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Prey availability and predation pressure are thought to be key constraints on larval growth, especially in low-productivity, subtropical environments. Yet, measuring their effects on larval fishes has been challenging, given the dynamic biophysical drivers of plankton distributions and small scales of interactions. We integrated fine-scale net tows (10s of meters) within situimaging to explore how predator-prey interactions influence larval fish growth in the Straits of Florida. Otolith-derived recent growth was analyzed for 3 ecologically important fishes: 2 coral reef labrids (Thalassoma bifasciatumandXyrichtys novacula) and 1 tuna (Katsuwonus pelamis), with differing mean growth rates (labrids 0.25 mm d-1,K. pelamis0.44 mm d-1) and prey (labrids-copepods; tuna-appendicularians). We used generalized additive models to examine the interactive effect of background density and frequency of elevated (>2 SD above background) prey and predators on recent (last 3 d) larval growth. For all taxa, recent growth increased with prey background density. Recent growth of labrids was also higher when copepod densities were more often elevated (14% of transect >20 ind. m-3) above otherwise low background densities (2 ind. m-3). Predators (chaetognaths and hydromedusae) had a growth-selective effect: stronger selection in transects with high-density predator patches, although the direction of the effect was species-specific. The effect of temperature was taxa-specific: growth increased with temperature for the labrids and peaked at an optimum (28°C) for the rapidly growing tuna. Integration of these fine-scale sampling methods improves our understanding of the variable influence of prey and predators on larval growth and, consequently, larval survival.

To quantitatively evaluate the distribution of tuna larvae relative to oceanographic conditions, we conducted investigations off the Nansei Islands in the western North Pacific in June from 2015 to 2017. Five species, namely Pacific bluefin tuna Thunnus orientalis (PBF), yellowfin tuna T. albacares (YFT), skipjack tuna Katsuwonus pelamis (SKJ), frigate tuna Auxis thazard, and bullet tuna A. rochei (BT), were collected in each year. The most dominant species was BT throughout the 3 yr period, followed by SKJ in 2015 and YFT in 2016 and 2017. The horizontal larval distributions of the 5 species were largely influenced by the Kuroshio Current: larvae of the 2 Auxis species were distributed in the Kuroshio and the Kuroshio inshore waters, whereas those of the other species were found in the Kuroshio offshore waters. These differences are consistent with the differences in spawner distributions among the tunas. Generalized additive models (GAMs) indicated that the larval densities were affected by the sea surface height anomaly and that the larvae were not always amassed by horizontal transport. Sea surface temperature (SST) and salinity possibly influenced the larval physiology and survival, thereby determining their densities. In the GAMs, PBF and YFT showed similar responses to SST, and YFT and SKJ similarly responded to salinity. To avoid overlapping their ecological niches, the larvae of 3 species (PBF, YFT, and SKJ) are expected to differ in other ways, including their vertical distributions and feeding habits.

As part of the endeavor aiming at the domestication of Atlantic bluefin tuna (BFT; Thunnus thynnus), first sexual maturity in captivity was studied by documenting its occurrence and by characterizing the key hormones of the reproductive axis: follicle stimulating hormone (FSH) and luteinizing hormone (LH). The full length sequence encoding for the related hormone b-subunits, bftFSHb and bftLHb, were determined, revealing two bftFSHb mRNA variants, differing in their 50 untranslated region. A quantitative immuno-dot-blot assay to measure pituitary FSH content in BFT was developed and validated enabling, for the first time in this species, data sets for both LH and FSH to be compared. The expression and accumulation patterns of LH in the pituitary showed a steady increase of this hormone, concomitant with fish age, reaching higher levels in adult females compared to males of the same age class. Conversely, the pituitary FSH levels were elevated only in 2Y and adult fish. The pituitary FSH to LH ratio was consistently higher (>1) in immature than in maturing or pubertal fish, resembling the situation in mammals. Nevertheless, the results suggest that a rise in the LH storage level above a minimum threshold may be an indicator of the onset of puberty in BFT females. The higher pituitary LH levels in adult females over males may further support this notion. In contrast three year-old (3Y) males were pubertal while cognate females were still immature. However, it is not yet clear whether the advanced puberty in the 3Y males was a general feature typifying wild BFT populations or was induced by the culture conditions. Future studies testing the effects of captivity and hormonal treatments on precocious maturity may allow for improved handling of this species in a controlled environment which would lead to more cost-efficient farming Versión del editor

