Introduction RESEARCH ARTICLE. K. M. Schaefer Æ D. W. Fuller

Transcription

1 Marine Biology (2005) 146: DOI /s x RESEARCH ARTICLE K. M. Schaefer Æ D. W. Fuller Behavior of bigeye (Thunnus obesus) and skipjack (Katsuwonus pelamis) tunas within aggregations associated with floating objects in the equatorial eastern Pacific Received: 2 December 2003 / Accepted: 14 September 2004 / Published online: 18 November 2004 Ó Springer-Verlag 2004 Abstract The horizontal and vertical movements of bigeye (Thunnus obesus Lowe, 1839) and skipjack (Katsuwonus pelamis Linnaeus, 1758) tunas within large multi-species aggregations associated with moored buoys or a drifting vessel were investigated, using ultrasonic telemetry and archival tags, along with sonar imaging, in the equatorial eastern Pacific Ocean (at 2 S 95 W and 2 N 95 W). Four sets of observations, each consisting of the concurrent monitoring of pairs of skipjack and/or bigeye with implanted acoustic or archival tags, were conducted in May 2002 and Ultrasonic telemetry data were not collected until 24 h or more after the fish were tagged and released, to avoid any abnormal behavior as a consequence of tagging. The pairs of acoustically tagged bigeye and skipjack, and also the entire aggregations, were primarily upcurrent of the moored buoy and downcurrent of the drifting vessel during the day. At night the aggregations were observed to be more diffuse, and the fish were feeding on organisms of the deep scattering layer. The aggregations returned to positions upcurrent of the buoy or downcurrent of the drifting vessel at dawn, commonly breezing at the surface within cohesive monospecific schools. The bigeye and skipjack had concurrent changes in depth records, occupying significantly greater mean depths at night than during the day, in most cases. When associated with a moored buoy, bigeye depth distributions were deeper during the day and night than those of skipjack, but bigeye depth distributions were Electronic Supplementary Material Supplementary material is available in the online version of this article at /s x Communicated by J.P. Grassle, New Brunswick K. M. Schaefer (&) Æ D. W. Fuller Inter-American Tropical Tuna Commission, 8604 La Jolla Shores Drive, La Jolla, CA , USA kschaefer@iattc.org Fax: shallower during the day and night than those of skipjack when associated with the drifting vessel. Simultaneous depth records of a large and a small bigeye with archival tags associated with a moored buoy also indicated diel changes in depth. The mean depth at night was significantly less than during the day for the larger bigeye, but the mean depth during the day was significantly less than during the night for the smaller bigeye. The mean depths during the day and night were significantly greater for the larger bigeye than the smaller. Introduction Many pelagic fish species of tropical and subtropical oceans associate with floating objects (Hunter and Mitchell 1966; Tsukagoe 1981; Freon and Dagorn 2000; Castro et al. 2002). Schools of tuna within large multi-species aggregations associated with moored and drifting fish-aggregating devices (FADs) have been commercially exploited since 1994 by tuna purse-seine vessels in the eastern Pacific Ocean (EPO) (Lennert- Cody and Hall 2000; Bayliff 2002). Over the past decade this practice has escalated between 5 N and 15 S, from the coast of the Americas to 150 W. This has resulted in a dramatic increase in the catch of bigeye (Thunnus obesus) by the EPO surface fishery, although the greatest component of the catch of the FAD fishery is skipjack (Katsuwonus pelamis) tuna (Bayliff 2002). In addition to the primary commercial tuna species, other species are also captured in sets on FADs, most of which are discarded at sea (Bayliff 2002). Justifiable concerns with FAD fishing include overexploitation of juvenile bigeye tuna and discards of unmarketable tunas and other species. This mode of fishing has also created problems for fisheries stock assessments, including deriving reasonable indices of species-specific catch per unit of effort (CPUE). This creates uncertainty in estimates of relative abundance

