Juvenile Yellowfin Research Articles

Movements of juvenile yellowfin tuna (Thunnus albacares) within the coastal FAD network adjacent to the Palau National Marine Sanctuary: Implications for local fisheries development

Pacific Island nations and territories must build their capacity to harvest pelagic fishes to ensure domestic food security into the future. The Republic of Palau recently created the Palau National Marine Sanctuary, a Marine Managed Area that was intended to conserve marine resources and enhance local pelagic fisheries. However, the capacity of the nation’s domestic fishery to exploit pelagic species must be built to meet these objectives and, to this aim, the development of a coastal network of Fish Attracting Devices (FADs) has been proposed. To inform the development of the nation’s FAD program, we used acoustic telemetry to study the movements of yellowfin tuna (Thunnus albacares) within Palau’s existing FAD network. Subadult yellowfin tuna (50−79 cm FL) remained within the FAD network for up to 175 days and young-of-the-year yellowfin tuna spent significantly more time in association with FADs in comparison to year-1 fish. A network analysis suggests that the Ngardmau and Peleliu FADs were the most important components of the current FAD network, and a mixed effects Poisson generalized linear model indicated that the residency of yellowfin tuna to these FADs was significantly related to mooring depth and distance from the reef. These results provide a description of yellowfin tuna movements within Palau’s FAD network and recommendations to improve the nation’s FAD program that are also applicable to other Pacific Island nations and territories that desire to improve domestic access to pelagic fish stocks.

STUDY ON SPECIES COMPOSITION OF JUVENILE TUNA CAUGHT BY PURSE SEINE FLEET LANDING IN TAMPERAN FISHING PORT, JAVA, INDONESIA

Juvenile yellowfin tuna (Thunnus albacares) and bigeye tuna (T. obesus) are very similar in morphological characteristics, hence it is difficult to distinguish between these two species, especially when they are landed in frozen or defect conditions. The presence of juvenile bigeye tuna in yellowfin catch was first noticed in the 1980s from pole and line fisheries in Maldives. We analyzed the monthly composition structure of juvenile tuna caught by purse seine fleet operating in the South Indian Ocean. Tamperan fishing port was chosen for benchmarking to other small-scale fisheries. The result is expected to be used as an index for increasing the accuracy of juvenile tuna proportion for national catch statistics. A total of 4760 juvenile tunas were examined during monitoring activities. The length ranged 19-65 cm FL with median 40 cm FL for juvenile yellowfin tuna and 44 cm for juvenile bigeye. The whole weight of juvenile tuna ranged from 5 to 7 kg. The length-weight conversion for both species are W = 0.0184*FL3.0086 (R2 = 0.95, n = 4144) for juvenile yellowfin tuna and W = 0.018*FL3.0047 (R2 = 0.93, n = 346) for juvenile bigeye tuna. It can be inferred from the study that in terms of catch proportion of juvenile tuna, it consists of ratio 10:1, whereas for every 10 kg of juvenile tuna contains approximately one kg of bigeye tuna. Besides, the length-weight equation for both species is interchangeable, which means either equation can be performed to convert length to weight for both juvenile yellowfin and bigeye tuna.

Physiological and behavioural thermoregulation of juvenile yellowfin tuna Thunnus albacares in subtropical waters

Fish of the genus Thunnus are unusual because they are regional endotherms. In this study, archival tag data were used to demonstrate behavioural and physiological thermoregulation in juvenile yellowfin tuna, Thunnus albacares (35−52 cm fork length). Tags inserted into the peritoneal cavity were recovered from 23 yellowfin tuna caught mainly around Ishigaki Island, Japan, in 2009−2012. Peritoneal and environmental temperatures and depth data revealed that juvenile yellowfin tuna made daytime bounce dives repeatedly with a median dive duration of 16 min, and they remained at the surface during night-time. Body temperature daily averages were slightly higher than those of ambient temperature, and the body temperatures varied according to ambient temperature changes associated with vertical movements. Heat exchange coefficients (λ) during dives were estimated by a heat budget model. The thermal inertia component of λ was estimated in an experiment with dead juvenile yellowfin tuna. The value of λ exceeded the measured thermal inertia and varied between ascents and descents, thus providing evidence that physiological thermoregulation contributes to λ in juvenile yellowfin tuna and varies according to environmental temperature. Physiological thermoregulation helps maintain relatively stable body temperatures during dives in which tuna behaviourally escape cool deep waters and enter warm surface waters. Comparison of the obtained physiological parameters of λ and heat production ($${\dot{T}}_{\mathrm{m}}$$) with those previously recorded in Pacific bluefin tuna indicate that yellowfin tuna has a higher λ and lower $${\dot{T}}_{\mathrm{m}}$$ than Pacific bluefin tuna, which may restrict yellowfin tuna to warm tropical and subtropical surface waters.