Foraging with tuna is a well-documented seabird strategy, referred to as facilitated foraging. However, despite this behaviour being considered almost obligatory in nutrient-poor tropical waters, little data exist on its relative importance to individual colonies. Therefore, to examine facilitated foraging under different patterns of nutrient availability, we tracked wedgetailed shearwaters Ardenna pacifica from 2 colonies, one tropical and one subtropical, situated in waters of contrasting productivity. Shearwater foraging behaviour was assessed relative to oceanographic covariates and predicted distributions for multiple tropical tuna species and ageclasses, simulated by an existing ecosystem model (SEAPODYM). Shearwaters from both colonies undertook long trips to deep, pelagic waters close to seamounts and foraged most often at fronts and eddies. Micronektonic and adult tuna age classes were highly correlated in space. Predation between these tuna age classes represents a likely source of facilitated foraging opportunities for shearwaters. At broad spatio-temporal scales, shearwaters consistently foraged in areas with higher predicted adult skipjack and micronektonic tuna densities and avoided adult bigeye tuna. At finer spatio-temporal scales, dynamic ocean features aggregated tuna of all sizes. Enhanced tuna density at these locations increased the likelihood of shearwater foraging activity. Long trips in the tropics targeted oligotrophic waters with higher tuna densities. Long trips in the subtropics targeted enhanced productivity, but in some years shifted to target the same oligotrophic, tunadense waters used by tropical conspecifics. We conclude that facilitated foraging with tuna is consistently important to the tropical breeding population and becomes increasingly important to the subtropical population in years of low marine productivity.

Monitoring live and dead discarded catch contributes to effective fisheries management and ecological and socioeconomic sustainability. We determined contemporary rates and levels of discards in global tuna fisheries. An estimated 265 279 t (52 283 to 478 275 t 95% CI) is annually discarded by global tuna fisheries, composing about 5% of the weight of the total catch. Pelagic longline and purse seine fisheries contributed about 64 and 36% of discards, respectively. Other gear types composed < 1% of discards. Discards in tuna fisheries are now 63% lower than estimated 1 decade earlier, mainly due to large declines in discards in longline tuna fisheries, possibly from increased retention of formerly discarded species and sizes of catch and increased gear selectivity. The decline also resulted, in part, from employing different methods to categorize caught sharks whose fins were retained and carcasses discarded following processing. Discard rates were greater in shallow-than deep-set longline fisheries, and higher in purse seine sets associated with fish aggregating devices and other floating objects than in other purse seine set types. The quality and availability of data on discards in global tuna fisheries were extremely limited. Filling gaps in monitoring, improving observer data fields and collection protocols, and providing public access to amalgamated discard data held by fisheries management organizations will improve the certainty of future discard estimates, supporting effective management of discards in global tuna fisheries.

We investigated the foraging ecology of southern bluefin tuna Thunnus maccoyii in open-ocean habitats of temperate waters in the southern hemisphere by analyzing their stomach contents. Samples were collected from longline vessels over 15 yr (n = 4649). Of the prey, 51% by weight were cephalopods and 46% were teleosts. These values differ from those in the literature for other top predators in the open oceans, for which teleosts compose the largest portion of prey. The dominance of cephalopods also differs from the pattern for juveniles in previous studies in their coastal habitat, where most of the prey are teleosts. Thus, a distinct shift of prey occurs along with the habitat shift due to ontogenetic development. By weight, important prey were omma - strephid (18%), lycoteuthid (12%), and argonautid (1%) cephalopods and nomeid (8%, mainly Cubiceps caeruleus), paralepidid (7%), bramid (6%), and alepisaurid (6%) teleosts. The prey composition was relatively consistent among tuna sizes, sea surface temperatures, and years; changes in prey composition were due largely to differences in the cephalopod prey. Cephalopods belonging to the families Lycoteuthidae and Argonautidae contributed to the prey only off the southern coast of Africa and near Tasmania, respectively. Lycoteuthids occurred at lower sea sur- face temperatures than ommastrephids off the southern coast of Africa. Small ommastrephids were dominant in smaller tuna in the southeastern Indian Ocean. Our data provide basic informa- tion that will improve our understanding of the oceanic food webs in southern temperate waters.

