Periodic behavior and residence time of yellowfin and bigeye tuna associated with fish aggregating devices around Okinawa Islands,as identified with automated listening stations

In order to better understand the associative behavior of yellowfin tuna (Thunnus albacares) and bigeye tuna (T. obesus) with anchored fish aggregating devices (FADs), we conducted long-term monitoring of these tuna tagged with ultrasonic transmitters, using automated receivers deployed on seven FADs around the Okinawa Islands. Current and surface water temperature were also monitored by data loggers attached to the stations as a way to examine the influence of these factors on the associative behavior of tuna with FADs. We monitored a total of 52 yellowfin and 11 bigeye tuna at monitoring FADs for a period of 2.5 years. We found that the majority of tuna remained continuously at the monitoring stations for a certain period (max.=55 days) without day-scale (>24 h) absences, until they left the stations completely. The residence time at a single FAD was estimated to be about 7 days, as the half-life for both yellowfin and bigeye tuna. No inter-specific differences were seen, though there was a significant difference in residence time between two size classes: the residence time of the larger size class was shorter than that of the smaller size class. We also found there was a periodicity of approximately 24 h and regularity of associative behavior, estimated based on the fluctuation pattern of the detection rate and of short-term (<24 h) absences. In particular, absences of several hours occurring once a day with high temporal precision were considered to be excursions within several nautical miles from the FADs. These results indicate that tuna express periodic behavior in relation to the FADs and can locate the FADs precisely enough to return to them after a certain time. No relationship was seen between associative behavior and abiotic oceanographic conditions. Therefore, the biological environment (prey availability, the presence of predators, etc.) and the internal state of the individual (hunger, etc.) may be more important than abiotic environmental cues for inducing changes in associative behavior and/or the departure from FADs. In addition, the strong association of tuna with a single FAD and the relatively prolonged residence time observed in the present study may relate to the vigorous activity of FAD fisheries in Okinawa and their utilization of a large amount of bait.Communicated by T. Ikeda, Hakodate     

Size-dependent behavior of tuna in an array of fish aggregating devices (FADs)

Several lines of evidence indicate that aggregations of yellowfin tuna associated with floating objects are more frequently composed of small animals than larger ones. Also, the diet of small yellowfin tuna caught at anchored fish aggregating devices (FADs) around Oahu, Hawaii, was found to shift quite rapidly when these fish reached approximately 50 cm FL. In order to test for ontogenetic changes in aggregation behavior, we tagged and released two distinct size classes of yellowfin tuna in an array of anchored FADs around Oahu, Hawaii. Twenty-four yellowfin tuna 30–39 cm FL and 16 yellowfin tuna 63–83 cm FL were tagged with acoustic transmitters and released near anchored FADs equipped with automated acoustic receivers. Fish in the smaller size class stayed about 2.5 times longer at individual FADs than the larger fish (mean 4.05 days vs. 1.65 days) and displayed larger horizontal movements within the array. However, the durations of unassociated phases, residence times in the entire FAD array, percentage of time spent associated with FADs and numbers of movements between FADs did not show any difference between the two size groups. The observed size-dependent behavior is discussed in terms of physiological abilities, diet segregation and anti-predator behavior.     

Behavior of yellowfin (<Emphasis Type="Italic">Thunnus albacares</Emphasis>) and bigeye (<Emphasis Type="Italic">T. obesus</Emphasis>) tuna in a network of fish aggregating devices (FADs)