Ontogenetic change in the amount and position of slow-oxidative myotomal muscle in relationship to regional endothermy in juvenile yellowfin tuna Thunnus albacares.

Myotomal slow-oxidative muscle (SM) powers continuous swimming and generates heat needed to maintain elevated locomotor muscle temperatures (regional endothermy) in tunas. This study describes how the amount and distribution of myotomal SM increases with fish size and age in juvenile yellowfin tuna Thunnus albacares in relationship to the development of regional endothermy. In T. albacares juveniles 40-74 mm fork length (LF ; n = 23) raised from fertilised eggs at the Inter-American Tropical Tuna Commission Achotines Laboratory in Panama and larger juveniles (118-344 mm LF ; n = 5) collected by hook and line off of Oahu, Hawaii, USA, SM was identified by histochemical staining for the mitochondrial enzyme succinic dehydrogenase or by colour (in the two largest individuals). The cross-sectional area of myotomal SM at 60% LF , a position with maximal percentage of SM in larger T. albacares, increased exponentially with LF . The percentage of total cross-sectional area composed of SM at 60% LF increased significantly with both LF and age, suggesting that SM growth occurs throughout the size range of T. albacares juveniles studied. In addition, the percentage of SM at 60% LF that is medial increased asymptotically with LF . The increases in amount of SM and medial SM, along with the development of the counter-current heat-exchanger blood vessels that retain heat, allow larger tuna juveniles to maintain elevated and relatively stable SM temperatures, facilitating range expansion into cooler waters.

Short Communication: The species and body size composition of pelagic fishes that caught by troll line in the fish landing of Sendang Biru, East Java, Indonesia

Abstract. Hidayati D, Oclandhi B, Maulidinia N, Sa’adah NN, Nurhayati APD. 2019. Short Communication: The species and body size composition of pelagic fishes that caught by troll line in the fish landing of Sendang Biru, East Java, Indonesia. Biodiversitas 20: 1764-1769. Troll line is the most used pelagic fishing gear for fishermen in Sendang Biru, the part of Indian ocean at East Java, which have an impact on the catch composition. The study was aimed to obtain data on species composition and size of pelagic fish caught in troll line in Sendang Biru, East Java, Indonesia. Fish samples were collected from nine different vessels landed in Sendang Biru for three days in April 2018. The composition of fish species was measured based on relative abundance. The fish size composition was obtained by measuring fork length which is also used to determine the stage of development of juvenile (Jv) or adult (Ad) fish. The results of the research showed there were four pelagic fish species which dominated by Katsuwonus pelamis or skipjack tuna (50.05%) and Thunnus albacares or yellowfin tuna (47.95%) and a small amount of Coryphaena hippurus and Makaira mazara. According to the body size, the fish caught by troll line in Sendang Biru is dominated by adult skipjack tuna (41-60 cm) and juvenile yellowfin tuna (21-78 cm).