We thank Maria Lema (Unidad de Tecnicas Instrumentales de Analisis, Universade da Coruna) for isotopic analysis, Oriol Ribalta and collaborating sports fishermen from the CEPRR for help in tuna sampling in the Cambrils area. This work was funded by the European Community (contract no. 212797), the Spanish Ministry of Science and Innovation (contracts CTM2011-29525-C04-01 and CTM2011-27505) and the Andalusian government (contract RNM-02469), and was conducted as part of the CLIOTOP programme. J.L.V. benefitted from a grant under the Prometeo Project programme (SENESCYT, Ecuador). This is contribution 84 of CEI.MAR and 700 of AZTI-Tecnalia (Marine Research Division).

Large marine predators such as tunas and sharks play an important role in structuringmarine food webs. Their future populations depend on the environmental conditions they en -counter across life history stages and the level of human exploitation. Standard predator−prey rela-tionships suggest favorable conditions (high prey abundance) should result in successful foragingand reproductive output. Here, we demonstrate that these assumptions are not invariably validacross species, and that somatic condition of Atlantic bluefin tuna Thunnus thynnus in the Gulf ofMaine declined in the presence of high prey abundance. We show that the paradox of decliningbluefin tuna condition during a period of high prey abundance is explained by a change in the sizestructure of their prey. Specifically, we identified strong correlations between bluefin tuna bodycondition, the relative abundance of large Atlantic herring Clupea harengus , and the energeticpayoff resulting from consuming different sizes of herring. This correlation is consistent withoptimal foraging theory, explaining why bluefin tuna condition suffers even when prey is abundant.Furthermore, optimal foraging principles explain a shift in traditional bluefin tuna foraging areas,toward regions with a higher proportion of large herring. Bluefin tuna appear sensitive to changesin the size spectrum of prey rather than prey abundance, impacting their distribution, reproductionand economic value. Fisheries managers will now face the challenge of how to manage for highabundance of small pelagic fish, which benefits benthic fishes and mammalian predators, andmaintain a robust size structure beneficial for top predators with alternative foraging strategies.KEY WORDS: Bluefin tuna · Herring · Optimal foraging · Condition ·

International audience; Fisheries bycatch is considered to be one of the most significant causes of mortality for many marine species, including vulnerable megafauna. In the open ocean, tuna purse seiners are known to use several cetacean species to detect tuna schools. This exposes the cetaceans to encirclement which can lead to incidental injury or death. While interactions between fishers and cetaceans have been well documented in the eastern tropical Pacific Ocean, little is known about these interactions and potential mortalities in the tropical Atlantic and Indian Oceans. Here, we provide the first quantification of these interactions in both oceans by analyzing a large database of captain’s logbooks (1980 to 2011) and observations collected by onboard scientific observers (1995 to 2011). Distribution maps of sightings per unit effort highlighted main areas of relatively high co-occurrence: east of the Seychelles (December to March), the Mozambique Channel (April to May) and the offshore waters of Gabon (April to September). The percentage of cetacean-associated fishing sets was around 3% in both oceans and datasets whereas 0.6% of sets had cetaceans encircled. Of the 194 cetaceans encircled in a purse seine net (122 baleen whales, 72 delphinids), immediate apparent survival rates were high (Atlantic: 92%, Indian: 100%). Among recorded mortalities, 8 involved pantropical spotted dolphins Stenella attenuata and 3 involved humpback whales Megaptera novaeangliae. These high survival rates suggest that setting nets close to cetaceans has a low immediate apparent impact on the species involved. Our findings will contribute to the development of an ecosystem approach to managing fisheries and accurate cetacean conservation measures.

