School fidelity and homing synchronicity of yellowfin tuna, Thunnus albacares

Thirty-eight yellowfin tuna (Thunnus albacares) were tagged with coded ultrasonic beacons between 6 March and 4 December 1996 near two buoys off the western coast of Oahu, Hawaii. Two to four tuna were captured, tagged, and released on the same day in as rapid succession as possible in an effort to tag members of the same school. Automated “listening” monitors attached to the buoys recorded when these marked individuals entered within a radius of ≤1.1 km of the buoys during a 13 mo period. Twenty-seven of the tuna returned to the site of tagging. The mean number of returns was 4.2 per tuna (max. = 17), and visits ranged from 1 to 910 min (median = 2.7 min, mean = 40.1 min). The intervals between successive returns varied from 1 to 257 d (median = 3.0 d, mean = 17.4 d). Seventy-three percent of the tuna returned together with tunas tagged on the same day, exceeding the frequency of returns of tuna tagged on another day or arriving alone. This social cohesion is supported by the pattern of return visits by five tuna tagged on 6 March at Monitoring Station R. Two or more of these tuna arrived together on 24 of 35 d when tagged tuna were detected. All five individuals visited R on 11 April, a month after tagging, three arrived together 5 mo later on 4 August, and three returned 6 mo later on 1 December 1996. Tuna often arrived at the same time of day, e.g. Individuals 1 and 3 visited R at 09:15 hrs on 12 April and at 09:00 hrs 8 mo later. The returns were also site-specific. The 22 tuna tagged at R made 182 return visits to R (92.4%) and only 15 visits to Monitoring Station K (7.6%), 10 km away. An allegiance of tuna to one school, a predilection for returning to the site of tagging, and precise timing when visiting sites, are consistent with tuna having migratory pathways consisting of “way-points” that are visited with temporal regularity. Received: 30 April 1998 / Accepted: 27 October 1998     

Allozyme and mitochondrial DNA variation in yellowfin tuna (Thunnus albacares) from the Pacific Ocean

Samples of yellowfin tuna (Thunnus albacares) collected in 1991 and 1992 from the western, central and eastern regions of the Pacific Ocean were examined for genetic variability. Four polymorphic allozyme loci (ADA ^*, FH ^*, GPI-S ^* and GPI-F ^*) were examined in all samples and a fifth polymorphism (GDA ^*) was examined in western and central samples only. Samples were also screened for mitochondrial DNA variation following restriction analysis by two enzymes (BcII and EcoRI) detecting polymorphic cut sites. Eighteen mtDNA haplotypes were revealed, with an overall nucleon diversity of 0.678. A subset of individuals screened for eight restriction enzymes had an overall nucleon diversity of 0.724 and a mean nucleotide diversity per sample of 0.359%. No significant spatial heterogeneity was detected for alleles at the ADA ^*, FH ^*, GPI-S ^* and GDA ^* loci nor for the mtDNA haplotypes. Significant heterogeneity was detected for GPI-F ^*. At this locus, the two eastern samples (southern California and northern Mexico) were not significantly different from each other but were significantly different (P<0.001) from the five western/central samples (Philippines, Coral Sea, Kiribati, Hawaii-91 and Hawaii-92). GPI-F ^* 100 was the most common allele in western and central regions, GPI-F ^* 75 the most common in eastern samples.     

Population structure of yellowfin tuna (Thunnus albacares) in the western Pacific Ocean, inferred from microsatellite loci

Five polymorphic microsatellite loci were examined in 1391 yellowfin tuna (Thunnus albacares) from eight regions of the western (Coral Sea, eastern Australia, Fiji, Indonesia, Philippines and Solomon Islands) and eastern (California and Mexico ) Pacific Ocean. Across all samples, numbers of alleles per locus ranged from 7 to 30 (mean: 17.0), and observed heterozygosities per locus ranged from 0.223 to 0.955 (mean: 0.593). Temporal collections were available for three areas: no significant temporal heterogeneity was observed for the Coral Sea (1991/1992 and 1995/1996 collections) or eastern Australia (1994/1995, 1995/1996, 1996/1997 and 1997/1998), but there was slight but significant heterogeneity at one locus (cmrTa-161) between the two Philippines collections (1994/1995 and 1996/1997). Genotypes generally showed a good fit to Hardy-Weinberg expectations within populations; only cmrTa-208 in the pooled Coral Sea population gave a significant deviation after Bonferroni correction for 40 tests, with a small but significant excess of homozygotes. Four loci showed no evidence of population differentiation following contingency Chi-squared and F_ST analyses. The fifth locus, cmrTa-161, showed small but significant differentiation (F_ST=0.002, P<0.001). This heterogeneity was largely a result of the Philippines 1994/1995 and Fiji collections; there was no correlation with geographic distance. The average F_ST across all five loci was very low (F_ST=0.002), but it was significant (P<0.001). It is unclear whether this low but significant differentiation reflects noise in the dataset, perhaps arising from experimental error, or real population differentiation. The finding of very limited population heterogeneity accords with most of the earlier allozyme and mitochondrial DNA studies of yellowfin tuna in the Pacific Ocean.     