The influence of multiple anchored fish aggregating devices (FADs) on the spatial behavior of yellowfin (Thunnus albacares) and bigeye tuna (T. obesus) was investigated by equipping all thirteen FADs surrounding the island of Oahu (HI, USA) with automated sonic receivers (“listening stations”) and intra-peritoneally implanting individually coded acoustic transmitters in 45 yellowfin and 12 bigeye tuna. Thus, the FAD network became a multi-element passive observatory of the residence and movement characteristics of tuna within the array. Yellowfin tuna were detected within the FAD array for up to 150 days, while bigeye tuna were only observed up to a maximum of 10 days after tagging. Only eight yellowfin tuna (out of 45) and one bigeye tuna (out of 12) visited FADs other than their FAD of release. Those nine fish tended to visit nearest neighboring FADs and, in general, spent more time at their FAD of release than at the others. Fish visiting the same FAD several times or visiting other FADs tended to stay longer in the FAD network. A majority of tagged fish exhibited some synchronicity when departing the FADs but not all tagged fish departed a FAD at the same time: small groups of tagged fish left together while others remained. We hypothesize that tuna (at an individual or collective level) consider local conditions around any given FAD to be representative of the environment on a larger scale (e.g., the entire island) and when those conditions become unfavorable the tuna move to a completely different area. Thus, while the anchored FADs surrounding the island of Oahu might concentrate fish and make them more vulnerable to fishing, at a meso-scale they might not entrain fish longer than if there were no (or very few) FADs in the area. At the existing FAD density, the ‘island effect’ is more likely to be responsible for the general presence of fish around the island than the FADs. We recommend further investigation of this hypothesis.

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Blood oxygen-binding characteristics of bigeye tuna (Thunnus obesus), a high-energy-demand teleost that is tolerant of low ambient oxygen

 We found blood from bigeye tuna (Thunnus obesus) to have a significantly higher O₂ affinity than blood from other tunas. Its P₅₀ (partial pressure of oxygen, P_O2 required to reach 50% saturation) was 1.6 to 2.0 kPa (12 to 15 mmHg) when equilibrated with 0.5% CO₂. Previous studies employing similar methodologies found blood from yellowfin tuna (T. albacares), skipjack tuna (Katsuwonus pelamis), and kawakawa (Euthynnus affinis) to have a P₅₀ of 2.8 to 3.1 kPa (21 to 23 mmHg). These observations suggest that bigeye tuna are more tolerant of low ambient oxygen than other tuna species, and support similar conclusions derived from laboratory whole-animal studies, depth-of-capture data, and directly-recorded vertical movements of fish in the open ocean. We also found the O₂ affinity of bigeye tuna blood to be essentially unaffected by a 10 C° open-system temperature change (as is the blood of all tuna species studied to date). The O₂ affinity of bigeye tuna blood was, however, more affected by a 10 C° closed-system temperature change than the blood of any tuna species yet examined. In other words, bigeye tuna blood displayed a significantly enhanced Bohr effect (change in log P₅₀ per unit change in plasma pH at P₅₀) when subjected to the inevitable changes in partial pressure of carbon dioxide (P_CO2) and plasma pH that accompany closed-system temperature shifts, than when subjected to changes in plasma pH accomplished by changing P_CO2 alone. In vivo, the resultant large decrease in O₂ affinity (i.e. the increase in P₅₀) that occurs as the blood of bigeye tuna is warmed during its passage through the vascular counter-current heat exchangers ensures adequate rates of O₂ off-loading in the swimming muscles of this high-energy-demand teleost. Received: 12 March 1999 / Accepted: 18 December 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