Juvenile silky sharks Carcharhinus falciformis comprise the largest component of the incidental elasmobranch catch taken in tropical tuna purse seine fisheries. During a chartered cruise on board a tuna purse seine vessel conducting typical fishing operations we investigated the post-release survival and rates of interaction with fishing gear of incidentally captured silky sharks using a combination of satellite linked pop-up tags and blood chemistry analysis. To identify trends in survival probability and the point in the fishing interaction when sharks sustain the injuries that lead to mortality, sharks were sampled during every stage of the fishing procedure. The total mortality rates of silky sharks captured in purse seine gear was found to exceed 84%. We found survival to precipitously decline once the silky sharks had been confined in the sack portion of the net just prior to loading. Additionally, shark interactions recorded by the scientists were markedly higher than those recorded by vessel officers and the fishery observer. Future efforts to reduce the impact of purse seine fishing on silky shark populations should be focused on avoidance or releasing sharks while they are still free swimming.

Physical barriers can have profound impacts on dispersal in marine species. Here, we investigate population structure and levels of relatedness among individuals of the coral reef associated fish, the dogtooth tuna Gymnosarda unicolor, collected from 15 sites across the Indo-West Pacific region. We screened 92 individuals for genetic variation at 13 nuclear microsatellite loci and the cytochrome c oxidase subunit 1 (COI) mitochondrial DNA (mtDNA) gene. We detected no genetic differentiation between ocean basins or between sites within ocean basins, suggesting G. unicolor possesses a highly mobile larval or juvenile stage. In addition, the lack of deep evolutionary mtDNA divergences suggests gene flow was also not limited historically. However, comparisons of relatedness between pairs of individuals revealed that individuals collected from the same site were more related to each other, on average, than individuals collected from different sites. Such patterns are consistent with chaotic genetic patchiness, a possible consequence of high variance in reproductive success and patchy, local recruitment.

Fishers have exploited the associative behavior displayed by several pelagic fish species with floating objects for decades, through the use of man-made fish aggregating devices (FADs), which facilitate the capture of such species. However, our understanding of this associative behavior and its adaptive value is poor and the scientific community is ill-equipped to provide fishery managers with science-based recommendations on the impacts of FADs on ecosystems. In an array of 13 anchored FADs around Oahu, Hawaii, USA, 72 yellowfin tuna Thunnus albacares were equipped with internal acoustic tags, which facilitated the continuous monitored of their presence and absence around each FAD using automated acoustic receivers. Data were analyzed using survival curves with the objective of determining the behavioral dynamics of fish joining and leaving the FADs. Residence times at FADs were characterized by 4 behavioral modes: briefly passing near a FAD (average 13.1 min), short association (average 2.9 d), and 2 long association behaviors (13.8 and 23.2 d, respectively). Statistical analyses suggest that different behavioral modes were likely dependent upon local conditions around the FAD at a given time (environmental factors or social interactions). We observed 2 behavioral modes for absence times from FADs: short (2.8 d) and long (infinite). More importantly, individuals exhibited behavioral variability, switching between short and long residence times at FADs. This suggests that large pelagic fish can display a range of behavioral responses while in an array of FADs, challenging the common hypothesis of a single behavioral pattern, which could ultimately lead to an ecological trap. Survival curves were best fitted with exponential models, suggesting that underlying behavioral processes were time independent.

We thank the purse-seine vessel owners for access to log-book data and to the Indian Ocean Tuna Commission and the Institut de Recherche pour le Developpement (France), in particular A. Fonteneau, M. Herrera and R. Pianet for data extraction and D. Rowat for data availability and help with interpretation. Our study was funded by Fundacao para a Ciencia e Tecnologia and European Social Funds through A.M.M.S.'s PhD scholarship (SFRH/BD/47465/2008), The University of Adelaide, Australian Institute of Marine Science, Flying Sharks and the Portuguese Association for the Study and Conservation of Elasmobranchs.