2 782 (Clark and Mangel 1979; Lauck 1996; Maunder and Harley 2002). This partially results from a lack of understanding of the characteristics and dynamics of these aggregations. Knowledge of the fine-scale spatial and temporal dynamics of tuna aggregations associated with moored and drifting FADs is needed to quantify several important aspects of the fishery, including size-specific species behavior and their vulnerability to fishing gear. Ultrasonic telemetry studies, utilizing depth-sensitive tags, can provide precise information on the fine-scale horizontal and vertical movements of free-swimming pelagic fish, with respect to their environment (Carey 1983; Arnold and Dewar 2001; Gunn and Block 2001). Such studies have provided some insights on the influence of anchored FADs on the behavior of skipjack (K. pelamis), yellowfin (Thunnus albacares), and bigeye (T. obesus) tunas (Holland et al. 1990; Cayré 1991; Klimley and Holloway 1999; Ohta et al. 2001). These studies were conducted around islands in relatively nearshore environments and have described the behavior of individuals, not of large multi-species aggregations. The vertical behaviors of bigeye tuna associated and unassociated with floating objects, based on archival tag datasets, have been validated in the paper by Schaefer and Fuller (2002). There are no published studies on the fine-scale behavior of large multi-species tuna aggregations associated with moored or drifting floating objects in areas of large-scale tuna purse-seine fisheries. The objectives of the present study were to evaluate the fine-scale spatial and temporal dynamics of bigeye and skipjack tunas concurrently within large multi-species aggregations associated with moored and drifting floating objects, hundreds of miles from land in the equatorial EPO, using electronic tags coupled with sonar imaging. Materials and methods Field studies were conducted in close proximity to two Tropical Atmosphere Ocean (TAO) moorings at 2 S; 95 W and 2 N; 95 W in the equatorial EPO, in May 2002 and 2003, respectively, utilizing the chartered M.V. Her Grace, a live-bait, pole-and-line vessel. The approximate depths of the seafloor at the 2 S and 2 N TAO moorings are 3438 and 3091 m, respectively (L. Stratton, personal communication). The ultrasonic telemetry studies consisted of passive monitoring of tunas within aggregations rather than active tracking of individual tunas. For the telemetry studies in 2002 and 2003, the vessel remained moored within 20 m of the buoy, with the main engine off, so as to not create acoustical interference or influence the behavior of the tuna aggregations (Appendix 1a of the Electronic Supplementary Material to this paper). The orientation of the drifting vessel to that of the tuna aggregation during the telemetry studies in 2003 is illustrated in Appendix 1b of the Electronic Supplementary Material to this paper. Telemetry was conducted using a Vemco VR-28 monitoring system, containing four separate receivers, in conjunction with a Vemco V-41 (28 50 khz) four-hydrophone 360 directional array mounted on a V-fin depressor. Incoming data were decoded and displayed on a portable computer connected to the receiver, using the proprietary Vemco software TRACK 28 (Vemco, ver. 2003, available from The software logs and displays data for the bearing, range, and depth of a tagged tuna. The computer was also connected to a Furuno differential global positioning system (DGPS). The V-fin with attached hydrophone array was deployed off the starboard side of the vessel utilizing the boom with a 22-cm block through which the hydrophone cable was suspended. The V-fin was fixed at a depth of 5 m and always pointed directly into the current of knots, which provided a constant bearing for the VR-28 tracking system. The VR-28 receiver was not capable of receiving simultaneous ultrasonic signals from multiple tags, so a 15-min rotation between each transmitter was adopted throughout to collect concurrent data sets for pairs of tunas. The TRACK 28 software stored depth data at 1-s intervals and geographical coordinates of the vessel every 10 s. The Vemco ultrasonic transmitters (model V16p: 50 khz, 1000 psi and model V22p: 34 khz, 1000 psi) were equipped with pressure sensors, and telemetered data on fish depth and direction, relative to that of the bow hydrophone, within the hydrophone array mounted on the V-fin. Trials were conducted with V22p and V16p tags, attached to a weighted line, at a depth of about 20 m towed completely around the vessel, behind a 2.4-m Avon inflatable boat with oars and no engine, to ensure the hydrophone array was at a depth sufficient to provide 360 unobstructed detection of the tags. Using the same technique, but with a radar reflector mounted on a vertical pole, it was determined that the maximum detection range of the V16p and V22p transmitters should be no more than 0.93 and 1.85 km, respectively. Bigeye (Thunnus obesus Lowe, 1839) and skipjack (Katsuwonus pelamis Linnaeus, 1758) tunas were also tagged with archival and/or conventional tags, and released at the 2 S and 2 N TAO moorings on the 95 W meridian, during these studies. Mk9 archival tags (Wildlife Computers, ver. 2003, available from were used. The Mk9 is designed for implantation into the peritoneal cavity of the fish, with the sensor stalk protruding outside the fish through an incision in the abdominal wall. The depth, internal and ambient temperatures, and light level were stored in the memory of the tags at 1-min intervals. A label, printed in Spanish, with information on tag recovery and the associated reward (US$ 500) was encased in the tag. Fish were also tagged with 12.5-cm yellow or green plastic dart tags (PDTs) (Hallprint, ver. 2003, available from using tubular stainless steel applicators. Tags were inserted into the dorsal musculature with the barbed heads

3 783 passing between the pterygiophores below the base of the second dorsal fin, from either side of the fish. Information for reporting fish recapture and a reward (US$ 5) was printed in Spanish on these tags. Bigeye and skipjack tunas were captured with fishing rods and reels and immediately placed in a padded aluminum cradle, covered with wetted smooth vinyl, in preparation for implantation of an ultrasonic or archival tag. The bigeye were most often scooped into a net, using a large aluminum-framed brail hung with knotless webbing, and then placed into the cradle, whereas skipjack were lifted directly into the cradle. The eyes of the fish were immediately covered with a wet synthetic chamois, the hook was removed, and the overall condition of the fish determined. If the fish had no damage to the eyes or gills and no significant bleeding, surgery was initiated. An incision of about 2 cm length was made in the abdominal wall, about 5 10 cm anterior to the anus and about 2 cm to the left of the centerline of the fish. Special care was taken to cut only through the dermis and partially through the muscle, but not into the peritoneal cavity. A gloved finger was then inserted into the incision and forced through the muscle into the peritoneal cavity. Vemco V-16P (50 khz) tags were inserted into skipjack and V-22P (34 khz) tags into bigeye tunas. The ultrasonic and archival tags, sterilized by soaking in Betadine solution, were inserted through the incision into the peritoneal cavity. Two sutures were used to close the incision, using a sterile needle and suture material [Ethicon (PDS II) size 0, cutting cp-1, 70 cm]. Bigeye tuna with archival tags also received green PDTs, but fish with acoustic tags did not. The fork lengths of fish were measured to the nearest centimeter, using marked graduations on the liner of the cradle. The fish were then released back into the sea. Three bigeye and two skipjack tunas received ultrasonic transmitters (henceforth referred to as acoustic tags) in 2002, and six bigeye and two skipjack received acoustic tags in Some of the fish were never detected with the VR-28 receiver, and others were detected infrequently, making them poor candidates for the study. The bigeye and skipjack tunas selected for ultrasonic monitoring during both years provided strong detection signals throughout the observation periods, remaining within the aggregations associated with the moored buoys (Table 1). Fish with acoustic tags were monitored and data collected with the VR-28 receiver for about 48 h in 2002 and for about 24 h in 2003, beginning 24 h or more after the fish had been tagged and released. Data files for the acoustic tags were filtered to remove outliers, such as depths greater than the rated depth of the tag s pressure sensor, and all negative data points. To provide objective filtering of the data, the length in meters of each fish was multiplied by 6.85, to estimate the maximum allowable swimming speed in meters per second. The filter then calculated the distance traveled (in meters) and the amount of time it took to travel that distance. If the calculated velocity between data points was determined to be greater than the maximum sustainable swimming speed for that fish, the data point was removed. The amount of data removed per fish ranged from 3.1% to 14.3%. The two archival tags from which data are presented (Table 1) were recovered from bigeye tunas recaptured by a purse-seine vessel during a set adjacent to the TAO buoy at 2 S; 95 W, on 19 April 2003, 15 days after release. During each set of telemetry observations the relative orientation and movements of the schools making up the aggregation were monitored through scanning sonar (Wesmar HD 600E-6 series, 160 khz) and echosounder (Furuno FCV 1000) imaging, from which selected images were recorded with a digital camera. The Wesmar HD 600E-6 was set at a range of 200 m with a tilt angle of 12. The Furuno FCV 1000 is a 50/200-kHz dual beam 1-kW echosounder. The settings used were 200 khz at 80 m depth. The sonar was used for observing the aggregation s bearing, range, size, and behavior. The echosounder was used to observe the aggregation s vertical distribution and behavior, identify presence or absence of bigeye tunas, and to monitor the diel vertical migrations of the deep scattering layer (DSL). In addition, a hard-wired Fisheye underwater Table 1 Thunnus obesus and Katsuwonus pelamis. Size, tagging, and tracking data for six bigeye and two skipjack tunas with implanted electronic tags Tuna (no.) Length (cm) Tag Association Deployment Duration of data collected (date/time) Location Date/Time 2002 Bigeye (1) 90 V22p Buoy 2 S; 95 W 5 May/1900 hours 7 May/1100 hours 9 May/1130 hours Skipjack (1) 61 V16p Buoy 2 S; 95 W 6 May/1000 hours 7 May/1100 hours 9 May/1130 hours 2003 Bigeye (2) 85 V22p Vessel 2 N; 95 W 12 May/1900 hours 14 May/0600 hours 15 May/0600 hours Skipjack (2) 67 V16p Vessel 2 N; 95 W 12 May/1600 hours 14 May/0600 hours 15 May/0600 hours Bigeye (3) 119 V22p Buoy then vessel 2 N; 95 W 12 May/2000 hours 15 May/2000 hours 16 May/2200 hours Bigeye (4) 67 V16p Buoy then vessel 2 N; 95 W 12 May/2015 hours 15 May/2000 hours 16 May/2200 hours Bigeye (5) 108 Mk9 Buoy 2 S; 95 W 4 April/2000 hours 4 April/2000 hours 19 April/1430 hours Bigeye (6) 59 Mk9 Buoy 2 S; 95 W 4 April/2000 hours 4 April/2000 hours 19 April/1430 hours

