Introduction
The main catches of yellowfin tuna (YFT) (Thunnus albacares) in the eastern Pacific Ocean (EPO) are obtained by fisheries that use purse-seine fishing gear. In the YFT fishery, the Mexican tuna fleet lands the largest annual catch volumes (IATTC 2018). The Mexican fleet mainly detects and captures YFT in the EPO when the tuna are associated with dolphins, are found as free schools of fish, and are associated with floating objects, although catch volumes are low in the case of the latter (Aldana 2000, Dreyfus 2008, IATTC 2016). YFT movements in the EPO have been of interest to the Inter-American Tropical Tuna Commission (IATTC); their research began in 1950 with a fish tagging program that involved the use of conventional tags to study the movements of YFT of different sizes (Schaefer et al. 1963). Recently, tagging studies have been conducted using archival tags to analyze the horizontal and vertical movements of YFT and other tuna species. Bigeye tuna (Thunnus obesus) reportedly prefer swimming at shallow depths (0-25 m) at night and deeper depths (200-350 m) during the day (Schaefer and Fuller 2010). By contrast, the vertical movement of YFT is characterized by extended periods spent at a depth of 100 m both during the day and at night, and brief visits to depths of 150-250 m and occasionally 500 m (Schaefer et al. 2007, 2011, 2014).
The YFT is an epipelagic species that inhabits tropical and subtropical waters (Collette and Nauen 1983), with physiological adaptations to maintain its internal temperature above water temperature (Graham 1975). Internal temperature regulation rates in different fish size groups and the ability to remain at deeper depths with minimal oxygen concentrations determine the frequency and dwell time of YFT in deep zones (Brill et al. 1998, Schaefer et al. 2011). This behavior was observed in tuna from age groups I and II, which spent a higher percentage of time (64.5-84.5%) at depths of less than 100 m than fish from age groups III, IV, and V, which spent a lower percentage of time in the first 100 m (43-56 %) and showed higher tolerance to lower temperatures and oxygen concentrations (Schaefer et al. 2011, Schaefer et al. 2014).
The EPO is characterized by a shallow oxygen minimum zone and by an anoxic water layer below the thermocline. The depth of the thermocline decreases towards the coast, but the maximum depth in the area from 120 ºW to the coast does not exceed an average depth of 100 m (Fiedler and Lynne 2006, Hinton 2015, Portela et al. 2016). The physiological adaptations of YFT allow them to regularly descend to deeper waters in order to feed, but they remain there for short periods because of the synergistic effects of low ambient temperatures and minimal oxygen concentrations (Schaefer et al. 2014, Hinton 2015). The objective of this study was to determine the depth preferences, during day and night hours, of YFT from different age groups and tagged with archival tags in the Revillagigedo Archipelago Biosphere Reserve, a protected natural area.
Materials and methods
This study used data from the project “Tagging of Yellowfin Tuna (Thunnus albacares) in the protected natural area of the Revillagigedo Archipelago Biosphere Reserve, Mexico,” carried out jointly by IATTC and the National Fisheries and Aquaculture Institute (Mexico) during the 2006-2011 period. YFT were captured, tagged, and released in waters near the 4 islands that compose the Revillagigedo Archipelago Biosphere Reserve (DOF 1994): San Benedicto, Socorro, Roca Partida, and Clarión. The sport fishing vessel M/B Royal Star (US flag) was used for fish catching and tagging. Each fishing trip involved an average of 18 crew members (captain, crew, and scientists), who carried out catching and tagging activities. The fishing trips were conducted annually in February, with a duration of 9 effective days of fishing. The 2 models of archival tags used, LTD_2310 and Mk9, were manufactured by Lotek Wireless (Ontario, Canada) and Wildlife Computers (Redmond, Washington, USA), respectively. The characteristics of archival tags and the tagging procedure for YFT were previously described by Schaefer et al. (2007) and Schaefer and Fuller (2016). Fork length (in centimeters) of each tagged was recorded. Fish age was calculated in years and days using the obtained fork length and the growth equation outlined by Wild (1986). The total number of days of freedom was calculated as the number of days that were added to the number of years of the age group in which the YFT was classified at the time of its release.