The association of yellowfin tuna and pantropical spotted dolphins in the eastern tropical Pacific Ocean (ETP) has been exploited by tuna fishermen and has intrigued scientists for decades, yet we still have questions about what the benefits of the association are—whether the association is obligatory or facultative, why the tuna are most often found with spotted dolphins, and why the species associate most strongly in the ETP. We review the hypotheses that have been proposed to explain the bond and present results from 3 studies conducted to address these hypotheses: a simultaneous tracking study of spotted dolphins and yellowfin tuna, a trophic inter- actions study comparing their prey and daily foraging patterns, and a spatial study of oceano- graphic features correlated with the tuna-dolphin association. These studies demonstrate that the association is neither permanent nor obligatory and that the benefits of the association are not based on feeding advantages. These studies do support the hypothesis that one or both species reduce the risk of predation by forming large, mixed-species groups. The association is most prevalent where the habitat of the tuna is compressed to the warm, shallow, surface waters of the mixed layer by the oxygen minimum zone, a thick layer of oxygen-poor waters underlying the mixed layer. The association has been observed in other oceans with similar oceanographic con- ditions, but it is most prevalent and consistent in the ETP, where the oxygen minimum zone is the most hypoxic and extensive in the world.

Pacific bluefin tuna Thunnus orientalis has 2 spawning grounds with different spawning seasons. One is in the northwest region of the Philippine Sea during May to June and the other is in the Japan Sea from late July to August. The oxygen isotope level in otoliths is affected by water oxygen isotope composition and negatively correlated to the ambient water temperature. We developed a technique using the temporal profiles of otolith δ18O to discriminate the natal origin of Pacific bluefin tuna. For Age-0 tuna caught in the Japan Sea (n = 35), the δ18O values varied from –2.5 to –2.2‰ between the otolith core and a distance of ca. 600 μm from the core, and then the values increased (enriched) to about –0.8 to –1‰ at 700 to 1000 μm. For Age-0 tuna caught in the Pacific Ocean (n = 41), the otolith δ18O values varied between –2.8 and –2.2‰ at 0 to 1000 μm from the otolith core and then increased to ca. –1.5 to –1‰ at around 1200 to 1600 μm from the core. The different profiles of otolith δ18O values between tuna caught in the Pacific Ocean and the Japan Sea evidently reflect different water temperatures along the migratory route from their natal origin. Fish hatched in the Japan Sea experience water temperatures of

The relationship between pelagic fish and ocean temperature is cited in many studies, the majority of which investigate correlations of pelagic species and sea surface temperatures (SST). While appropriate for surface-associated species, this may not be accurate for deep-diving fishes. A different way to examine this relationship is to model spatial autocorrelation of fish species and temperatures at an appropriate range of depths. Spatial autocorrelation, the distance at which data are interdependent, is a potential descriptor of the patch size of an organism. Here we modeled spa- tial autocorrelation for 5 pelagic species that inhabit different depths in the Gulf of Mexico: dolphin- fish Coryphaena hippurus, wahoo Acanthocybium solandri, yellowfin tuna Thunnus albacares, swordfish Xiphias gladius, and bigeye tuna Thunnus obesus. Additionally, we modeled spatial auto- correlation for ocean temperatures at the surface, at 200, and 400 m. We hypothesized that autocor- relation distances will be greater for deeper water temperatures and for species that live at deeper depths due to greater homogeneity of deep waters over greater spatial ranges. Results show average distances of autocorrelation on the order of 55 to 60 km for wahoo and dolphinfish, 90 km for yel- lowfin tuna, and 135 to 145 km for swordfish and bigeye tuna; the same data for temperature were 75, 135, and 300 km for SST, and at 200 and 400 m depth, respectively. Autocorrelation distances of dolphinfish, wahoo, and yellowfin were correlated with that of of SST, while the autocorrelation dis- tance of swordfish and bigeye were correlated with that of temperatures at 200 m. Results suggest that autocorrelation distances may be useful as a proxy for habitat delineation.