Movement of juvenile tuna deduced from parasite data

Abstract The movements, and hence stock structures, of bigeye tuna, Thunnus obesus, and yellowfin tuna, Thunnus albacares, remain poorly defined despite their importance to food security, livelihoods and government revenue in many nations. We examined the parasite fauna of juvenile bigeye tuna and juvenile yellowfin tuna from areas within and outside Indonesia to determine how far they may have moved since metamorphosis and acquisition of a piscivorous diet. Patterns in parasite data between collection areas were consistent between the two tuna species. Fish from two outlier areas, Maldives and the Solomon Islands, harboured a distinct parasite fauna from those from adjacent Indonesian areas, although occasionally showed similarity with those from within the Indonesian archipelago. Within Indonesia, the parasite data indicated that few juvenile fish moved west from the Pacific Ocean into the archipelago and few moved west from the archipelago into the eastern Indian Ocean. While sampling of adult fish is required to fully resolve the stock structure of both tuna species, the results provide a necessary first-step in examining structuring of the two tuna species in Indonesia and indicate that juveniles of both species may have more restricted movement than is recognized in current management arrangements.

Behavioural and brain biomarker responses in yellowfin bream (Acanthopagrus australis) after inorganic mercury ingestion

To assess the effects of inorganic mercury ingestion on fish general behaviour and brain biochemical markers (acetylcholinesterase – AChE; lipid peroxidation – LPO; glutathione S-transferase – GST; catalase – CAT), juvenile yellowfin bream (Acanthopagrus australis) were fed mercury dosed food (low = 0.7 mg kg−1, medium = 2.4 mg kg−1 and high = 6 mg kg−1) or undosed food (control = 0.2 mg kg−1) for up to 16 days (5 replicates per treatment). Behavioural indicators, LPO levels and GST activity significantly changed overtime, but were not affected by mercury concentration. In contrast, CAT activity was higher in exposed fish in comparison to controls after 4 days, but not after 8 and 16 days. An additional depuration treatment was evaluated and fish from this treatment were less active and had significantly lower LPO levels and CAT activity than fish constantly exposed to the medium treatment. Overall, results from the present study indicate that a diet containing inorganic mercury impaired yellowfin bream growth and initially affected fish brain CAT activity, however fish were able to recover even with continuous exposure.

Catch and bycatch captured by tropical tuna purse-seine fishery in whale and whale shark associated sets: comparison with free school and FAD sets

In an ecosystem approach to fisheries management (EAFM) framework of the tuna purse-seine fishery, the assessment of target species, but also that of bycatch species, is essential. In the Atlantic and Indian oceans, purse-seine nets are sometimes set around tuna schools associated with whale sharks and baleen whales, although less frequently than around free-swimming tuna schools or those associated with fish aggregating devices (FAD). However, knowledge on the targeted catch and bycatch in these megafauna associated fishing sets is still relatively limited. Therefore, the aims of this study were to assess species and size composition of the target tuna species, as well as the diversity of bycatch species in whale and whale shark associated sets. Whale associated sets were found to be very similar to free school sets in terms of tuna catch (large yellowfin tuna), bycatch occurrence (presence in half the sets) and species assemblage (alpha and beta diversity). Whale shark associated sets were intermediate between FAD and free school sets, with tuna catch (skipjack and juvenile yellowfin) closer to FAD than to free school sets. However, the presence of large yellowfin, the bycatch composition (with almost no finfish, abundantly captured in FAD sets) and the species assemblage showed similarity with free school sets. This study highlights the need for an EAFM in the tuna purse-seine fishery by providing knowledge on pelagic multi-specific catches and bycatches.

Association of early juvenile yellowfin tuna <i>Thunnus albacares</i> with a network of payaos in the Philippines

To understand how early juvenile yellowfin tuna use the habitat and environment provided by fish aggregating devices (FADs), fish (19–31 cm FL) implanted with ultrasonic transmitters into their abdominal cavities were released in a network of payaos in Panay Gulf, the Philippines. Self-recording receivers were attached to the anchor ropes of the payaos to detect the presence of the fish. Some aspects of the behavior of juveniles were similar to those reported in adults. One juvenile showed a diurnal vertical swimming pattern, swam within a limited shallow range during the nighttime, and dived to deeper waters during the daytime. Two juveniles performed deep dives over 100 m during payao-to-payao excursion. Three juveniles showed a diurnal horizontal swimming pattern that was synchronized. In contrast, juveniles stayed <6 days in the network, shorter than adults. No juveniles returned to the same payao after an interruption of over 24 h. It is suggested that juveniles in this area are just starting to migrate and are temporarily staying around a payao for a few days to forage before continuing their migration.

