. This information has direct applications for variation has provided evidence of discrete northern and improving stock assessments of yellowfin in the EPO. southern populations of yellowfin in the EPO (Diaz-Jaimes and Uribe-Alcocer 2006). Yellowfin tuna possess both central and lateral counter- Materials and methods current heat exchangers, which provide the ability to con- serve metabolic heat and elevate their body temperatures Tag releases above that of ambient water temperatures (Carey 1973; Graham 1975; Dizon and Brill 1979). This enhanced Twenty-five yellowfin were captured, tagged, and released thermal inertia slows the body temperature cooling rate, during 12–13 October 2002 on ‘‘the ridge’’ (25.73 N providing the capability for the fish to undertake brief dives 113.13 W) about 111 km northwest of Magdalena Bay, into cooler waters below the thermocline to exploit deep and 43 during 9–16 October 2003 at Guadalupe Island prey resources or escape predators (Neill et al. 1976; Ste- (29.07 N 118.23 W), Alijos Rocks (24.97 N 115.76 W), vens and Neill 1978). and on ‘‘the ridge’’ (25.73 N 113.13 W) from about 37– Apparent depth distributions, temperature preferences, 111 km northwest of Magdalena Bay (Fig. 1a). Tagging and vertical movements of yellowfin in the Pacific, by size was conducted during regularly-scheduled 10-day trips on and time of day, have been reported based on studies using the FV Royal Star, a 28-m long-range sportfishing vessel. ultrasonic telemetry (Carey and Olson 1982; Holland et al. The ATs used in this study were model LTD_2310 1990; Block et al. 1997, 1999) and analyses of longline manufactured by Lotek Wireless Inc. (St John’s, NF, fishing records (Sund et al. 1981; Boggs 1992). Those Canada) (Lotek Wireless Inc. 2006). The tag body is studies have indicated vertical movements of yellowfin to cylindrical, measuring 16 mm in diameter and 70 mm in be predominantly restricted to depths of the mixed layer, length, and weighing 40 g in air. The tag is designed for but occasionally below the thermocline for short periods. implantation into the peritoneal cavity of the fish so that the Elucidating the physiological and behavioral constraints, sensor stalk protrudes outside the fish through an incision along with the environmental variables that define habitat in the abdominal wall. Information about reporting the for yellowfin, including depth and temperature distribu- recovery of the tag and the associated reward (US$500) tions, can improve stock assessments through standardi- was printed in English, Spanish, and Japanese on the 316 zation of purse-seine and longline catch and effort data stainless steel casing of the tag body. (Hinton and Nakano 1996; Brill and Lutcavage 2001). The depth, internal, and ambient temperatures, and light Geolocating archival tags (ATs) can vastly improve our level were stored in the memory of the tag at 1-min understanding of yellowfin movements, behavior, and intervals. At this sampling rate, the memory of each tag habitat (Arnold and Dewar 2001; Gunn and Block 2001; (8 MB) is capable of storing 979 days (2.7 years) of those 123 Mar Biol (2007) 152:503–525 505 resolution of 1 and 0.05% of full scale, respectively. The temperature sensing is a range of 0–30°C, with an accuracy and resolution of 0.1 and 0.05°C, respectively. The light sensing has nine decades of sensitivity, a resolution of 32 points/decade, and is capable of detecting light to a depth of 440 m (Schaefer and Fuller 2006). Yellowfin tuna were captured during the day on rod and reel with live sardines on circle hooks (sizes 1/0–7/0). Each fish was brailed from the water at the side of the vessel with a heavy-gauge aluminum rigid-framed net of knotless webbing and placed either in a tagging cradle ventral side up or on a foam pad covered with smooth vinyl, depending upon the size of the fish.