Macroalgae are modular organisms that can express different morphologies depending on the environment to which they are exposed. Their growth under varying light, temperature, and nutrient regimes, interacting with disturbance factors such as herbivory and hydrodynamics, leads to particular morphological types. The present study illustrates the potential of using morphological variations of benthic modular and/or clonal organisms as indicators of the factors and processes influ- encing them in their particular location. The morphogenetic agent-based model SPREAD (spatially explicit reef algae dynamics) was used to determine the range of potential morphological types in 3 dominant macroalgal species (Halimeda tuna, H. opuntia, and a species of Dictyota) in the Florida Reef Tract. Simulations of growth under a range of light, nutrient, and disturbance conditions simi- larly found at inshore patch and offshore bank reefs led to 6 potential morphological types for H. tuna, 2 for H. opuntia, and 3 for a species of Dictyota. From these potential sets derived from the model, we observed that particular morphological types corresponded to the morphologies found in the 2 reef habitat types. The simulated conditions that led to the formation of these morphologies in the model were similar to the environmental conditions at these sites. In addition to relating combinations of environmental and disturbance factors to macroalgal growth morphologies, the present study pro- vides insights into the differing life-history strategies among the species, and the adaptive value of plasticity expressed by these macroalgae. The morphologies of the successful fragmenters H. opun- tia and a generalized species of Dictyota were more influenced by disturbance. On the other hand, the morphology of the less successful fragmenter H. tuna was strongly influenced by the growth factors of light and nutrients.

Stable carbon and oxygen isotopes in the otoliths of Atlantic bluefin tuna Thunnus thynnus were used to investigate trans-Atlantic movement and natal homing behaviors. Otolith δ 13C and δ 18O of yearling bluefin tuna from 6 different year classes (1999 to 2004) were characterized to validate the approach and assess inter-annual variation in these markers for individuals from eastern (Mediterranean Sea/eastern Atlantic Ocean) and western (USA Atlantic Ocean) nurseries. Otolith δ 18O of yearlings from the eastern nursery were enriched relative to their western counterparts, while otolith δ 13C was relatively similar between nurseries. Temporal variability in otolith δ 13C and δ 18O of bluefin tuna was observed ; nevertheless, classification success to eastern and western nurseries was relatively high (87%) when all year classes were pooled. Otolith δ 13C and δ 18O of yearlings were then used as a baseline sample (‘ eastern’ and ‘ western’ nursery signature) to predict the origin of adolescents (20 to 70 kg) from 1 foraging area (USA Atlantic Ocean) and adults (>140 kg) from 1 spawning ground (Mediterranean Sea) based on otolith core signatures corresponding to yearling period). Mixed-stock analysis indicated that approximately 60% of the adolescent bluefin tuna collected from foraging areas in the USA Atlantic Ocean originated from the eastern nursery, suggesting that substantial trans-Atlantic movement of adolescents from east to west occurred. In addition, natal homing was well developed, with 94% of the adult bluefin tuna collected in the Mediterranean Sea derived from the eastern nursery. Findings from this study suggest that USA fisheries depend upon migrants of Mediterranean origin and that mixing across the 45°W management boundary is substantially higher than assumed previously.

In the Northeast Atlantic, off the Bay of Biscay, the pelagic top predator community is mainly composed of the blue shark Prionace glauca, the swordfish Xiphias gladius, the albacore tuna Thunnus alalunga, the common dolphin Delphinus delphis and the striped dolphin Stenella coeru- laeoalba. The present study is aimed at determining the patterns of feeding niche segregation among this oceanic top predator assemblage. Overlaps were measured in terms of prey taxa and prey sizes. Preferred foraging depth ranges and diel patterns were inferred from prey compositions and diges- tion conditions. In terms of prey taxa, the blue shark, the albacore and the swordfish segregated fairly well from each other and from the 2 dolphins, whereas the 2 dolphins showed considerable overlap. In terms of prey sizes, substantial overlap was found between the blue shark and the swordfish, but these predators differed from the 2 dolphins and the albacore, which, in turn, overlapped consider- ably. Spatio-temporally, the blue shark and the swordfish appeared to be predominantly diurnal mesopelagic predators, while the albacore and the dolphins were mostly nocturnal epipelagic feed- ers. Prey diversity was higher in the 2 dolphins, which also showed a lower interindividual variabil- ity in stomach content composition. The 2 dolphins have the highest energy needs and are bound to the surface for breathing; the albacore is also bound to the surface layer for physiological reasons (swim bladder development and body temperature control): all 3 rely on small gregarious epi- to ver- tically migrating mesopelagic prey species of high energy content. The swordfish and the blue shark have much lower energy needs and are not restricted to the surface layer; they are better able to for- age on scattered, deep-living, large-size and low-energy prey. Hence, within the whole community, the energetics of predation and constraints relative to the sea surface are the main structuring factors, and not the relationship between predator size and prey size.