To determine the vertical circadian habitat preferences of YFT by age group and time of day, depths were divided into the following strata: 0-25 m, 26-50 m, 51-75 m, 76-100 m, 101-125 m, 126-150 m, 151-175 m, 176-200 m, 200-500 m, and >500 m. The number of records collected per hour in each stratum was counted by age group. The hours were grouped into hours of the day (6:00 to 18:00) and night (19:00 to 5:00), depending on the activity of the Mexican fleet (DOF 1999, DOF 2014). A “deep zone” was defined as a zone that was deeper than 50 m, which is the average depth of the mixed layer (Portela et al. 2016). The ambient temperatures were used to describe the prevailing environmental conditions at different depths.
Dwell time of YFT by hour and age group at the different depth strata was analyzed by means of the type III Wald χ2 test (Montgomery 1991, Lyman 1993, Quinn and Keough 2002).
Results
A total of 68 archival tags were analyzed out of 88 tags recovered between 2006 and 2012. Seven tags recovered by a recreational fleet were discarded, as well as all tags recovered in less than 30 d after the fish were released and tags with corrupted data. Of the 68 tags, 38 were recovered from YFT tagged on Clarion Island, 10 from YFT tagged on Roca Partida Island, 16 from YFT tagged on Socorro Island, and 4 from YFT tagged on San Benedicto Island. The number of days after the release varied from 32 to 1,139, with those extremes corresponding to one individual tagged on Clarion Island and another on San Benedict Island, respectively. The age groups with the highest representation were, in order of importance, II, I, and III, with 44, 13, and 11 specimens, respectively. The fish were recaptured by the purse-seine fleet, being 41 from dolphin sets and 27 from school sets (Fig. 1, Table 1).
Release | Recapture | Age | Days | |||||||||||||||
Type of tag | Tag no. | FL (cm) | Date | Latitude (N) | Longitude (W) | Date | Latitude (N) | Longitude (W) | Year | Days | At liberty | Analyzed | Set | |||||
Clarion | ||||||||||||||||||
LTD2310 | D0604 | 102 | 19 Feb 06 | 18.32 | 114.75 | 21 May 07 | 18.58 | 114.70 | II | 845 | 456 | 258 | US | |||||
LTD2310 | D0098 | 117 | 20 Feb 06 | 18.34 | 114.69 | 7 Jun 07 | 16.98 | 114.30 | II | 988 | 472 | 115 | D | |||||
LTD2310 | D0742 | 110 | 20 Feb 06 | 18.34 | 114.69 | 24 Jul 06 | 20.77 | 114.83 | II | 920 | 154 | 154 | D | |||||
LTD2310 | D0755 | 113 | 20 Feb 06 | 18.34 | 114.69 | 4 Jun 07 | 18.33 | 114.83 | II | 949 | 469 | 154 | D | |||||
LTD2310 | D0622 | 113 | 20 Feb 06 | 18.34 | 114.69 | 14 Nov 07 | 18.40 | 114.67 | II | 949 | 632 | 154 | US | |||||
LTD2310 | D0065 | 129 | 20 Feb 06 | 18.34 | 114.69 | 19 May 06 | 19.48 | 113.35 | III | 1,115 | 88 | 88 | US | |||||
LTD2310 | D0606 | 140 | 20 Feb 06 | 18.34 | 114.69 | 18 Nov 07 | 18.43 | 114.43 | III | 1,247 | 636 | 223 | US | |||||