We measured the horizontal and vertical movements of five adult yellowfin tuna (Thunnus albacares, estimated body mass 64 to 93 kg) near the main Hawaiian Islands, while simultaneously gathering data on oceanographic conditions and currents. Fish movements were recorded by means of ultrasonic depth-sensitive transmitters. Depth–temperature and depth–oxygen profiles were measured with vertical conductivity–temperature–depth (CTD) casts, and the current-velocity field was surveyed using an acoustic Doppler current profiler (ADCP). Large adult yellowfin tuna spent ≃60 to 80% of their time in or immediately below the relatively uniform-temperature surface-layer (i.e. above 100 m), a behavior pattern similar to that previously reported for juvenile yellowfin tuna, blue marlin (Makaira nigricans), and striped marlin (Tetrapturus audax) tracked in the same area. In all three species, maximum swimming depths appear to be limited by water temperatures 8 C° colder than the surface-layer water temperature. Therefore, neither large body mass, nor the ability to maintain elevated swimming-muscle temperatures due to the presence of vascular counter-current heat exchangers in tunas, appears to permit greater vertical mobility or the ability to remain for extended periods below the thermocline. In those areas where the decrease in oxygen with depth is not limiting, the vertical movements of yellowfin tuna, blue marlin and striped marlin all appear to be restricted by the effects of water temperature on cardiac muscle function. Like juvenile yellowfin tuna, but unlike blue marlin and striped marlin, adult yellowfin tuna remained within 18.5 km of the coast and became associated with floating objects, including anchored fish-aggregating devices (FADs) and the tracking vessel. Like juvenile yellowfin tuna, large adult yellowfin repeatedly re-visit the same FAD, and appear able to navigate precisely between FADs that are up to 18 km apart. The median speed over ground ranged from 72 to 154 cm s⁻¹. Neither speed nor direction was strongly influenced by currents. Received: 27 March 1998 / Accepted: 13 November 1998     

Tuna food habits related to the micronekton distribution in French Polynesia

Stomach content analyses are commonly used to study both fish feeding behaviour and trophic conditions. However, the interpretation of such data depends on fish foraging behaviour for a given environment and how representative the stomach contents are to the prey distribution. Tuna feeding behaviour was studied within the context of a research programme conducted in French Polynesia. Tuna prey distribution was characterised using acoustic measurements and pelagic trawls; thereafter, this distribution was compared with the stomach contents of tuna caught using an instrumented longline. Acoustic, pelagic trawling and stomach content analyses give complementary elements to describe the pelagic trophic habitat and to better understand tuna-prey relationships. The classic concept of a reduced food availability for tunas in the tropical pelagic environment seems relative. Tunas able to dive enough during daytime to exploit the migrant micronektonic species secure a source of regular food. This is particularly true of bigeye tuna (Thunnus obesus), which have ecophysiological capacities for this purpose. The behaviour of albacore tuna (T. alalunga), which dive >400 m in depth, remains less clear, as little is known about their vertical behaviour. Lastly, yellowfin tuna (T. albacares), which are distributed in more superficial waters, can better exploit the biomass of juvenile fish and crustaceans exported from the reefs. Analysis of the stomach fullness of tuna caught by longline, a passive gear, generally showed an empty state. This result suggests that most tuna foraging on large prey aggregations present in the study area are quickly satiated and escape longline capture and sampling. A consequence is that studies of tuna feeding behaviour based on longlining may be biased, particularly when large aggregations of prey are present such as in convergence zones. Another potential consequence is that longline tuna catch rates could differ according to prey richness. Longline tuna catch rates may sometimes reflect the relative abundance of prey rather than relative tuna abundance. Electronic supplementary material to this paper can be obtained by using the Springer LINK server located at http://dx.doi.org/10.1007/s00227-001-0776-3.     

Stable isotopic evidence for trophic groupings and bio-regionalization of predators and their prey in oceanic waters off eastern Australia