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     

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     

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     

Movements of albacore tuna (Thunnus alalunga) in the South Pacific: Evidence from parasites

Parasites were collected from over 400 albacore (Thunnus alalunga) caught by surface trolling and longlining in the south-west Pacific between 1985 and 1988. Parasites found included 1 apicomplexan, 3 nematode species, 4 cestode species, 1 acanthocephalan, 12 digenean species and 3 copepod species. Twelve of these parasite species which could be accurately recognised and counted were used in the subsequent analyses. Parasite data from albacore caught around New Zealand show a decrease in prevalence of three didymozoid parasites with increasing fish length up to a fork length of 70 to 79 cm. The subsequent increase in prevalence of these didymozoids in large longline-caught fish is consistent with fish returning from spawning in tropical waters where re-infection is presumed to occur. Albacore collected at widely separate locations in the south-west Pacific have differences in parasite prevalence, supporting an hypothesis that juvenile albacore move south to New Zealand from the tropics and do not return until the onset of sexual maturity. Albacore appear to move along the subtropical convergence zone, as indicated by a decline in prevalence and abundance ofAnisakis simplex andHepatoxylon trichiuri from New Zealand to the central South Pacific. This is supported by tagging and seasonal movements of the fishery.     

Genetic population structure of southern bluefin tuna (Thunnus maccoyii )

Samples of southern bluefin tuna, Thunnus maccoyii (Castelnau), taken from off the coasts of South Africa, Western Australia, South Australia and Tasmania from 1992 to 1994 were analysed for six polymorphic allozyme loci (ADA ^*, GDA ^*, GPI-A ^*, MPI ^*, PGDH ^* and PGM-1 ^*, n = 595 to 733 per locus) and for mitochondrial DNA variants revealed by three restriction enzymes (Bam HI, Bcl I and Eco RI) detecting polymorphic cut sites (n = 555). No significant spatial heterogeneity was detected. There were no sex-related differences in allele or mtDNA haplotype frequencies. Juveniles (30 to 35 cm and 46 to 54 cm) from what are thought to be two temporally-separated spawning peaks showed no significant genetic differentiation. There were also no significant differences in allele or haplotype frequencies between fish smaller than 70 cm and those larger than 70 cm. These data are consistent with the null hypothesis of a single unit stock of southern bluefin tuna, with a single spawning area. This is located to the south of Java and off the north-west coast of Australia. Received: 28 August 1996 / Accepted: 30 September 1996     

Simultaneous behavior of skipjack (Katsuwonus pelamis), bigeye (Thunnus obsesus), and yellowfin (T. albacares) tunas, within large multi-species aggregations associated with drifting fish aggregating devices (FADs) in the equatorial eastern Pacific Ocean

Ten separate experiments monitoring the simultaneous behaviors of 26 skipjack (Katsuwonus pelamis), 26 bigeye (Thunnus obesus), and 33 yellowfin (T. albacares) tunas within large multi-species aggregations associated with drifting fish aggregating devices (FADs) were investigated using ultrasonic telemetry in the equatorial eastern Pacific Ocean. Experiments were conducted during a research cruise aboard a chartered purse seine vessel. Purse seine sets were made on the tuna aggregations associated with FADs at the termination of six of the ten experiments. Seventeen of the 44 tagged tunas were not recaptured indicating the transient nature of the associative behavior of tunas with FADs. Although there was considerable overlap in the depths of the three species, by day and night, there were some species-specific differences and diel differences within species. While we documented spatial and temporal differences in the schooling behavior of the three tuna species, the differences do not appear sufficient such that modifications in purse seine fishing practices could effectively avoid the capture of small bigeye and yellowfin tunas, while optimizing the capture of skipjack tuna in purse seine sets on FADs.     