Telemetry Study on Juvenile Yellowfin Tuna Thunnus albacares Around a Payao in the Philippines

Telemetry Study on Juvenile Yellowfin Tuna Thunnus albacares Around a Payao in the Philippines

Local Ecological Knowledge (LEK) on Fish Behavior Around Anchored FADs: the Case of Tuna Purse Seine and Ringnet Fishers from Southern Philippines

Abstract The Fishing Industry in the Philippines plays an important role in the food and employment need of Filipino fishers. By using anchored Fish Aggregating Devices (FADs or payao), the Philippine tuna fisheries was transformed into a million-dollar industry. Minimal studies on exploitation rates and fish behaviour around anchored FADs hampered further understanding of this fishery practice. Studies on fish behavior using Local Ecological Knowledge (LEK) are good complement where data is limited. A study using semi-structured interview (n=46) and three focus group discussions (n=39 participants) to record fishers’ knowledge and observations on the behavior of different fish species around anchored FADs was conducted. This particularly focused on attraction, retention, and departure behaviour of fishes in identified FAD sites. Based on the fishers’ knowledge, tuna schools are attracted to anchored FADs at 10 km distance. In anchored FADs, tuna form schools segregated by species and size. There was no relationship between the attraction distance and the reported school size and the various waiting times for fish to aggregate below the FADs. There was no variation between the species present at day or night time although fishers have reported a distinction of species found near the surface (0-10 m) and those found at other depths (11-20 m). Juvenile yellowfin tuna (Thunnus albacares), skipjack (Katsuwonus pelamis) and frigate and bullet tunas (Auxis spp.) are found to stay at 25-50 m from the FAD at a depth of >20 m. Adult oceanic tunas reside in deeper waters (75 m). The fish visual census produced similar results with the semi-structured interviews and FGDs but did not observe oceanic tunas at depths of 15-20 m in the anchored FADs examined.

Feeding habits of juvenile yellowfin tuna (Thunnus albacares) in Ecuadorian waters assessed from stomach content and stable isotope analysis

Size-related shifts in feeding habits of juvenile yellowfin tuna (YFT), Thunnus albacares (Bonnaterre, 1788), in Ecuadorian waters were assessed using stomach content analysis (SCA) (n=339) and stable isotope analysis (n=83). In all size classes, fishes were the predominant prey group in the diet (Alimentary Index, %AI=96.86) followed by cephalopods (%AI=3.29) and crustaceans (%AI=0.03). PERMANOVA analysis revealed significant variations in diet with body size: i) Class I YFT (≤50cm in fork length, FL) fed mainly on laternfishes, family Myctophidae (%AI=37.34), and jumbo squid, Dosidicus gigas (%AI=36.06), Class II YFT (50–60cm, FL) consumed bullet and frigate tunas, Auxis spp., unidentifiable fish and jumbo squid (%AI=47.75, %AI=25.06 and%AI=21.67, respectively), and Class III YFT (≥60cm, FL) preyed almost exclusively on bullet and frigate tunas (%AI=90.85). Mean±SD muscle isotope values were 12.14±1.95‰ for δ15N and −17.42±0.27 for δ13C‰; mean (±SD) liver values were 11.30‰ for δ15N and −17.42±0.27‰ for δ13C. Overall, liver and muscle samples showed significant size-related differences in δ15N and δ13C. Stable isotope Bayesian ellipses, SEAc, did not show trophic overlap among size classes. A significant positive linear correlation was observed between δ15N and δ13C measured in liver and muscle tissues with body size, suggesting an increase of prey size as tuna grew. These results indicate that the observed size-related diet shifts are not only related to prey type, but also to prey size.