4 784 color video camera (Humminbird, ver. 2003, available from connected to a monitor and VCR recorder was suspended horizontally at a depth of 20 m under the vessel. The camera maintained a constant bearing into the current because of the large vertical fin fixed to the trailing edge of the camera housing. The video provided additional confirmation of species identification and behavior. Water temperature-depth profiles taken at 12-h intervals with a temperature-depth probe (model SBE 39) (Sea-Bird Electronics, ver. 2003, available from provided estimates of temperatures at average depths for each fish with an acoustic tag. Horizontal movements Bigeye 1 and skipjack 1 were monitored by ultrasonic telemetry for 48 h (Table 1), while associated with a moored buoy (Appendix 1a of the Electronic Supplementary Material to this paper). The first 24-h period of concurrent observations of the horizontal orientation of those fish, relative to the bow hydrophone mounted on the V-fin and the current direction, are presented in twelve 2-h directional plots (Fig. 1a l). Throughout this 24-h sequence, the directional orientations of bigeye 1 and skipjack 1 showed considerable correspondence in most of the 2-h intervals, with consistent orientation upcurrent of the vessel. Only in Fig. 1b was there a lack of correspondence in the proportion of time within the same directional sectors. Bigeye 1 was oriented between Results Data are presented for four independent sets of observations on pairs of tunas, a total of six bigeye (Thunnus obesus) and two skipjack (Katsuwonus pelamis). Fish length, type of electronic tag, association, date tag was implanted, and duration of data collection are provided (Table 1). Fig. 1a l Thunnus obesus and Katsuwonus pelamis. Twelve 2-h concurrent directional plots from ultrasonic telemetry of bigeye 1 (green) and skipjack 1 (black) within a large multi-species aggregation associated with a moored buoy. Panels a l show hourly intervals from 1100 hours, 7 May to 1100 hours, 8 May Dotted concentric rings represent proportion of time within a directional sector. Black arrow indicates direction of current; v-fin pointed into the current

5 785 0 and 45, whereas skipjack 1 was oriented between 75 and 105 to the bow hydrophone. During the night-time period of hours, the directional orientation of these two fish correspondingly shifted from that of the previous 10 h to about 90 to starboard. Preceding dusk ( hours) (Fig. 1d) and, more clearly, at predawn ( hours) (Fig. 1j) bigeye 1 and skipjack 1 oriented themselves in relatively tight formations, primarily between 0 and 15, directly upcurrent. Similar patterns were observed in the 2-h directional plots (not presented) for the following 24-h period for bigeye 1 and skipjack 1. Selected images to illustrate particularly distinct behaviors of the aggregation, as observed from the continuous sonar imaging conducted throughout the 48-h ultrasonic telemetry for bigeye 1 and skipjack 1, are presented (Fig. 2). The estimated size of this multispecies aggregation, consisting primarily of bigeye and skipjack, with some yellowfin tuna (Thunnus albacares) as well, based on the sonar imaging and visual observations, was estimated as >100 metric tons (mt). The aggregation of tunas was tightly clustered around and under the vessel in an apparent response to several large marlins attempting to feed on the tunas during the day at 1330 hours (Fig. 2a). The aggregation was tightly clustered and oriented upcurrent of the buoy at dusk, 1815 hours (Fig. 2b). During the early evening the aggregation ceased their upcurrent orientation to the moored buoy and spread out around the vessel, within about m from upcurrent to downcurrent, while actively feeding at the surface on organisms of the DSL (Fig. 2c, d). During predawn, from 0537 to 0544 hours, the aggregation of tunas ceased feeding as the DSL migrated downward, and the aggregation again began to orient upcurrent of the moored buoy (Fig. 2e, f). The distribution and orientation of the aggregation near dawn, from 0552 to 0605 hours, was tightly clustered upcurrent and within 50 m of the buoy. Adjacent, monospecific schools of skipjack and bigeye spread out near the surface exhibited some breezing behavior (Scott 1969) (Fig. 2g, h). Bigeye 2 and skipjack 2 were monitored by ultrasonic telemetry for 24 h (Table 1), while associated with the drifting vessel (Appendix 1b of the Electronic Supplementary Material to this paper). Concurrent observations of their orientation relative to the bow hydrophone on the V-fin and current direction are presented in twelve 2-h directional plots (Fig. 3a l). The directional orientations of bigeye 2 and skipjack 2 throughout this 24-h sequence were generally downcurrent. There was consistent directional separation between the bigeye and the skipjack, by about 40 throughout the day, as seen in the 2-h intervals from 0700 to 1900 hours (Fig. 3a f). Fig. 2a h Thunnus obesus and Katsuwonus pelamis. Images from a WESMAR HD 600 side-scan sonar (frequency 160 khz) of a large multi-species aggregation of bigeye and skipjack associated with a moored buoy, 7 9 May The center of each image is the position of the sonar dome on the hull of the vessel. Concentric rings represent 150-ft (46-m) increments. Black dot indicates position of buoy relative to the vessel. Each image was taken during the ultrasonic telemetry of bigeye 1 and skipjack 1. a Daytime aggregation, 1330 hours, 7 May; b dusk aggregation, 1815 hours, 7 May; c night aggregation, 1930 hours, 7 May; d night aggregation, 2100 hours, 8 May; e predawn aggregation, 0537 hours, 9 May; f predawn aggregation, 0544 hours, 9 May; g dawn aggregation, 0552 hours, 9 May; h dawn aggregation, 0605 hours, 9 May