Muscle tissue was collected for stable isotope analysis (SIA) from the main fish predators and their fish and cephalopod prey from oceanic waters off eastern Australia between 2004 and 2006. SIA of δ¹⁵N and δ¹³C revealed that the species examined could be divided into three main trophic groups. A “top predator” group consisted mainly of large billfish (Xiphias gladius and Tetrapturus audax), yellowfin (Thunnus albacares), bigeye (T. obesus) and southern bluefin (T. maccoyii) tunas and sharks; with mako (Isurus oxyrinchus) the highest. Below this tier was a second group composed of mid-trophic level fishes including albacore tuna (Thunnus alalunga), lancet fish (Alepisaurus ferox), mahi mahi (Coryphaena hippuris) and ommastrephid squid. Underlying both groups was a grouping of small fishes including myctophids, small scombrids and nomeids as well as surface fishes including macrorhamphosids. These groupings were based largely on mean animal size which showed a positive linear relation to δ¹⁵N (r ² = 0.58). Some species showed significant ontogenetic variation in either δ¹⁵N (swordfish, lancet fish, yellowfin and albacore tuna) or δ¹³C (mako shark). We also noted a consistent latitudinal change in δ¹⁵N and δ¹³C at ~28°S for the top predator species, particularly albacore and yellowfin tuna. The differences were consistent with a change from oligotrophic Coral Sea to nutrient rich Tasman Sea waters. These differences suggest that predatory fishes may have extended residence time in distinct regions off eastern Australia.     

Trophic niche overlap among dolphinfish and co-occurring tunas near the northern edge of their range in the western North Atlantic

We present the first quantitative analyses of dolphinfish (Coryphaena hippurus) foraging habits and trophic interactions with co-occurring yellowfin (Thunnus albacares) and albacore (T. alalunga) tunas in the Southern New England region of the western North Atlantic Ocean. Fish caught by recreational anglers in offshore waters of Massachusetts were sampled during the summers of 2007–2010. Diet analysis revealed that shortfin squid (Illex illecebrosus) and small pelagic crustaceans were principal prey to dolphinfish, yellowfin tuna, and albacore tuna. A wide variety of Sargassum-associated fishes were also important to dolphinfish and yellowfin tuna diets. Dietary (Schoener’s index: 0.82–0.86) and isotopic niche (isotopic ellipse overlap: 53.6–64.7 %) overlap was high, and dolphinfish and tunas occupied equivalent trophic positions (TP = 3.4–3.6). Relative prey size in dolphinfish and yellowfin tuna diets exhibited convergence with ontogeny. Overall, dolphinfish had the greatest isotopic niche width, which was twice as large as yellowfin tuna and three times as large as albacore tuna; dolphinfish also consumed the greatest range of prey sizes. Results quantify dolphinfish trophic interactions in the western Atlantic near the northern extent of their geographical range, and are relevant for ecosystem-based management of the offshore pelagic guild in the context of shifting fish populations and fisheries in response to climate and ecological change.     

Gill and intestinal Na+-K+ ATPase activity, and estimated maximal osmoregulatory costs, in three high-energy-demand teleosts: yellowfin tuna (Thunnus albacares), skipjack tuna (Katsuwonus pelamis), and dolphin fish (Coryphaena hippurus)

We hypothesize that the morpho-physiological adaptations that permit tunas to achieve maximum metabolic rates (MMR) that are more than double those of other active fishes should result in high water and ion flux rates across the gills and concomitant high osmoregulatory costs. The high standard metabolic rates (SMR) of tunas and dolphin fish may, therefore, be due to the elevated rates of energy expenditure for osmoregulation (i.e. teleosts capable of achieving exceptionally high MMR necessarily have SMR). Previous investigators have suggested a link between activity patterns and osmoregulatory costs based on Na⁺-K⁺ ATPase activity in the gills of active epipelagic and sluggish deep-sea fishes. Based on these observations, we conclude that high-energy-demand fishes (i.e. tunas and dolphin fish) should have exceptionally elevated gill and intestinal Na⁺-K⁺ ATPase activity reflecting their elevated rates of salt and water transfer. To test this idea and estimate osmoregulatory costs, we measured Na⁺-K⁺ ATPase activity (V _max) in homogenates of frozen samples taken from the gills and intestines of skipjack and yellowfin tunas, and the gills of dolphin fish. As a check of our procedures, we made similar measurements using tissues from hybrid red tilapia (Oreochromis mossambicus ×O. niloticus). Contrary to our supposition, we found no difference in Na⁺-K⁺ ATPase activity per unit mass of gill or intestine in these four species. We estimate the cost of osmoregulation to be at most 9% and 13% of the SMR in skipjack tuna and yellowfin tuna, respectively. Our results, therefore, do not support either of our original suppositions, and the cause(s) underlying the high SMR of tunas and dolphin fish remain unexplained. Received: 7 September 2000 / Accepted: 4 December 2000     