It has been suggested that seamounts hold higher abundances of some 'visiting' ani- mals, such as tuna, sharks, billfishes, marine mammals, sea-turtles and even seabirds, but this has been based on sparse records, and warrants further examination. In this paper we use data from a fishery observer program to examine whether the predicted higher abundances of tuna, marine mammals, sea turtles and seabirds actually occur around Azores seamounts and to map the distribu- tion of the various species. Our results indicate that some marine predators (skipjack Katsuwonus pelamis and bigeye tuna Thunnus obesus, common dolphin Delphinus delphis and Cory's shearwater Calonectris diomedea borealis) were significantly more abundant in the vicinity of some shallow- water seamount summits. Our methodology, however, failed to demonstrate a seamount association for bottlenose dolphins Tursiops truncatus, spotted dolphin Stenella frontalis, sperm whale Physeter macrocephalus, terns Sterna hirundo and S. dougalli, yellow-legged gull Larus cachinnans atlantis and loggerhead sea turtles Caretta caretta. Seamounts may act as feeding stations for some of these visitors. Not all seamounts, however, seemed to be equally important for these associations. Only seamounts shallower than 400 m depth showed significant aggregation effects. These seamounts may be considered hotspots of marine life in the Azores, and a special effort should be made in order to ensure a sustainable management of these habitats.

Although tunas and billfishes are of substantial economic importance and conservation concern, global patterns of diversity and distribution remain poorly understood. Many species are highly migratory and able to tolerate a wide thermal range. In the present study, ambient water temperature data for 18 species of tuna and billfish from 190 literature sources were combined according to geographical location. An empirical modelling approach was used to relate temperature tolerances of tunas and billfishes to their global diversity patterns. Mean preferred and tolerated temperature ranges were calculated for each species in the adult and juvenile life stages. Mean tolerance data were then overlaid in order to fit models relating the species richness of tunas and billfishes to ambient water temperature. The best-fit model was used in conjunction with gridded water temperature data to predict global species richness patterns. Cumulative species richness pre- dictions from water temperature data were positively correlated with observed longline-derived rich- ness data (r = 0.577, p < 0.0001). Diversity consistently peaked at intermediate latitudes (10 to 35° N and S) in a manner similar to other pelagic taxa. This analysis provides evidence that the ambient water temperature tolerances of tunas and billfishes can be used to predict broad species richness patterns on a global scale.

The Strait of Gibraltar, the only passage between the Mediterranean Sea and the Atlantic Ocean, and characterised by a surface inflow of Atlantic waters and a deep outflow of Mediterranean waters, is inhabited by a large number of cetacean species. The present study focuses on the occurrence and the spatial distribution of cetacean species within the strait in relation to oceanographic features. Shipboard visual surveys were conducted during the summers of 2001 to 2004, covering 4926 km. A total of 616 sightings of 7 cetacean species were made. The spatial distri- butions of 6 species (short-beaked common dolphins Delphinus delphis, striped dolphins Stenella coeruleoalba, bottlenose dolphins Tursiops truncatus, long-finned pilot whales Globicephala melas, sperm whales Physeter macrocephalus and killer whales Orcinus orca) were examined with respect to depth and slope. The analyses indicate that these species could be ordered into 3 groups. A first group, with a northward tendency, is composed of common and striped dolphins. Due to its at-sea location and feeding habits, this group is likely to feed on mesopelagic fishes or squids associated with the surface Atlantic waters. The second group, constituted by bottlenose dolphins, long-finned pilot whales and sperm whales, is mainly found over the deep waters of the central part of the strait. While the foraging ecology of bottlenose dolphins is still unclear, both sperm whales and pilot whales are most likely to feed on squids occurring in deep Mediterranean waters. The third group, formed by killer whales Orcinus orca, was associated with blue fin tuna Thunnus thynnus fisheries in the southwestern part of the strait.