LTD2310 | D0741 | 102 | 20 Feb 06 | 18.34 | 114.69 | 14 Mar 07 | 18.62 | 114.88 | II | 845 | 387 | 258 | D | |||||
LTD2310 | D0729 | 97 | 20 Feb 06 | 18.34 | 114.69 | 9 Apr 06 | 18.30 | 114.97 | II | 799 | 48 | 48 | US | |||||
LTD2310 | D0744 | 139 | 20 Feb 06 | 18.34 | 114.69 | 19 Mar 07 | 16.85 | 117.50 | III | 1,235 | 392 | 235 | US | |||||
LTD2310 | D0632 | 92 | 21 Feb 06 | 18.33 | 114.70 | 14 Mar 07 | 18.62 | 114.88 | II | 754 | 386 | 236 | D | |||||
LTD2310 | C0174 | 117 | 22 Feb 07 | 18.32 | 114.72 | 21 May 07 | 18.58 | 114.70 | II | 988 | 87 | 87 | D | |||||
LTD2310 | C0908 | 147 | 22 Feb 07 | 18.32 | 114.72 | 18 Nov 07 | 18.43 | 114.43 | III | 1,344 | 269 | 126 | US | |||||
LTD2310 | D3412 | 108.5 | 20 Feb 09 | 18.31 | 114.73 | 11 Oct 09 | 09.90 | 99.25 | II | 901 | 233 | 203 | D | |||||
Mk9 | 0490916 | 140 | 23 Feb 07 | 18.32 | 114.72 | 13 Nov 07 | 18.50 | 114.08 | III | 1,247 | 264 | 223 | D | |||||
LTD2310 | D3051 | 74 | 17 Feb 08 | 18.32 | 114.62 | 12 May 08 | 18.13 | 114.67 | I | 595 | 85 | 85 | US | |||||
LTD2310 | D3055 | 76 | 17 Feb 08 | 18.32 | 114.62 | 8 Apr 08 | 18.33 | 114.67 | I | 613 | 51 | 51 | US | |||||
LTD2310 | D3099 | 122 | 18 Feb 08 | 18.32 | 114.62 | 24 May 08 | 18.50 | 114.65 | II | 1,039 | 96 | 64 | US | |||||
LTD2310 | D3063 | 120 | 18 Feb 08 | 18.32 | 114.62 | 24 Feb 10 | 11.00 | 98.05 | II | 1,019 | 737 | 84 | D | |||||
LTD2310 | D3050 | 120 | 19 Feb 08 | 18.32 | 114.67 | 23 May 08 | 18.38 | 114.65 | II | 1,019 | 94 | 84 | US | |||||
LTD2310 | D1477 | 79 | 19 Feb 08 | 18.32 | 114.67 | 24 May 08 | 18.50 | 114.65 | I | 640 | 95 | 95 | US | |||||
LTD2310 | D1474 | 118 | 20 Feb 08 | 18.32 | 114.62 | 23 May 08 | 18.38 | 114.65 | II | 998 | 93 | 93 | US | |||||
LTD2310 | D3412 | 61 | 22 Apr 08 | 18.35 | 114.68 | 24 May 08 | 18.53 | 114.60 | I | 478 | 32 | 32 | US | |||||
LTD2310 | D2050 | 117 | 22 Feb 09 | 18.32 | 114.73 | 6 Oct 09 | 14.72 | 115.37 | II | 988 | 226 | 115 | D | |||||
Mk9 | 1090003 | 111 | 15 Feb 10 | 18.33 | 114.75 | 13 Sep 10 | 17.92 | 112.70 | II | 929 | 210 | 174 | D | |||||
Mk9 | 1090024 | 123 | 15 Feb 10 | 18.33 | 114.75 | 26 Jul 10 | 18.55 | 114.48 | II | 1,050 | 161 | 53 | US | |||||
Mk9 | 1090025 | 76 | 15 Feb 10 | 18.33 | 114.75 | 15 Sep 11 | 18.47 | 114.50 | I | 613 | 577 | 123 | US | |||||
Mk9 | 1090007 | 108 | 15 Feb 10 | 18.33 | 114.75 | 27 Jul 11 | 23.38 | 111.17 | II | 901 | 527 | 202 | US | |||||
Mk9 | 1090012 | 111 | 15 Feb 10 | 18.33 | 114.75 | 27 Jul 10 | 18.55 | 114.48 | II | 929 | 162 | 162 | US | |||||
Mk9 | 1090051 | 86 | 22 Apr 10 | 18.32 | 114.75 | 5 Jun 10 | 18.83 | 116.33 | I | 701 | 44 | 34 | D | |||||
Mk9 | 1090064 | 114 | 22 Apr 10 | 18.32 | 114.75 | 26 Jul 10 | 18.55 | 114.48 | II | 958 | 95 | 95 | US | |||||
Mk9 | 1090075 | 117 | 22 Apr 10 | 18.32 | 114.75 | 27 Jun 10 | 18.57 | 114.70 | II | 988 | 66 | 66 | US | |||||
Mk9 | 0990500 | 148 | 24 Apr 10 | 18.32 | 114.75 | 29 Jun 10 | 18.15 | 115.35 | III | 1,358 | 66 | 66 | D | |||||