A rapid ontogenetic shift in the diet of juvenile yellowfin tuna from Hawaii

Within the tropical and subtropical oceans, tuna forage opportunistically on a wide variety of prey. However, little is known about the trophic ecology of the smallest size classes which play an important role in stock assessments and fisheries management. The foraging behavior of yellowfin tuna, Thunnus albacares (23.5–154.0 cm FL), collected from nearshore Fish Aggregating Devices (FADs) around Oahu was studied using stable isotope and stomach contents analyses. Emphasis was placed on small juveniles. Yellowfin tuna changed their diets significantly between 45 and 50 cm forklength (ca. 1.5 kg). Smallest size classes fed on planktonic organisms inhabiting the shallow mixed layer, primarily larval stomatopod and decapod crustaceans, whereas larger tuna fed on teleosts and adult Oplophorus gracilirostris, a vertically migrating mesopelagic species of shrimp. When interpreting the variation in prey δ ¹⁵N values, we considered both their relative trophic position and δ ¹⁵N values of the nitrogen at the base of the food web. Based on the distinct diet shift of the yellowfin tuna, demonstrated by both isotope and stomach content analyses, we propose a critical mass threshold was reached at about 45 cm FL that enabled sufficient endothermic capability to allow tuna to access prey dwelling in deeper, colder water. These ontogenetic changes in foraging range and commensurate shift in diet of small tunas would affect their vulnerability to fishing pressure.     

Feeding ecology and niche segregation in oceanic top predators off eastern Australia

We examined the feeding ecology and niche segregation of the ten most abundant fish species caught by longline operations off eastern Australia between 1992 and 2006. Diets of 3,562 individuals were examined. Hook timer data were collected from a further 328 fish to examine feeding behaviour in relation to depth and time of day. Prey biomass was significantly related to predator species, predator length and year and latitude of capture. Although the fish examined fed on a mix of fish, squid and crustacea, fish dominated the diet of all species except small albacore (Thunnus alalunga) which fed mainly on crustacea and large swordfish (Xiphias gladius) and albacore which fed mainly on squid. Cannibalism was observed in lancetfish (Alepisaurus spp.). Multidimensional scaling identified three species groups based on their diet composition. One group consisted of yellowfin tuna (T. albacares), striped marlin (Tetrapturus audax) and dolphinfish (Coryphaena hippurus); a second group consisted of bigeye tuna (T. obesus), swordfish and albacore; and a third consisted of southern bluefin tuna (T. maccoyii) and blue shark (Prionace glauca). Of note was the separation of mako shark (Isurus oxyrhynchus) and lancetfish from all other predators. Prey length generally increased with increasing predator length although even large predators fed on a wide range of prey lengths including very small prey. Overall, differences in prey type and size, feeding times and depths were noted across the range of species examined to the extent that predators with overlapping prey, either in type or size, fed at different times of the diel period or at different depths. Taken together these data provide evidence for feeding niche segregation across the range of oceanic top predators examined.     

