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).

Figure 2 Graphical representation of percent time spent by the different yellowfin tuna, Thunnus albacares, age groups in the upper depth strata (0-25 m, 26-50 m, 51-75 m, y 76-100 m).  

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).

Figure 3 Average ambient and internal temperatures for yellowfin tuna, Thunnus albacares, age groups I, II, and III in different depth strata.  

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).

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.