Mk9 | 1090392 | 119 | 18 Feb 11 | 18.37 | 114.661 | 25 Jun 11 | 19.13 | 114.25 | II | 1,008 | 127 | 95 | D | |||||
Mk9 | 1090391 | 113 | 18 Feb 11 | 18.33 | 114.74 | 26 Oct 11 | 18.08 | 114.87 | II | 949 | 249 | 154 | US | |||||
Mk9 | 1090400 | 122 | 18 Feb 11 | 18.33 | 114.74 | 23 Jun 11 | 19.73 | 114.27 | II | 1,039 | 124 | 64 | D | |||||
Mk9 | 1090467 | 106 | 22 Apr 11 | 18.37 | 114.65 | 15 Sep 11 | 18.47 | 114.50 | II | 882 | 145 | 52 | US | |||||
Mk9 | 1090469 | 84 | 23 Apr 11 | 18.37 | 114.67 | 6 Jun 11 | 18.58 | 114.02 | I | 684 | 44 | 44 | D | |||||
Roca Partida | ||||||||||||||||||
LTD2310 | D1479 | 60 | 16 Feb 08 | 19.00 | 112.07 | 24 Apr 08 | 22.67 | 109.97 | I | 468 | 68 | 68 | US | |||||
LTD2310 | D1497 | 74 | 16 Feb 08 | 19.00 | 112.07 | 17 Jul 08 | 19.00 | 112.07 | I | 595 | 153 | 140 | US | |||||
LTD2310 | D1567 | 75 | 16 Feb 08 | 19.00 | 112.07 | 17 Jul 08 | 19.00 | 112.07 | I | 604 | 153 | 131 | US | |||||
LTD2310 | D3379 | 111 | 19 Feb 09 | 19.01 | 112.01 | 14 Apr 09 | 23.17 | 112.63 | II | 929 | 54 | 54 | D | |||||
LTD2310 | D5107 | 111 | 20 Apr 09 | 19.01 | 112.02 | 26 May 09 | 18.98 | 112.42 | II | 929 | 36 | 36 | US | |||||
LTD2310 | D5480 | 77 | 14 Feb 10 | 19.01 | 112.08 | 26 Jul 10 | 18.55 | 114.48 | I | 622 | 162 | 114 | US | |||||
Mk9 | 1090072 | 93 | 21 Apr 10 | 18.97 | 112.05 | 25 Oct 11 | 9.47 | 110.33 | II | 763 | 552 | 340 | D | |||||
Mk9 | 1090342 | 114 | 17 Feb 11 | 19.02 | 112.04 | 9 Apr 11 | 18.97 | 112.08 | II | 958 | 51 | 51 | D | |||||
Mk9 | 1190051 | 66 | 10 May 11 | 18.98 | 112.07 | 28 Aug 11 | 23.00 | 111.25 | I | 524 | 110 | 110 | D | |||||
Mk9 | 1090397 | 65 | 16 Feb 11 | 19.02 | 112.04 | 20 Jul 11 | 24.00 | 112.22 | I | 515 | 155 | 155 | D | |||||
Socorro | ||||||||||||||||||
LTD2310 | D1580 | 104 | 16 Feb 07 | 18.77 | 110.90 | 27 Sep 07 | 11.18 | 109.70 | II | 863 | 223 | 223 | D | |||||
LTD2310 | D2050 | 103 | 16 Feb 07 | 18.77 | 110.90 | 16 Apr 08 | 17.68 | 109.30 | II | 854 | 425 | 249 | D | |||||
LTD2310 | D1589 | 145 | 16 Feb 07 | 18.77 | 110.90 | 7 Jul 08 | 10.62 | 124.55 | III | 1,315 | 507 | 155 | D | |||||
LTD2310 | D1593 | 140 | 16 Feb 07 | 18.77 | 110.90 | 24 Mar 08 | 14.52 | 112.27 | III | 1,247 | 402 | 223 | D | |||||
LTD2310 | D2030 | 142 | 17 Feb 07 | 18.71 | 110.90 | 12 Mar 08 | 6.27 | 101.55 | III | 1,274 | 389 | 196 | D | |||||
LTD2310 | D2036 | 144 | 17 Feb 07 | 18.71 | 110.90 | 27 Mar 08 | 16.07 | 112.72 | III | 1,301 | 404 | 169 | D | |||||
LTD2310 | D3436 | 98 | 24 Apr 08 | 18.72 | 111.00 | 3 Jun 10 | 18.98 | 113.65 | II | 808 | 770 | 295 | D | |||||
LTD2310 | D3438 | 88 | 24 Apr 08 | 18.72 | 111.00 | 18 Feb 10 | 13.90 | 109.48 | II | 719 | 665 | 384 | D | |||||
LTD2310 | D4383 | 113 | 17 Feb 09 | 18.70 | 110.90 | 7 Jun 09 | 18.33 | 111.08 | II | 949 | 110 | 110 | D | |||||
LTD2310 | D5158 | 118 | 17 Apr 09 | 18.70 | 110.90 | 3 Nov 09 | 13.70 | 117.12 | II | 998 | 201 | 105 | D | |||||