Environmental preferences of yellowfin tuna (Thunnus albacares) at the northern extent of its range

We used acoustic telemetry to examine the small-scale movement patterns of yellowfin tuna (Thunnus albacares) in the California Bight at the northern extent of their range. Oceanographic profiles of temperature, oxygen, currents and fluorometry were used to determine the relationship between movements and environmental features. Three yellowfin tuna (8 to 16 kg) were tracked for 2 to 3 d. All three fish spent the majority of their time above the thermocline (18 to 45 m in depth) in water temperatures >17.5 °C. In the California Bight, yellowfin tuna have a limited vertical distribution due to the restriction imposed by temperature. The three fish made periodic short dives below the thermocline (60 to 80 m), encountering cooler temperatures (>11 °C). When swimming in northern latitudes, the depth of the mixed layer largely defines the spatial distribution of yellowfin tuna within the water column. Yellowfin prefer to spend most of their time just above the top of the thermocline. Oxygen profiles indicated that the tunas encountered oceanic water masses that ranged most often from 6.8 to 8.6 mg O₂ l⁻¹, indicating no limitation due to oxygen concentrations. The yellowfin tuna traveled at speeds ranging from 0.46 to 0.90 m s⁻¹ (0.9 to 1.8 knots h⁻¹) and frequently exhibited an oscillatory diving pattern previously suggested to be a possible strategy for conserving energy during swimming. Received: 14 February 1997 / Accepted: 14 April 1997     

Behavior of bigeye (<Emphasis Type="Italic">Thunnus obesus</Emphasis>) and skipjack (<Emphasis Type="Italic">Katsuwonus pelamis</Emphasis>) tunas within aggregations associated with floating objects in the equatorial eastern Pacific

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 2003. 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 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.Electronic Supplementary Material  Supplementary material is available in the online version of this article at http://dx.doi.org/10.1007/s00227-004-1480-xCommunicated by J.P. Grassle, New Brunswick     

Genetic evidence for inter-oceanic subdivision of bigeye tuna (Thunnus obesus) populations

Bigeye tuna (Thunnus obesus Lowe, 1839) are a commercially important species of tuna found in the Atlantic, Indian, and Pacific oceans. To initiate an analysis of bigeye tuna population-structure, three PCR–RFLP assays were developed based on the published mtDNA control-region sequences of four bigeye tuna. Population analyses using these three restriction assays on a total of 248 individuals resulted in an array of 13 composite haplotypes. A total of 347 nucleotides of mtDNA control-region sequence was characterized for 11 of the 13 composite haplotypes. Phylogenetic analyses demonstrated that the DNA sequences belong to two monophyletic clades. However, only one of the three restriction assays was able to discriminate between the two clades. The other two assays were confounded by excessive homoplasy. Both parallel (independent occurrences of the same nucleotide change) and convergent (different nucleotide changes within the same restriction site) changes of restriction sites were observed. These results emphasize the importance of DNA sequence-analysis for the interpretation of restriction-site polymorphism data. Analyses of the frequency distribution indicated that samples of bigeye tuna from the Atlantic Ocean were genetically distinct from those found in the Indian and Pacific oceans. Thus, these results reject the null hypothesis of a single global population of bigeye tuna. Received: 16 June 1997 / Accepted: 25 July 1998     

Isotopic evidence of distinct feeding ecologies and movement patterns in two migratory predators (yellowfin tuna and swordfish) of the western Indian Ocean

Ecologists primarily use δ¹⁵N values to estimate the trophic level of organisms, while δ¹³C, and even recently δ¹⁵N, are utilized to delineate feeding habitats. However, many factors can influence the stable isotopic composition of consumers, e.g. age, starvation or isotopic signature of primary producers. Such sources of variability make the interpretation of stable isotope data rather complex. To examine these potential sources of variability, muscle tissues of yellowfin tuna (Thunnus albacares) and swordfish (Xiphias gladius) of various body lengths were sampled between 2001 and 2004 in the western Indian Ocean during different seasons and along a latitudinal gradient (23°S to 5°N). Body length and latitude effects on δ¹⁵N and δ¹³C were investigated using linear models. Both latitude and body length significantly affect the stable isotope values of the studied species but variations were much more pronounced for δ¹⁵N. We explain the latitudinal effect by differences in nitrogen dynamics existing at the base of the food web and propagating along the food chain up to top predators. This spatial pattern suggests that yellowfin and swordfish populations exhibit a relatively unexpected resident behaviour at the temporal scale of their muscle tissue turnover. The body length effect is significant for both species but this effect is more pronounced in swordfish as a consequence of their different feeding strategies, reflecting specific physiological abilities. Swordfish adults are able to reach very deep water and have access to a larger size range of prey than yellowfin tuna. In contrast, yellowfin juveniles and adults spend most of their time in the surface waters and large yellowfin tuna continue to prey on small organisms. Consequently, nitrogen isotopic signatures of swordfish tissues are higher than those of yellowfin tuna and provide evidence for different trophic levels between these species. Thus, in contrast to δ¹³C, δ¹⁵N analyses of tropical Indian Ocean marine predators allow the investigation of complex vertical and spatial segregation, both within and between species, even in the case of highly opportunistic feeding behaviours. The linear models developed in this study allow us to make predictions of δ¹⁵N values and to correct for any body length or latitude differences in future food web studies.     