PERIKANAN PUKAT CINCIN TUNA SKALA KECILYANG BERBASIS DI PELABUHAN PERIKANAN PANTAI (PPP) TAMPERAN (Small Scale Tuna Purseseine Fisheries Based in Tamperan Fishing Port)

&lt;p&gt;ABSTRACT&lt;br /&gt;In general, fisheries in Indonesia is still dominated by small-scale fisheries, one of themis the purse seine fishery based in Tamperan Fishing Port. This study aims to determine aspects of purse seine fishery based on Tamperan Fishing Port such as vessel size and fishing gear, FADs , fishing ground and catches. The study was conducted in 2012 in Tamperan Fishing Port with measurement methods and direct observation and interviews with owners, skippers and crew. The results showed that the vessel of small-scale tuna purse seine fishing based in Tamperan Fishing Port have tonnage between 28-45 GT and made of wood with length 17.21-28 m, width 6-7 m, and depth 2-3 m. Nets is used have length between 250-300 m and width 8-10 m. Fishing ground is FADs at coordinates 10º-12º S and 100º-110º E, 60-100 miles from the Pacitan Bay with distance to the location of FADs about one day. The catch is dominated by skipjack about 57.27%, followed by scad26.31% and juvenile yellowfin 10.05%.&lt;/p&gt;&lt;p&gt;&lt;br /&gt;Keywords: Purse seine, small scale tuna fisheries, Tamperan&lt;/p&gt;&lt;p&gt;-------&lt;/p&gt;&lt;p&gt;ABSTRAK&lt;/p&gt;&lt;p&gt;Secara umum, perikanan tangkap di Indonesia masih didominasi oleh usaha perikanan skala kecil, salah satunya adalah nelayan pukat cincin yang berbasis di PPP Tamperan. Penelitian ini bertujuan untuk mengetahui aspek penangkapan perikanan pukat cincin yang berbasis di PPP Tamperan berupa ukuran kapal dan alat tangkap, rumpon, daerah penangkapan dan hasil tangkapan. Penelitian dilakukan pada tahun 2011 di PPP Tamperan Pacitan dengan metode pengukuran dan pengamatan langsung di lapangan serta wawancara dengan nelayan pemilik, nakhoda dan awak kapal. Hasil penelitian menunjukkan bahwa armada pukat cincin perikanan tuna skala kecil yang berbasis di PPP Tamperan memiliki ukuran tonase antara 28-45 GT dan terbuat dari kayu dengan ukuran panjang antara 17,21-28 m, lebar 6-7 m, dan dalam 2-3 m. Jaring yang digunakan memiliki panjang antara 250-300 m dan lebar 8-10 m dengan menggunakan rumpon. Daerah penangkapan berada pada koordinat 10º-12º LS dan 100º-110º BT dan berjarak sekitar 60-100 mil dari Teluk Pacitan dengan lama perjalanan menuju lokasi rumpon sekitar 1 hari. Hasil tangkapan didominasi oleh cakalang sebesar 57,27%, diikuti oleh layang 26,31% dan yuwana madidihang 10,05%.&lt;/p&gt;&lt;p&gt;&lt;br /&gt;Kata kunci: Pukat cincin, perikanan tuna skala kecil, Tamperan&lt;/p&gt;

Effects of Modified Handling on the Physiological Stress of Trawled-and-Discarded Yellowfin Bream (Acanthopagrus australis).

Modified handling is often claimed to reduce (sub-)lethal impacts among organisms caught-and-released in fisheries. Improving welfare of discarded fish warrants investigation, when their survival is of both economic and ecological importance. In this study, juvenile yellowfin bream (Acanthopagrus australis) were trawled in an Australian penaeid fishery and then discarded after on-board sorting in either dry or water-filled (modified) trays and with delays in starting sorting of either 2 or 15 mins. Blood plasma cortisol, glucose and potassium were sampled immediately from some yellowfin bream, while others were placed into cages (with controls) and sampled after five days. Irrespective of their on-board handling, all trawled fish incurred a relatively high acute stress response (i.e. an increase in Mean ± SE cortisol from a baseline of <4 to 122.0 ± 14.9 ng/mL) that was mostly attributed to the trawling process, and exacerbated by variation in key parameters (low salinity, changes in water temperature and the presence of jellyfish Catostylus mosaicus in catches). When C. mosaicus was present, the potassium concentrations of fish sampled immediately after sorting were significantly elevated, possibly due to nematocyst contact and subsequent inhibition of ion pumps or cytolysis. Stress also increased during handling in response to warmer air temperatures and longer exposure. While most fish had substantially recovered by 120 hours after discarding, deploying selective trawls (to reduce jellyfish) for short periods and then quickly sorting catches in water would benefit discard welfare.