LTD2310 | D5146 | 109 | 17 Apr 09 | 18.70 | 110.90 | 8 May 10 | 16.52 | 109.27 | II | 910 | 386 | 109 | D | |||||
Mk9 | 990284 | 97 | 19 Apr 10 | 18.70 | 110.93 | 18 Jul 10 | 18.98 | 109.60 | II | 799 | 90 | 90 | D | |||||
Mk9 | 1090357 | 122 | 14 Feb 11 | 18.73 | 110.90 | 20 Apr 11 | 18.45 | 111.13 | II | 1,039 | 65 | 64 | D | |||||
Mk9 | 1090441 | 108 | 18-Apr-11 | 18.78 | 110.90 | 17 Mar 12 | 19.50 | 110.63 | II | 901 | 350 | 202 | D | |||||
Mk9 | 1190063 | 122 | 8 May 11 | 18.70 | 110.92 | 23 Apr 12 | 16.13 | 110.18 | III | 1,039 | 350 | 350 | D | |||||
Mk9 | 1190040 | 93 | 8 May 11 | 18.77 | 110.90 | 22 Apr 12 | 16.70 | 109.43 | II | 763 | 334 | 334 | D | |||||
San Benedicto | ||||||||||||||||||
LTD2310 | D1481 | 120 | 14 Feb 08 | 19.33 | 110.78 | 2 Apr 08 | 15.72 | 112.97 | II | 1,019 | 48 | 48 | D | |||||
LTD2310 | D1159 | 91 | 14 Feb 08 | 19.33 | 110.78 | 23 Apr 08 | 17.35 | 109.88 | II | 746 | 69 | 68 | D | |||||
LTD2310 | D5511 | 97 | 11 Feb 10 | 19.33 | 110.80 | 6 Apr 10 | 17.30 | 107.85 | II | 799 | 53 | 53 | D | |||||
LTD2310 | D4396 | 114 | 15 Feb 09 | 19.33 | 110.78 | 30 Mar 12 | 10.33 | 109.17 | II | 958 | 1,139 | 145 | D |
Age group vs depth
The number of records in different depth strata was counted by age group (I, II, and III). The sum of records for the 3 age groups was concentrated in the first 2 strata (0-25 m and 26-50 m), whereas the remaining records were distributed in the other strata in descending order, with the smallest number being found in the deepest stratum (>500 m). Age groups I, II, and III had the longest dwell time in the 0-25 m stratum, with 72%, 65%, and 63% of the time (corresponding to 17, 15, and 15 h), respectively. The stratum with the second longest dwell time was 26-50 m, with 16%, 17%, and 17% of the time for age groups I, II, and III, respectively. The 51-75 m and 76-100 m depth strata were frequently visited by the 3 age groups, but dwell time there decreased to less than 12%. Overall, YFT from age groups I, II, and III spent 98%, 94%, and 91% of the daytime, respectively, at a depth of 0-100 m. Age groups II and III visited the remaining depth strata (101-500 m) more often than age group I, although the percentage of time spent at these depths was minimal.
Deep zone (>500 m)
The total number of tuna that dove beyond 500 m depth by age group was 1 for group I, 5 for group II, and 5 for group III. Maximum recorded depth was 746 m for group I, 1,240 m for group II, and 1,432 m for group III. The number of visits to deep zones made by each age group was 3 for group I, 10 for group II, and 5 for age group III. Average dwell time was 10 min for group I, 24 min for group II, and 47 min for group III (Table 2).
Behavior and habitat use by time of day
There were significant differences in the number of fish in different depth strata during day and night hours (χ2[18] = 38.963, P < 0.00288) between age groups I, II, and III.