6 786 Fig. 3a l Thunnus obesus and Katsuwonus pelamis. Twelve 2-h concurrent directional plots from ultrasonic telemetry of bigeye 2 (green) and skipjack 2 (black) within a large multi-species aggregation associated with drifting vessel. Panels a l show 12 consecutive 2-h intervals between 0700 hours, 14 May and 0700 hours, 15 May Dotted concentric rings represent proportion of time within a directional sector. Black arrow indicates direction of current; v-fin pointed directly downcurrent However, from dusk until dawn ( hours) (Fig. 3g l), the gap closed, and there was a relatively tight correspondence in the directional orientations of the bigeye and the skipjack. Bigeye 3 and bigeye 4 were monitored using ultrasonic telemetry for 26 h (Table 1), while in association with a moored buoy and then the drifting vessel. The concurrent observations of those tunas relative to the bow hydrophone and current direction are presented in twelve 2-h directional plots (Appendix 2, a l of the Electronic Supplementary Material to this paper). The directional orientations of bigeye 3 and bigeye 4 throughout this 24-h sequence showed considerable correspondence in most of the 2-h intervals. From 2200 to 0500 hours (Appendix 2a d of the Electronic Supplementary Material to this paper), these bigeye were oriented upcurrent of the vessel in association with the moored buoy. At 0500 hours, when the vessel began to drift away from the buoy with the large multi-species aggregation associated with it, the initial shift in orientation from upcurrent of the vessel and the buoy to downcurrent from the vessel was reflected in a bifurcation of the distributions of the bigeye in Appendix 2d of the Electronic Supplementary Material to this paper. The directional orientations of both bigeye from 0600 to 2200 hours (Appendix 2e l of the Electronic Supplementary Material to this paper) corresponded well in most of the 2-h intervals, with consistent orientation downcurrent of the vessel. Vertical movements The vertical movements of bigeye 1 and skipjack 1 while associated with a moored buoy are presented in Fig. 4. Both species remained primarily above 30 m depth and 20 C. There was a pronounced downward shift in the 20 C isotherm during the second 24-h period (Fig. 4c, d), but this had no noticeable effect on the behavior of either fish. Both fish showed an average range in depth of 20 m during each 15-min sampling interval, and minimum and maximum depths were comparable for both fish during day and night. There was an apparent concurrent diel change in the depth records of the two

7 787 Fig. 4a d Thunnus obesus and Katsuwonus pelamis. Concurrent depth records from ultrasonic telemetry for bigeye 1 and skipjack 1 within a large multi-species aggregation associated with a moored buoy, during a 48-h period in May 2002, divided into four 12-h windows. Grey panels indicate night time: a hours, 7 May; b hours, 8 May; c hours, 8 May; d hours, 9 May tunas, resulting from significantly greater mean depths at night than during the day (Tables 2, 3). The mean depths during day and night were significantly greater for bigeye 1 than skipjack 1 (Tables 2, 4). Skipjack 1 was located within the upper 5 m, near dawn on both days (Fig. 4b, d), within a tightly packed school of skipjack, with strong cohesion and synchronized sequential behaviors commonly referred to as breezer, black spot, and shiner (Scott 1969). The vertical movements of bigeye 2 and skipjack 2 throughout the 24-h period of ultrasonic telemetry (Table 1), while associated with the drifting vessel, are presented in Appendix 3 of the Electronic Supplementary Material to this paper. Both fish remained primarily above 30 m depth and 25 C, with an average range in their depths of only 10 m during each 15-min sampling interval. Minimum and maximum depths were comparable for both fish during the day, but the skipjack showed greater maximum depths than the bigeye at night (Appendix 3b of the Electronic Supplementary Material to this paper). Again there was an apparent concurrent diel change in depth for both tunas, resulting from significantly deeper mean depths at night than during the day (Tables 2, 3). However, in this association of the bigeye and skipjack with the drifting vessel, Table 2 Thunnus obesus and Katsuwonus pelamis. Depths (m) of bigeye and skipjack tunas by day and night Tuna (no.) Depth/Day (m) Depth/Night (m) Mean (±SD) Min. Max. Mean (±SD) Min. Max. Bigeye (1) 15.7 (5.9) (6.7) Skipjack (1) 9.7 (6.4) (6.5) Bigeye (2) 7.8 (4.3) (7.1) Skipjack (2) 10.6 (3.3) (5.1) Bigeye (3) 7.4 (7.9) (5.4) Bigeye (4) 11.1 (3.8) (5.5) Bigeye (5) 21.9 (36.4) (22.5) Bigeye (6) 8.7 (13.2) (11.4)