Comparative Efficacy of Anesthetics among MS-222, 2-Phenoxyethanol, and Clove Oil in Early Juvenile Yellowfin Tuna Thunnus albacares

Comparative Efficacy of Anesthetics among MS-222, 2-Phenoxyethanol, and Clove Oil in Early Juvenile Yellowfin Tuna Thunnus albacares

Feeding Ecology of Juvenile Yellowfin Tuna from Waters Southwest of Taiwan Inferred from Stomach Contents and Stable Isotope Analysis

AbstractThe Yellowfin Tuna Thunnus albacares is one of the major fish species caught around subsurface fish aggregation devices (FADs) in the waters southwest of Taiwan. However, how it interacts with other organisms around FADs is poorly known. In this study, the diet and feeding habits of juvenile Yellowfin Tuna were estimated from the analysis of stomach contents from 1,477 specimens with FLs ranging from 24 to 108 cm and stable isotope analysis (202 specimens) collected around FADs in the waters southwest of Taiwan. The analysis of stomach contents indicated that juvenile Yellowfin Tuna with FL &lt; 50 cm mainly feed on larval purpleback flying squid Sthenoteuthis oualaniensis, larval shrimps, and zooplanktonic organisms such as amphipods. Yellowfin Tuna with FL of ∼50 cm switch their diet to teleost fishes such as Japanese Barracudina Lestrolepis japonica, Skinnycheek Lanternfish Benthosema pterotum, and fishes in the families Exocoetidae and Scombridae. Stable isotope analysis indicated that the δ15N values ranged between 6.2‰ and 12.6‰, and the estimated trophic position varied from 3.18 ± 0.24 for tuna with FL &lt; 30 cm, while it reached 4.59 ± 0.50 for those with FL &gt; 50 cm and 4.75 ± 0.06 for those with FL &gt; 90 cm. Based on the distinct diet shift of the juvenile Yellowfin Tuna, demonstrated by both stomach contents and stable isotope analyses, this study concluded that the tuna shift their diet at approximately 50 cm FL.Received March 31, 2015; accepted September 9, 2015

Effects of partial substitution of dietary fish meal by fermented soybean meal on growth performance, body composition and activity of digestive enzymes of juvenile yellowfin sea bream (Acanthopagrus latus)

A feeding trial was conducted to evaluate the effect of replacing fish meal protein with fermented soybean meal (FSM) on the growth performance, feed utilization, body composition and digestive enzymes activity of yellowfin sea bream (Acanthopagrus latus) juvenile. Five isonitrogenic and isolipidic diets were prepared with levels of 0 (control), 100, 150, 250 and 300 g kg -1 FSM. Triplicate groups (20 fish per tank) of yellowfin sea bream with initial weight of 2.51±0.01 g were hand -fed to visual satiation at three meals per day for 56 days. The fish fed diets containing different levels of FSM had no significant differences regarding Final weight, Weight gain, SGR, FCR and Survival with control group. Whole body proximate compositions of fish were not affected by dietary FSM level. The activity of digestive enzymes in the intestine was not affected by dietary FSM level. This study showed that up to 30% fish meal in the diets of juvenile yellowfin (Acanthopagrus latus) could be replaced by fermented soybean meal.