Age group I
Age group I remained longer in the 0-25 m depth stratum, corresponding to 30 to 53 min·h-1 (average of 42 min). The time spent in this stratum was longer (50 min) from 03:00 to 11:00, but subsequently decreased from 12:00 to 24:00, with no more than 35 min·h-1. The stratum with the second longest dwell time was 26-50 m, corresponding to 6-15 min·h-1 (average of 10 min). The longest times in this stratum were recorded from 12:00 to 03:00, with a limit of up to 15 min·h-1. In these 2 strata (0-25 and 26-50 m), the total dwell time was 53 min. In the remaining strata (51-500 m), dwell time decreased with depth, with visits to the deep zones being more frequent from 13:00 to 01:00. With respect to the deepest stratum (>500 m), age group I made 3 visits, 2 during the night (02:00 and 05:00) and 1 during the day (09:00). The time spent at these depths was 7-12 min, with an average of 10 min. The deepest swim (746 m) was made at 09:00 and lasted 12 min (Fig. 2).
Age group II
Dwell time was longer in the 0-25 m depth stratum, corresponding to 35 to 53 min (average of 38 min) spent per hour. The longest times spent in that stratum were recorded from 01:00 to 11:00 and varied from 40 to 53 min. Nonetheless, dwell time decreased from 12:00 to 24:00 a.m., corresponding to 23-34 min. The other depth stratum where age group II remained for a considerable time was at 26-50 m, corresponding to 6-15 min (average of 10 min). The longest dwell times in this stratum were recorded from 12:00 to 24:00 a.m. and varied from 12 to 15 min·h-1. In these 2 strata (0-25 and 26-50 m), the average dwell time was 48 min·h-1. In the remaining strata (51-500 m), dwell time decreased with depth, with dives to deep zones being more frequent from 13:00 to 24:00. Regarding the deepest stratum (>500 m), age group II made 10 visits, 6 during the day (08:00 to 16:00) and 4 during the night (20:00 to 04:00). The time spent at these depths was 8 to 65 min, with an average of 24 min. The deepest swim (1,240 m) was made at 02:00 and lasted 32 min (Fig. 2).
Age group III
Dwell time was longer at 0-25 m, corresponding to 21-54 min (average of 38 min) per hour. The longest dwell times were recorded from 01:00 to 12:00 and varied from 39 to 54 min. After 12:00, and until 21:00, dwell time decreased to 21-33 min. The other stratum where age group III remained a considerable time was 26-50 m, corresponding to 5-14 min·h-1 (average of 10 min). The longest dwell times in this stratum were recorded from 13:00 to 23:00 and varied from 12 to 14 min·h-1. In these 2 strata (0-25 and 26-50 m), the average dwell time was 48 min·h-1. In the remaining strata, dwell time decreased with depth, with longer times spent from 12:00 to 02:00. For depths >500 m, age group III made 7 visits, 6 during the day (06:00 to 18:00) and only 1 at night (05:00). The time spent at these depths was 20 to 70 min, with an average of 47 min. The deepest swim (1,432 m) was made during the day (17:36) and lasted 25 min (Fig. 2).
Internal and ambient temperatures
The internal and ambient temperatures that were individually recorded in the tags at the maximum depth reached by YFT were, respectively, 24.3 and 6.25 ºC at 746 m for group I, 23.3 and 2.0 ºC at 1,240 m for group II, and 24.7 and 3.7 ºC at 1,432 m for group III. The highest average internal temperature was 24.0, 25.0, and 25.0 ºC for age groups I, II, and III, respectively, and was recorded in the surface stratum. Starting at 50 m depth, the average internal temperature in the 3 age groups decreased by no more than 3 degrees. By contrast, the ambient temperature decreased with depth, with the highest average ambient temperatures (23.6 to 25.0 ºC) recorded at 0-25 m and the lowest (5.0 to 7.2 ºC) recorded at >500 m (Fig. 3).
The 3 age groups spent more than 80% of the time in the mixed layer, with age group I being the one that spent the most time (88%) in that stratum. Fish from age group III showed the highest number of visits to the mixed layer, with a dwell time of 20%. The 3 age groups preferred making visits below the mixed layer during the day rather than during the night. However, this preference was more pronounced in age groups I and II, with daytime dwell times higher than 60%, than in age group III, whose daytime dwell time was comparatively lower (54%), but higher during the night (Table 3).
Discussion
The use of archival tags to generate information on YFT movements in the water column proved to be an efficient support tool for understanding vertical swimming behavior during day and night hours. The recapture of YFT of different sizes and with records of more than one year in freedom evidenced that the type of archival tag did not affect fish health. In addition, following the capture, tagging, and release procedures for YFT described by Schaefer et al. (2007) and Schaefer and Fuller (2016) was critical to keep the handling time of the tuna aboard the vessel to a minimum and reduce stress; this was indicated by the signs of strength and vitality of the fish during their release.