Analysis of the eastern Pacific yellowfin tuna fishery based on multiple management objectives

Vector optimization techniques were used to generate arbitrary segments of a policy frontier for a dynamic yellowfin tuna (Thunnus albacares) fishery model assuming fixed technology and considering four policy objectives: minimizing dolphin mortality, minimizing incidental catch (all species except dolphins), maximizing sustainable yield, and minimizing biological risk for the yellowfin tuna stock. Results show that along the policy frontier: (1) reducing incidental dolphin mortality increases the incidental catch of other species in a nonlinear way; (2) yield increases (subject to a biomass precautionary level) can only be obtained at the expense of higher levels of dolphin mortality and incidental catch; (3) biological risk increases as the level of tunas caught increases, but this increase depends on the type of fishery (longline fishing and three different modes of purse-seining: log-sets, dolphin-sets or school-sets) that dominates the fishing effort; (4) there is an indirect relationship between the dolphin mortality levels and those of biological risk; (5) there is a direct relationship between the incidental catch levels and biological risk. Catch obtained with dolphin-sets dominates the Pareto-optimal solutions with highest dolphin mortality levels but is associated with lower biological risk, whereas catch obtained with log-sets dominates in Pareto-optimal solutions with higher incidental catch and higher biological risk. In general, trade-offs or shadow prices among objectives are not linear, indicating that marginal costs vary along the policy frontier. Results of the trade-off analysis may provide useful information for decision-makers and other policy actors. Complete information about the preferences of the decision-makers regarding the objectives is necessary to recommend a specific management policy.     

Predicting fish diet composition using a bagged classification tree approach: a case study using yellowfin tuna (<Emphasis Type="Italic">Thunnus albacares</Emphasis>)

We provided a classification tree modeling framework for investigating complex feeding relationships and illustrated the method using stomach contents data for yellowfin tuna (Thunnus albacares) collected by longline fishing gear deployed off eastern Australia between 1992 and 2006. The non-parametric method is both exploratory and predictive, can be applied to varying size datasets and therefore is not restricted to a minimum sample size. The method uses a bootstrap approach to provide standard errors of predicted prey proportions, variable importance measures to highlight important variables and partial dependence plots to explore the relationships between explanatory variables and predicted prey composition. Our results supported previous studies of yellowfin tuna feeding ecology in the region. However, the method provided a number of novel insights. For example, significant differences were noted in the prey of yellowfin tuna sampled north of 20°S in summer where oligotrophic waters dominate. The analysis also identified that sea-surface temperature, latitude and yellowfin size were the most important variables associated with dietary differences. The methodology is appropriate for delineating ecosystem-level trophic dynamics, as it can easily incorporate large datasets comprising multiple predators to explore trophic interactions among members of a community. Broad-scale relationships among explanatory variables (environmental, biological, temporal and spatial) and prey composition elucidated by this method then serve to focus and lend validity to subsequent fine-scale analyses of important parameters using standard diet methods and chemical tracers such as stable isotopes.     