Fine-scale vertical and horizontal movements of juvenile yellowfin tuna (Thunnus albacares ) associated with a subsurface fish aggregating device (FAD) off southwestern Taiwan

Summary An increase in yellowfin tuna (Thunnus albacares) catch by danish seine fisheries around the subsurface fish aggregating devices (FADs) in southern Taiwan waters has been a concern of local government and environmental groups. However, the attraction mechanism of aggregating tunas at the subsurface FADs is still poorly understood. The objective of this study is to examine the fine-scale vertical and horizontal movements of juvenile yellowfin tunas around a subsurface FAD. In total, 53 tunas (35–81 cm fork length) were tagged with ultrasonic telemetry tags and released at a subsurface FAD in the waters off Shiao-Liu-Chiu Island, southwestern Taiwan from October 2008 to December 2009. These tunas stayed at the subsurface FAD for up to 31 days, with daytime vertical movement depths averaging 60–80 m at a maximum depth of 250 m. At night, the tuna gathered at a shallow depth of 40 m. The mean depth of vertical movement in the daytime is significantly different from that of the nighttime (P < 0.05). The maximum detectable distance of horizontal movement was 1.6 km, with 80% of the long horizontal movements occurring in the daytime. It is likely that the purpose of these vertical and horizontal movements was for feeding or avoiding predators. Moreover, the tagged tunas did not depart from the subsurface FAD simultaneously, suggesting distinct behaviors in their movements.

Vertical behavior of juvenile yellowfin tuna Thunnus albacares in the southwestern part of Japan based on archival tagging

The behavior of juvenile yellowfin tuna Thunnus albacares in southwestern Japan was investigated using archival tag data from five fish (fork length 52.5–92 cm, days at liberty 26–280 days) released near the Nansei Islands (24–29°N, 122–130°E). Vertical behavior was classified into three patterns: “shallow” (≥50 % of daytime hours at depth of <50 m), “deep” (≥50 % of daytime hours at ≥100 m), and “intermediate” (other than “shallow” or “deep”). The pooled proportion of the number of days of each behavior was 29, 25 and 46 %, respectively. The proportion of “shallow” behavior increased with fish size. The proportion of time spent near the surface at nighttime increased in the colder season, when the thermal gradient was relatively small. Surface-oriented behavior (fish remained at a depth of <10 m for more than 10 min) occurred mainly during nighttime and between November and January. Dives exceeding 500 m were occasionally observed (0.02 day−1), and one fish dived to 1230 m. The results of our study show that yellowfin tuna were typically distributed in the mixed layer or upper thermocline where the water temperature was close to the sea surface temperature and that the vertical behavior was variable.

Schooling behavior of juvenile yellowfin tunaThunnus albacaresaround a fish aggregating device (FAD) in the Philippines

A fish aggregating device (FAD) called a payao is conventionally installed to catch pelagic species in the Philippines. The waters around the Philippines are important regions for yellowfin tuna stocks because they include spawning grounds and nurseries. To understand the schooling behavior of juvenile yellowfin tuna Thunnus albacares around a payao, 13 juveniles (20.5−24.0 cm fork length) double tagged with ultrasonic transmitters (V7-2L-R256; Vemco Ltd.) and data loggers (DST-micro; Star-Oddi Ltd.) were released around a payao. A self-recording receiver (VR2-DEL; Vemco Ltd.) was attached on the mooring rope of the payao to follow the horizontal movements and data loggers recorded the vertical movements of tagged juveniles. Nine juveniles were recaptured simultaneously by ring net at the same payao after 4-7 days. One juvenile was recaptured by hand line at another payao 12 km away from the tagging site after 6 days. Recaptured juveniles showed a diurnal schooling pattern suggesting different school shape and foraging strategy between daytime and nighttime. Juveniles showed a diurnal horizontal moving pattern, concentrated near the payao during daytime, while they were distributed around the payao at nighttime. The fluctuations of swimming depth were synchronized among fish. Juveniles also showed a diurnal vertical movement pattern in surface mixed layer. They concentrated in a shallow and narrow range (11.2±8.2 m, mean ± SD) at nighttime, while they were distributed to a deep and wide range (20.0 ± 11.8 m) during daytime. The maximum vertical neighbor distance indicated the vertical thickness of the school and showed a peak around noon. Higher vertical movement speed during daytime indicated vertical foraging in a water column, while at nighttime the juveniles might forage horizontally following the diurnal migration patterns of prey in the surface layer.