Tagged YFT scattered to areas outside the Revillagigedo Archipelago Biosphere Reserve, where they were vulnerable to commercial fishing by Mexican tuna vessels, mainly fishing on dolphin sets or school sets (Aldana 2000, Dreyfus 2008, IATTC 2016). The recapture rate by age group was variable, with medium and large YFT being predominantly caught, thus corroborating the results of Schaefer et al. (1963). This pattern could be explained by little or no presence of floating objects in the area. The vertical movements and the preference of YFT for shallow strata confirmed its classification as an epipelagic species, which spends most of the day in the 0-50 m water column and is more abundant at depths of up to 100 m (Collette and Nauen 1983). The 3 age groups complemented the rest of their swimming time with visits to depths of 100 to 500 m and devoted only a few minutes to occasional dives to depths beyond 500 m (Schaefer et al. 2007, Schaefer et al. 2011, Schaefer et al. 2014). Although the statistical analysis revealed significant differences between age groups I, II, and III by depth stratum during the day and night, the 3 age groups exhibited a very similar behavior pattern in the water column, spending more time at depths of 0-100 m. This observed pattern contrasts with the one exhibited by bigeye tuna (Schaefer and Fuller 2010). Regarding the depth preferences by age group, small fish (age group I) spent most of the time at 0-25 m and 26-50 m and were significantly less abundant in depth strata >100 m, whereas age groups II and III were more abundant in these deeper strata, agreeing with the results of Schaefer et al. (2011, 2014).
Deep zone (>500 m)
The deep-sea swimming activity proved the physiological ability of YFT to dive beyond 500 m and compensate for low temperatures and minimum oxygen concentrations in this zone. The deep zone is not exclusive to large-sized YFT (age groups II and III) as it was also visited by a small individual from age group I. The deep swimming activity, defined by the number of dives, depth, and dwell time, was associated with the size of YFT, i.e., the smaller the YFT, the shorter the dwell time in the deep zone. On the other hand, age groups II and III showed more visits to depths >1,000 m and longer dwell times, which coincides with the deep swimming behavior of YFT reported by Schaefer et al. (2011, 2014). By contrast, as reported for bigeye tuna (Schaefer and Fuller 2010), there was no time pattern or preference for day or night time for deep-sea exploration in any age group. The deep-sea swimming strategy is occasionally used to forage for food, including mesopelagic fish and squid (Graham 1975, Schaefer et al. 2011). A “rebound” swim strategy (from the shallow to the deep zone) was employed by the 3 age groups to compensate for the loss of body heat and oxygen requirements during deep dives (Schaefer et al. 2014).
Internal and ambient temperatures
The ability of YFT to regulate and maintain their internal temperature above ambient temperature and to explore deep zones is strongly associated with fish size. In deep zones, where there is a sharp difference between fish internal temperature and ambient temperature, small-sized YFT (group I) can lose body heat at a faster rate, explaining the low number of visits to these zones and the minimal dwell times. By contrast, large-sized tuna (older groups) can dive more frequently to deep zones and dwell there for longer periods. According to Brill et al. (1998), after a deep dive, YFT return to the shallow stratum and stay there as long as needed to raise their internal temperature to that of water.
Overall, the 3 age groups preferred spending more time, both during the daytime and nighttime, in the mixed layer, which is considered a comfort zone with appropriate temperature and dissolved oxygen conditions for YFT to satisfy their physiological needs (Schaefer et al. 2014, Hinton 2015). Solar intensity could play a role on these depth preferences. All age groups were noticeably present in the first 2 strata from 06:00 to 12:00. After 12:00 and until sunset, there was an increase in the number of visits to depths beyond the mixed layer, where internal temperature and ambient temperature were markedly different, explaining the minimal dwell time at these depths. As the day progressed and until the night hours, there was an increase in the number of visits to depths beyond 200 m in all age groups.
Given the importance of the YFT fishery (IATTC 2018), continued tagging studies in other Pacific regions off the Mexican coasts are recommended. Moreover, supplementary information should continue to be provided for use with data on YFT horizontal movements and vertical habitat use so the information can altogether be included in stock assessment models that enable managers to make recommendations for the sustainability of the resource.