8 788 Table 3 Thunnus obesus and Katsuwonus pelamis. Statistical comparisons using analysis of variance of daytime versus nighttime depths for bigeye and skipjack tuna data in Tables 1 and 2 Tuna (no.) F P Bigeye (1) < Skipjack (1) < Bigeye (2) < Skipjack (2) < Bigeye (3) < Bigeye (4) < Bigeye (5) < Bigeye (6) < Table 4 Thunnus obesus and Katsuwonus pelamis. Statistical comparisons using analysis of variance of depths of bigeye and skipjack tunas by day and night, for data in Tables 1and 2 Comparison Period F P Bigeye (1) vs. skipjack (1) Day < Bigeye (1) vs. skipjack (1) Night < Bigeye (2) vs. skipjack (2) Day < Bigeye (2) vs. skipjack (2) Night < Bigeye (3) vs. bigeye (4) Day < Bigeye (3) vs. bigeye (4) Night < Bigeye (5) vs. bigeye (6) Day < Bigeye (5) vs. bigeye (6) Night < the mean depths during the day and night were significantly greater for skipjack 2 than for bigeye 2 (Tables 2, 4). The vertical movements of bigeye 3 and bigeye 4 throughout the 24-h period of ultrasonic telemetry (Table 1), while initially associated with a moored buoy (until 0600 hours) and then the drifting vessel, are presented in Appendix 4 of the Electronic Supplementary Material to this paper. The data were somewhat sporadic for the first 10 h of the concurrent observations until about 0800 hours. Both bigeye remained primarily above 20 m depth and 24 C, with an average range in their depths of about 15 m during each 15-min sampling interval. Minimum and maximum depths were comparable for both fish during the night, but the larger bigeye (no. 3) was at greater maximum depths during the day. There was no obvious diel change in the concurrent depth records for these two bigeye. The mean depth at night was significantly greater than during the day for bigeye 3, but the reverse was true for bigeye 4 (Tables 2, 3). Mean depths during the day and night were significantly greater for the smaller bigeye (no. 4) than for the larger bigeye (no. 3) (Tables 2, 4). The simultaneous vertical movements of a large bigeye (no. 5) and smaller bigeye (no. 6), while associated with a moored buoy for 7 days from 12 to 18 April 2003 (Table 1), based on the depth records from recovered archival tags are presented in Fig. 5. Both bigeye remained primarily above 30 m depth and 20 C. The smaller bigeye showed an average range in depth of <10 m, whereas the larger one showed an average range in depth of about 20 m, during each 15-min sampling interval. Minimum depths were comparable for both fish throughout this 7-d period, but the maximum daily depths of the larger bigeye exceeded those of the smaller one throughout the period. There was an apparent diel change in the concurrent depth records for both bigeye. For the smaller bigeye the pattern was similar to that previously presented from the ultrasonic telemetry studies, with significantly greater mean depths at night than during the day (Tables 2, 3; Fig. 5). However, for the larger bigeye, the pattern was reversed, with significantly greater mean depths during the day than at night (Tables 2, 3; Fig. 5; 12 April). In this evaluation of the archival tag data from two bigeye, the mean depths during the day and night were significantly greater for the larger bigeye than for the smaller one (Tables 2, 4). Simultaneous dives by both bigeye to depths of m were recorded on 5 days (14 18 April 2003), either close to or just before dusk (Fig. 5; 18 April). Discussion The vertical movements of the pairs of bigeye (Thunnus obesus) and skipjack (Katsuwonus pelamis) with implanted ultrasonic transmitters or archival tags coupled with echosounder imaging in the present study provided interesting intra- and inter-species comparisons. The vertical behaviors of both the bigeye and skipjack associated with the moored buoy and the drifting vessel showed concurrent diel changes in depth, resulting from significantly greater mean depths at night than during the day. The mean depths during the day and night were significantly greater for the bigeye than for the skipjack associated with the moored buoy, but the opposite was observed with the drifting vessel. A plausible explanation for the greater depths at night than during the day for all but two bigeye in this study is the nighttime vertical distribution of the DSL in conjunction with the feeding behavior of the tunas while associated with the moored buoys and the drifting vessel in the study area. Previous studies of the diel patterns of vertical movements of bigeye and yellowfin using ultrasonic telemetry (Holland et al. 1990; Cayre 1991; Block et al. 1997; Josse et al. 1998; Brill et al. 1999) indicated that the vertical depth distributions were significantly shallower at night than during the day. Skipjack also demonstrated shallower nighttime depth distributions in waters off Hawaii (Yuen 1970; Dizon et al. 1978) and Tahiti (Cayre and Chabanne 1986), from ultrasonic telemetry studies. Bigeye with archival tags, exhibiting behavior associated with drifting FADs, generally remain above the thermocline, showing diel shifts in depth distributions of, on average, about 6 m shallower at night than during the day (Schaefer and Fuller 2002). The association of the tuna aggregations with moored buoys and the drifting vessel provided the opportunity to delay the collection of behavioral data beyond the first 24 h following capture and tagging. Implantation of ultrasonic transmitters, h preceding collection of telemetry data, apparently precluded any abnormal