Tracking adult North Atlantic bluefin tuna (Thunnus thynnus) in the northwestern Atlantic using ultrasonic telemetry

Ultrasonic, depth-sensitive transmitters were used to track the horizontal and vertical movements, for up to 48 h, of 11 adult (136 to 340 kg estimated body mass) North Atlantic bluefin tuna (Thunnus thynnus Linnaeus). Fish were tracked in October 1995, September and October 1996, and August and September 1997 in the Gulf of Maine, northwestern Atlantic. The objective was to document the behavior of these fish and their schools in order to provide the spatial, temporal, and environmental information required for direct (i.e. fishery-independent) assessment of adult bluefin tuna abundance using aerial surveys. Transmitters were attached to free-swimming fish using a harpoon attachment technique, and all fish remained within the Gulf of Maine while being followed. Most of the bluefin tuna tagged on Stellwagen Bank or in Cape Cod Bay (and followed for at least 30 h) held a predominately easterly course with net horizontal displacements of up to 76 km d⁻¹. Mean (±SD) swimming depth for all fish was 14 ± 4.7 m and maximum depth for individuals ranged from 22 to 215 m. All but one fish made their deepest excursions, often single descents, at dawn and dusk. In general, adult bluefin tuna spent <8% of their time at the surface (0 to 1 m), <19% in the top 4 m, but >90% in the uppermost 30 m. Mean (±SD) speed over ground was 5.9 km h⁻¹, but for brief periods surpassed 20 to 31 km h⁻¹. Sea surface temperatures during tracking were 11.5 to 22.0 °C, and minimum temperatures encountered by the fish ranged from 6.0 to 9.0 °C. Tagged bluefin tuna and their schools frequented ocean fronts marked by mixed vertebrate feeding assemblages, which included sea birds, baleen whales, basking sharks, and other bluefin schools. Received: 19 July 1999 / Accepted: 25 March 2000     

Mitochondrial DNA and electronic tracking reveal population structure of Atlantic bluefin tuna (Thunnus thynnus)

Population subdivision was examined in Atlantic bluefin tuna (Thunnus thynnus) through sequencing of the control region of the mitochondrial genome. A total of 178 samples from the spawning grounds in the Gulf of Mexico, Bahamas and Mediterranean Sea were analyzed. Among the samples from these locations were 36 electronically tagged bluefin tuna that were tagged in the North Atlantic and subsequently traveled to one of these known spawning grounds during the spawning season. Bluefin tuna populations from the Gulf of Mexico and the Mediterranean Sea were found to be genetically distinct based on Φ_st, and sequence nearest neighbor analyses, showing that these two major spawning areas support independent stocks. Sequence nearest neighbor analysis indicated significant population subdivision among the Gulf of Mexico, western Mediterranean and eastern Mediterranean Sea. However, it was not possible to find significant pairwise differences between any sampling areas when using all samples. If only samples that had a high likelihood of assignment to a specific spawning site were used (young of the year, spawning adults), the differentiation increased among all sampling areas and the Western Mediterranean Sea was distinct from the Eastern Mediterranean Sea and the Gulf of Mexico. It was not possible to distinguish samples from the Bahamas from those collected at any of the other sampling sites. These data support tagging results that suggested distinctness of the Gulf of Mexico, Eastern and Western Mediterranean Sea spawning areas. This level of stock differentiation is only possible if Atlantic bluefin tuna show strong natal homing to individual spawning grounds.     

Solitary and eusocial nests in a population of Augochlorella striata (Provancher) (Hymenoptera; Halictidae) at the northern edge of its range

Summary Augochlorella striata was studied at the northern limit of its range. The study population contained a mixture of solitary and social nest foundresses. Eusocial foundresses produced 1 or 2 workers before switching to a male biased brood. Solitary foundresses produced males first. Cells vacated by eclosed offspring were reused late in summer. A female biased brood resulted from cell reuse in both solitary and eusocial nests. Workers were slightly smaller than their mothers and were sterile although most of them mated. In comparison to published data from a Kansas population of this species, the Nova Scotia population had i) a lower proportion of multiple foundress nests, ii) a smaller worker brood and iii) a briefer period of foraging activity but iv) comparable overall nest productivity.     

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     

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.     

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.     

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.     