Movement patterns of large bigeye tuna (Thunnus obesus) in the open ocean, determined using ultrasonic telemetry

 The horizontal and vertical movements of large bigeye tuna (Thunnus obesus Lowe, 1839; 25 to 50 kg) captured in the south Pacific Ocean (French Polynesia) were determined using pressure-sensitive ultrasonic transmitters. Bigeye tuna swam within the first 100 m below the surface during the night-time and at depths between 400 and 500 m during the daytime. The fish exhibited clear relationships with the sound scattering layer (SSL). They followed its vertical movements at dawn and dusk, and were probably foraging on the organisms of the SSL. Bigeye tuna did, however, make regular rapid upward vertical excursions into the warm surface layer, most probably in order to regulate body temperature and, perhaps, to compensate for an accumulated oxygen debt (i.e. to metabolize lactate). The characteristics of these dives differ from those reported from previous studies on smaller bigeye tuna (∼12 kg) near the main Hawaiian Islands. During the daytime, the large fish in French Polynesia made upward excursions approximately only every 2.5 h, whereas smaller fish in Hawaiian waters made upward excursions approximately every hour. Our data are the first observations on the role of body size in the vertical behavior of bigeye tuna. Received: 9 September 1998 / Accepted: 25 November 1999     

Nuclear and mitochondrial DNA markers indicate unidirectional gene flow of Indo-Pacific to Atlantic bigeye tuna (<Emphasis Type="Italic">Thunnus obesus</Emphasis>) populations,and their admixture off southern Africa

A sharp genetic break separates Atlantic from Indo-Pacific bigeye tuna (Thunnus obesus) populations, as the frequencies of two major mitochondrial (mt) DNA types ( and ) found in this species are different across the tip of southern Africa. The level of nucleotide divergence between mtDNA types and is of the same order as that between reproductively isolated taxa. To further investigate the genetic structure of bigeye tuna over its distribution range and in the contact zone off southern Africa, bigeye tuna samples collected between 1992 and 2001 (including samples from a previous mtDNA survey) were characterized for four nuclear DNA loci and for mtDNA. Nuclear markers did not support the hypothesis that and mitochondria characterize sibling species. Significant allele-frequency differences at one intronic locus (GH2) and one microsatellite locus (µ208) were found between Atlantic and Indo-Pacific samples, although the level of nuclear genetic differentiation (Weir and Cockerhams =0.025 to 0.042) was much lower than in mtDNA ( =0.664 to 0.807). Probabilistic Bayesian assignment of individuals to a population confirmed that southern African bigeye tuna samples represent a simple mixture of individuals from Atlantic and Indian stocks that do not interbreed, with a higher contribution from Indian Ocean individuals (about 2/3 vs. 1/3).Communicated by M.S. Johnson, Crawley     

Simulating the temporal pattern of waste production in farmed gilthead seabream (Sparus aurata), European seabass (Dicentrarchus labrax) and Atlantic bluefin tuna (Thunnus thynnus)

Most fish farming waste output models provide gross waste rates as a function of stocked or produced biomass for a year or total culture cycle, but without contemplating the temporality of the discharges. This work aims to ascertain the temporal pattern of waste loads by coupling available growth and waste production models and developing simulation under real production rearing conditions, considering the overlapping of batches and management of stocks for three widely cultured species in the Mediterranean Sea: gilthead seabream (Sparus aurata), European seabass (Dicentrarchus labrax) and Atlantic bluefin tuna (Thunnus thynnus). For a similar annual biomass production, the simulations showed that waste output and temporal dumping patterns differ between the three species as a result of the disparities in growth velocity, nutrient digestibility, maintenance metabolic budget and husbandry. The simulations allowed the temporal patterns including the periods of maximum discharge and the dissolved and particulate nitrogen and phosphorus content in the wastes released to be determined, both of which were seen to be species-specific.