9 789 Fig. 5 Thunnus obesus. Simultaneous depth records from archival tags from a large bigeye (no. 5, blue) and a small bigeye (no. 6, red), within a large multi-species aggregation associated with a moored buoy, April Gray panels indicate night time behavior observed from these bigeye and skipjack specimens. This premise is based on observations following surgical implantations of archival tags in both captive and wild tunas (Schaefer and Fuller, unpublished data). The decision to surgically implant the transmitters in the peritoneal cavity, rather than attaching them externally or inserting them into the stomach, as previously employed in several ultrasonic telemetry studies on tunas (Arnold and Dewar 2001; Gunn and Block 2001), was based on recent studies with archival tags in bigeye tuna (Schaefer and Fuller 2002), the behavior of captive yellowfin tuna 24 h after surgical implantation of archival tags (Bayliff 2002), and studies of wild yellowfin (Klimley and Holloway 1999; Klimley et al. 2003) with ultrasonic transmitters implanted in their peritoneal cavities. Previous ultrasonic telemetry studies of tunas have either employed active tracking of individual tunas with a vessel under way or have used coded ultrasonic transmitters, along with automated listening monitors attached to buoys, which record presence or absence of the tagged tunas only within about 1 km of the buoy (Arnold and Dewar 2001; Gunn and Block 2001). Concerns about this approach include inability to determine whether the tuna is solitary or within a school, potential herding of the tuna by the vessel, and association of the tuna with the vessel (Dagorn et al. 2001; Davis and Stanley 2001). The accuracy in geolocation estimates for tunas with archival tags in the equatorial eastern Pacific (Schaefer and Fuller 2002) precludes the ability to determine whether the two bigeye tuna with archival tags in this study were in close proximity to the buoy for the period of time presented. Our determination that these two bigeye remained associated with the buoy for the duration of the study is based on the depth data from the archival tags. The bigeye tuna archival tag data presented in this study are indicative of fish exhibiting associative behavior with a floating object. If the fish had moved away from the buoy, they would have exhibited what is classified as unassociated type-1 behavior (Schaefer and Fuller 2002). Visual observations of the aggregations during the daytime, underwater observations with a video camera, and the sonar and echosounder imaging of the aggregations confirm that the acoustic and archival tag data are indicative of the behavior of tuna aggregations. The time-depth data in this study for bigeye associated with a moored buoy from the archival tags sampling at 1-min intervals are more informative than those obtained from acoustic tags sampling at 1-s intervals. The 1-min resolution provides more than adequate information on vertical distributions. The longer term data sets obtained from recovered archival tags show more dynamic behavior than those from 1- to 2-day data sets from ultrasonic telemetry. The archival tags provided simultaneous data on two different-sized bigeye, whereas the Vemco VR-28 receiver was capable of monitoring only one acoustic tag at a time. Furthermore, although the acoustic tags were transmitting at a 1-s sampling rate, the depth data captured by the VR-28 receiver contained a significant amount of erroneous data, which was filtered out. The archival tag data provide precise depth and temperature values. However, ultrasonic telemetry is required in order to obtain bearing and range of acoustically tagged individuals in conjunction with the sonar imaging in this type of investigation of fine-scale horizontal and vertical distributions. In a comparison of acoustic and archival tags, Boustany et al. (2001) showed that the 2-min sampling rate of their archival

10 790 tags was sufficient to accurately describe the behavior of Atlantic bluefin tuna. The tuna aggregations observed in this study fed at night near the surface on prey organisms of the DSL (Tont 1976; Kuznetsov et al. 1982). Recently ingested organisms of the DSL, consisting mostly of gonostomatids, myctophids, and cephalopods, in that order of occurrence, were present in stomachs of bigeye and skipjack examined from about 2000 to 0400 hours, whereas, throughout the day, most of the stomachs examined were relatively empty (Schaefer and Fuller, unpublished data). The prolonged residence times observed for those associated tuna aggregations appear to be related to the occurrence of suitable prey organisms of the DSL. In the equatorial EPO the gonostomatid Vinciguerria sp. is an extremely abundant mesopelagic species, common from the surface to about 100 m at night and descending to 500 m during the day (Blackburn 1968). Batalyants (1993) reported that tunas caught in association with FADs in the Atlantic Ocean had stomachs full of squid and mesopelagic fishes. However, stomach content analysis of FAD-associated bigeye tuna in the eastern Atlantic Ocean (Menard et al. 2000) showed that 83% of the stomachs were empty, whereas only 25% of the stomachs of bigeye tuna not associated with floating objects were empty. However, most studies of the feeding habits of FAD-associated and unassociated tuna schools are potentially biased, because most samples have come from commercial fishing vessels, which capture tunas primarily during the day. The apparent gastric evacuation rates by tunas of 6 8 h for cephalopods (Olson and Boggs 1986) and probably for small mesopelagic fishes could lead to the conclusion that FAD-associated tunas are not feeding, when they are actually gorging themselves on DSL organisms at night. Previous telemetry studies of the fine-scale behavior of tropical tunas have provided contrasting results on apparent residence times in association with floating objects. The telemetry studies of bigeye and yellowfin off Okinawa, Japan, indicated that 8 of the 11 tagged tunas remained around the FADs continuously for 5 24 days (Ohta et al. 2001). The study by Klimley and Holloway (1999) of the associations of yellowfin with moored FADs in Hawaiian waters showed very different results, as only a few of the tagged yellowfin repeatedly visited FADs a few times a day for short periods and some intermittently for about 9 months. In the study reported by Holland et al. (1990) for two bigeye and five yellowfin, following capture in association with moored FADs relatively near Oahu, Hawaii, the fish tended to stay close to the FADs during the day, moved away on short excursions around sunset, and returned the next day. The studies by Cayre (1991) of skipjack and yellowfin following capture in association with moored FADs in Comoros waters did not indicate any association of the skipjack with the FADs, although the yellowfin formed some short-term associations with the FADs. The directional orientations of the pair of bigeye 2 and skipjack 2 and two bigeye with acoustic tags showed general correspondence with the overall position of the tuna aggregations downcurrent of the drifting vessel and upcurrent of the buoy, as observed in There was some separation between the bigeye and skipjack schools throughout the day, but at night, when the tunas were observed to be feeding on organisms of the DSL, the schools were mixed and showed greater similarities. Although the behavior of multi-species tuna aggregations associated with a drifting vessel may be similar to that with a drifting FAD, that may not be the case. During an IATTC tuna tagging cruise in March May 2000 in the equatorial EPO, we observed large multispecies tuna aggregations associated with drifting FADs. Skipjack schools associated with the FADs at night would usually move away shortly after dawn, sometimes up to several kilometers, to either return the next night or not at all. The associated bigeye schools would typically remain within about 2 km of the drifting FAD throughout the day and cluster tightly under the FADs at night with the other tunas or disappear, commonly not remaining for more than about 2 days at a drifting FAD. Tsukagoe (1981) reported that the association of skipjack schools with drifting objects should be considered temporary, as the schools usually remain with the object for only a few days. The mean residence time reported for bigeye associated with floating objects in the equatorial EPO, ascertained from evaluations of time-depth records from archival tags, was 3.1 days (Schaefer and Fuller 2002). The residence times of large multi-species tuna aggregations associated with drifting FADs appear to be of much shorter duration than those observed at moored buoys in the present study. The tuna aggregations associated with both the moored buoys and the drifting vessel in the present study were often observed breezing near dawn. The significance of this commonly observed behavior is not known. However, there was some horizontal separation into monospecific breezing schools of bigeye and skipjack in association with these floating objects near dawn. This is possibly a species-specific segregative behavior commonly exhibited in large multi-species aggregations associated with floating objects. It is not uncommon for the skipjack schools to subsequently move away from drifting objects within a few hours following this breezing behavior at dawn. An interesting behavior of bigeye deduced from our archival tag data and those of Schaefer and Fuller (2002) is the tendency for relatively deep excursions of up to a few hours preceding dusk, while associated with floating objects. We presume that this behavior is indicative of bigeye diving to forage for prey in association with the DSL, rather than waiting for the DSL to ascend to the surface. Ultrasonic telemetry studies have shown that the diel vertical migrations of bigeye not associated with floating objects are closely associated with the vertical movements of organisms of the DSL (Josse et al. 1998).