Relationship between the growth and survival of larval Pacific bluefin tuna, Thunnus orientalis

We tested the hypothesis that a large body size and rapid growth rate affect the survival of larval Pacific bluefin tuna, Thunnus orientalis (PBT), and analyzed larval growth in relation to environmental conditions. Seven high density larval patches of PBT were tracked with reference buoys in the northwestern Pacific Ocean for 28–171 h in May–June from 2004 to 2008. The otolith radii and daily growth rates of the survivor larvae (collected on later tracking days of each tracking session) tended to be larger and more rapid, respectively, than those of original larvae (collected on earlier tracking days). A large body size was found to positively affect the survival of larval PBT, as did a rapid growth rate, even at an early larval stage (7 days after hatching). Generalized linear modeling showed that the otolith radius was influenced positively by the sea temperature, stratification parameter and food density, while the growth rate was influenced positively by the sea temperature and food density.     

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.     

Movements,behavior, and habitat utilization of yellowfin tuna (<Emphasis Type="Italic">Thunnus albacares</Emphasis>) in the northeastern Pacific Ocean,ascertained through archival tag data

Sixty-eight yellowfin tuna, Thunnus albacares, (60-135 cm fork length) were caught and released with implanted archival tags offshore off Baja California, Mexico, during October 2002 and October 2003. Thirty-six fish (53%) were recaptured and the data were downloaded from all 36 recovered tags. Time at liberty ranged from 9 to 1,161 days, and the data were analyzed for the 20 fish that were at liberty for 154 or more days. The accuracy in the position estimates, derived from light-level longitude data and sea-surface temperatures (SSTs) based latitude, is about 0.41° in longitude and 0.82° in latitude, in this region. The movement paths, derived from position estimates, for the 20 yellowfin indicated that 19 (95%) remained within 1,445 km of their release locations. The estimated mean velocity along movement paths was 77 km/day. The southern and northern seasonal movement paths observed for yellowfin off Baja California are influenced by the seasonal movements of the 18°C SST isotherm. Cyclical movements to and from suitable spawning habitat (≥24°C SST) was observed only for mature fish. For the 12 fish that demonstrated site fidelity, the mean 95 and 50% utilization distributions were 258,730 km² and 41,260 km², respectively. Evaluations of the timed depth records resulted in discrimination of four distinct behaviors. When exhibiting type-1 diving behavior (78.1% of all days at liberty) the fish remained at depths less than 50 m at night and did not dive to depths greater than about 100 m during the day. Type-2 diving behavior (21.2% of all days at liberty) was characterized by ten or more dives in excess of 150 m during the day. Type-2 diving behavior is apparently a foraging strategy for fish targeting prey organisms of the deep-scattering layer during the day, following nighttime foraging within the mixed layer on the same prey. Yellowfin tuna exhibited occasional deep-diving behavior, and some dives exceeded 1,000 m, where ambient temperatures were less than 5°C. Surface-oriented behavior, defined as the time fish remained at depths less than 10 m for more than 10 min, were evaluated. The mean number and duration of surface-oriented events per day for all fish was 14.3 and 28.5 min, respectively. Habitat utilization of yellowfin, presented as monthly composite horizontal and vertical distributions, indicates confined geographical distributions, apparently resulting from an affinity to an area of high prey availability. The vertical distributions indicate greater daytime depths in relation to a seasonally deeper mixed layer and a greater proportion of daytime at shallower depths in relation to a seasonally shallower mixed layer.     

Vertical movements, behavior, and habitat of bigeye tuna (Thunnus obesus) in the equatorial eastern Pacific Ocean, ascertained from archival tag data

The results presented in this report are based on analyses of 16,721 days of data downloaded from 96 archival tags recovered from bigeye tuna (Thunnus obesus; 54–159 cm in length, 0.97–5.44 years of age) at liberty from 31 to 1,508 days in the equatorial eastern Pacific Ocean. Analyses of daily timed depth and temperature records resulted in the classification of the data into three daily behavior types: characteristic, associative (associated with floating objects), and other. There is a significant positive correlation between the proportion of time fish exhibit characteristic behavior and increasing length, and significant negative correlations between the proportion of time bigeye exhibit associative and other behavior with increasing length. For the smallest (54–80 cm) to largest (100–159 cm) length classes, the vertical habitats utilized when exhibiting non-associative behaviors were 99 and 98% of the time above the thermocline depth (60 m) during the night, at the same average depth of 34 m, and 60 and 72% of the time below the thermocline during the day at average depths of 163 and 183 m, respectively. For the same smallest to largest length classes, when exhibiting associative behavior, the average nighttime and daytime depths were 25 and 21, and 33 and 37 m, respectively. The apparent effects of the environment on the behavior of the fish are discussed.