11 791 Understanding the recruitment and residence dynamics of tuna species that form large multi-species aggregations near drifting FADs is important. Fishing for tropical tunas associated with FADs by purse-seine vessels has become extremely efficient, and fishing techniques are continually evolving to further increase efficiency. There is a need to elucidate the diel differences in the behavior of bigeye and skipjack within large multispecies aggregations associated with drifting FADs. Such research may reveal horizontal and/or vertical separation between monospecific schools of these species, possibly providing a way of reducing the purseseine catch of bigeye while still capturing the schools of skipjack associated with drifting FADs. Acknowledgements We greatly appreciate the assistance by Captain B. Blocker and the crew aboard Her Grace while conducting these field studies. We thank B. Bayliff, S. Harley, and two anonymous reviewers for their comments on drafts of the manuscript. This research was part of the Inter-American Tropical Tuna Commission s bigeye tuna tagging program, made possible through financial support provided by the Japan Fisheries Agency. These studies were conducted on the high-seas, outside the 200 nm EEZ of any country; regardless, the experimental procedures complied with the current laws of the United States. References Arnold G, Dewar H (2001) Electronic tags in marine fisheries research: a 30-year perspective. In: Sibert JR, Nielsen JL (eds) Electronic tagging and tracking in marine fisheries. Kluwer, Dordrecht, The Netherlands, pp 7 64 Batalyants KY (1993) On the hypothesis of comfortability stipulation of tuna association with natural and artificial floating objects. ICCAT Coll Doc Sci 40: Bayliff WH (ed) (2002) Annual report for Inter-American Tropical Tuna Commission, La Jolla, Calif., USA Blackburn M (1968) Micronekton of the eastern tropical Pacific Ocean: family composition, distribution, abundance and relations to tuna. Fish Bull (Wash DC) 67: Block BA, Keen JE, Castillo B, Dewar H, Freund EV, Marccinek DJ, Brill RW, Farwell C (1997) Environmental preferences of yellowfin tuna (Thunnus albacares) at the northern extent of its range. Mar Biol 130: Boustany AM, Marcinek DJ, Keen J, Dewar H, Block BB (2001) Movements and temperature preferences of Atlantic bluefin tuna (Thunnus thynnus) off North Carolina: a comparison of acoustic, archival and pop-up satellite tags. In: Sibert JR, Nielsen JL (eds) Electronic tagging and tracking in marine fisheries. Kluwer, Dordrecht, The Netherlands, pp Brill RW, Block BA, Boggs CH, Bigelow KA, Freund EV, Marcinek DJ (1999) Horizontal movements and depth distribution of large adult yellowfin tuna (Thunnus albacares) near the Hawaiian Islands, recorded using ultrasonic telemetry: implications for the physiological ecology of pelagic fishes. Mar Biol 133: Carey FG (1983) Experiments with free-swimming fish. In: Brewer PG (ed) Oceanography the present and future. Springer, New York Berlin Heidelberg, pp Castro JJ, Santiago JA, Santana-Ortega AT (2002) A general theory on fish aggregation to floating objects: an alternative to the meeting point hypothesis. Rev Fish Biol Fish 11: Cayré P (1991) Behavior of yellowfin tuna (Thunnus albacares) and skipjack tuna (Katsuwonus pelamis) around fish aggregating devices (FADs) in the Comoros Islands as determined by ultrasonic tagging. Aquat Living Resour 4:1 12 Cayré P, Chabanne J (1986) Marquage acoustique et comportement de thons tropicaux (albacore: Thunnus albacares, et listao: Katsuwonus pelamis) au voisinage d un dispositif concentrateur de poisons. Oceanogr Trop 21: Clark CW, Mangel M (1979) Aggregation and fishery dynamics: a theoretical study of schooling and the purse seine tuna fisheries. Fish Bull (Wash DC) 77: Dagorn L, Josse E, Bach P (2001) Yellowfin tuna (Thunnus albacares) associated with tracking vessels during ultrasonic telemetry experiments: contribution to the tuna/floating objects issue. Fish Bull (Wash DC) 99:40 48 Davis TLO, Stanley CA (2001) Vertical and horizontal movements of southern bluefin tuna (Thunnus maccoyii) in the Great Australian Bight observed with ultrasonic telemetry. Fish Bull (Wash DC) 100: Dizon AE, Brill RW, Yuen HSH (1978) Correlations between environment, physiology, and activity and the effects on thermoregulation in skipjack tuna. In: Sharp GD, Dizon A (eds) The physiological ecology of tunas. Academic, New York, pp Freon P, Dagorn L (2000) Review of fish associative behavior: toward a generalization of the meeting point hypothesis. Rev Fish Biol Fish 10: Gunn J, Block B (2001) Advances in acoustic, archival, and satellite tagging of tunas. In: Block BA, Stevens ED (eds) Tunas: ecological physiology and evolution. Academic, San Diego, pp Holland KN, Brill RW, Chang RKC (1990) Horizontal and vertical movements of yellowfin and bigeye tuna associated with fish aggregating devices. Fish Bull (Wash DC) 88: Hunter JR, Mitchell CT (1966) Association of fishes with flotsam in the offshore waters of central America. Fish Bull (Wash DC) 66:13 29 Josse E, Bach P, Dagorn L (1998) Simultaneous observations of tuna movements and their prey by sonic tracking and acoustic surveys. Hydrobiologia 371/372:61 69 Klimley AP, Holloway CF (1999) School fidelity and homing synchronicity of yellowfin tuna, Thunnus albacares. Mar Biol 133: Klimley AP, Jørgensen S, Muhlia-Melo A, Beavers S (2003) The occurrence of yellowfin tuna (Thunnus albacares) at Espiritu Santo seamount in the Gulf of California. Fish Bull (Wash DC) 101: Kuznetsov IL, Stefanov SR, Savagov VI (1982) A migrating sound scattering layer in the equatorial Pacific Ocean. Oceanology 22: Lauck T (1996) Uncertainty in fisheries management. In: Gordon DV, Munro GR (eds) Fisheries and uncertainty: a precautionary approach to resource management. University of Calgary Press, Calgary, pp Lennert-Cody CE, Hall MA (2000) The development of the purse seine fishery on drifting fish aggregating devices in the eastern Pacific Ocean: In: Le Gall JY, Cayre P, Taquet M (eds) Pêche Thonie` re et Dispositifs de Concentration de Poissons, Colloque Caraïbe-Martinique, vol 28. Inst Fran Recherche Exploitation Mer (IFREMER), Plouzane, pp Maunder MN, Harley SJ (2002) Status of bigeye tuna in the eastern Pacific Ocean in 2001 and outlook for Stock Assessment Report 3, Inter-American Tropical Tuna Commission, La Jolla, Calif., USA, pp Menard F, Stequert B, Rubin A, Herrera M, Marchal E (2000) Food consumption of tuna in the equatorial Atlantic Ocean: FAD-associated versus unassociated schools. Aquat Living Res 13: Ohta I, Shinichiro K, Kiyoaki K (2001) Aggregating behavior of yellowfin and bigeye tuna tagged with coded ultrasonic transmitters around FADs in Okinawa, Japan. In: Sibert JR, Nielsen JL (eds) Electronic tagging and tracking in marine fisheries. Kluwer, Dordrecht, the Netherlands, pp Olson RJ, Boggs CH (1986) Apex predation by yellowfin tuna (Thunnus albacares): independent estimates from gastric evac-

12 792 uation and stomach contents, bioenergetics, and cesium concentrations. Can J Fish Aquat Sci 43: Schaefer KM, Fuller DW (2002) Movements, behavior, and habitat selection of bigeye tuna (Thunnus obesus) in the eastern equatorial Pacific, ascertained through archival tags. Fish Bull (Wash DC) 100: Scott JM (1969) Tuna schooling terminology. Calif Fish Game 55: Tsukagoe T (1981) Fishing skipjack tuna schools associated with shoals and drifting objects. Suisan Sekai 30: NOAA NMFS Translation no. 83 by Tamio Otsu Tont SA (1976) Deep scattering layers: patterns in the Pacific. Calif Coop Ocean Fish Investig Rep 18: Yuen HSH (1970) Behavior of skipjack tuna, Katsuwonus pelamis, as determined by tracking with ultrasonic devices. J Fish Res Board Can 27:

13 Electronic appendices Appendix 1 Thunnus obesus and Katsuwonus pelamis. Orientation of aggregations: a associated with the moored buoy and b the drifting vessel. The current direction is indicated by the arrows. Triangles indicate direction of V-fin and bow hydrophone Appendix 2 Thunnus obesus and Katsuwonus pelamis. Twelve 2-h concurrent directional plots from ultrasonic telemetry of large bigeye no. 3 (blue) and small bigeye no. 4 (red) within a large multi-species aggregation associated with a moored buoy from 20:00 hrs 15 May until 05:00 hrs 16 May 2003 when vessel began to drift away from moored buoy. A large proportion of the aggregation, including bigeye 5 and 6, were then associated with the drifting vessel. Panels a-d show four consecutive 2-h intervals between 22:00 hrs 15 May and 06:00 hrs 16 May 2003 when v-fin was pointing into the current. Panels e-l show 8 consecutive 2-h intervals between 06:00 hrs and 22:00 hrs 16 May when v-fin was pointing down current. Dotted concentric rings represent proportion of time within a directional sector. Black arrow indicates direction of current Appendix 3 Thunnus obesus and Katsuwonus pelamis. Concurrent depth records from ultrasonic telemetry for bigeye 2 and skipjack 2 within a large multi-species aggregation associated with a drifting vessel in May 2003 over a 24-h period, displayed in two 12-h windows. Grey panels indicate night time. a 06:00-17:59 hrs 14 May b 18:00 hrs 14 May to 06:00 hrs 15 May Appendix 4 Thunnus obesus. Concurrent depth records from ultrasonic telemetry for large bigeye 3 (blue) and small bigeye no 4 (red) within a large multi-species

14 aggregation associated with a moored buoy from 20:00 hrs 15 May until 06:00 hrs 16 May 2003, and thereafter associated with the drifting vessel for the remainder of the 24-h period, displayed in two 12-h windows. Grey panels indicate night time. a 22:00 hrs 15 May to 09:59 hrs 16 May b 10:00-22:00 hrs 16 May

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