Killer whales Orcinus orca occur in the area of the Strait of Gibraltar, where they prey on migrating bluefin tuna Thunnus thynnus. In the spring, killer whales were observed to chase tuna for up to 30 min at a relatively high sustained speed (3.7 ± 0.2 m s -1 ) until they captured them. Using simple models based on previous locomotor performance data on killer whales and thunnids, we investigated the hypothesis that killer whales push tuna beyond their aerobic limits to exhaust and capture them. To test this hypothesis, the endurance of bluefin tuna was estimated from data on maximum burst and aerobic swimming available for bluefin and yellowfin tuna T. albacares. The endurance performance of killer whales was evaluated on the basis of the maximal rate of oxygen uptake during exercise (VO2max). We modelled the maximum aerobic power output for a killer whale according to swimming speed using a VO2max ranging between 20 and 30 ml O2 kg -1 min -1 . The out- put of this model was compared to the observed sustained swimming speed of killer whales chasing prey over long durations. Our results support the hypothesis that killer whales may use an endurance-exhaustion technique to catch small to medium sized (up to 0.8 to 1.5 m) bluefin tuna, while larger tuna may be inaccessible to killer whales unless they use cooperative hunting tech- niques or benefit through depredation of fish caught on long lines, drop lines or trap nets.

Various air-breathing marine vertebrates such as seals, turtles and seabirds show dis- tinct patterns of diving behaviour. For fish, the distinction between different vertical behaviours is often less clear-cut, as there are no surface intervals to differentiate between dives. Using data from acoustic tags (n = 23) and archival depth recorders attached to cod Gadus morhua (n = 92) in the southern North Sea, we developed a quantitative method of classifying vertical movements in order to facilitate an objective comparison of the behaviour of different individuals. This method expands the utilisation of data from data storage tags, with the potential for a better understanding of fish behaviour and enhanced individual based behaviour for improved ecosystem modelling. We found that cod were closely associated with the seabed for 90% of the time, although they showed distinct seasonal and spatial patterns in behaviour. For example, cod tagged in the southern North Sea exhib- ited high rates of vertical movement in spring and autumn that were probably associated with migra- tion, while the vertical movements of resident cod in other areas were much less extensive and were probably related to foraging or spawning behaviours. The full reasons underlying spatial and tempo- ral behavioural plasticity by cod in the North Sea warrant further investigation.

Comparative histological and stereological analysis of gonads from longline- and purse seine-caught Atlantic bluefin tuna Thunnus thynnus was undertaken to assess the reproductive potential of the population in the western Mediterranean breeding area. Bluefin tuna sampled by purse seine clearly had higher gonadosomatic index values (GSI) than those caught by longline. While no remarkable histological differences were found between purse seine- and longline-caught males, the ovarian histology indicated a higher level of maturity in females from purse-seine samples, where most specimens were found to be in active spawning (AS). Within the longline sample the pro- portion of active spawning females was considerably higher at the end of the fishing season (mid- June to mid-July) than earlier in May to mid-June (52.8 vs. 24%). Stereological analyses revealed that the samples collected by the 2 sampling gears differed primarily in the numbers of migratory- nucleus oocytes, which were overwhelmingly more abundant in purse-seine catches, with average relative fecundity: 59 oocytes g -1 vs. 1.2 oocytes g -1 (purse-seine vs. longline). The frequency of ovaries with postovulatory follicles (POFs) in purse seine-caught bluefin tuna was 83.7%, equivalent to a spawning interval of 1.2 d; in contrast, only 32.6% of the females in longline catches had POFs, representing a spawning interval of 3.1 d. Assuming an average individual duration of the spawning process of 14 d, and considering the estimates of fecundity and spawning interval obtained from purse-seine samples, the average annual fecundity of bluefin tuna in the Balearic spawning ground would be approximately 77 × 10 6 eggs (relative fecundity of 702 eggs g -1 BW (body weight)).

The larvae of tropical pelagic and reef fishes were sampled from October 1995 to August 1997 using a very large fry pelagic trawl at sites between 4 and 20° S and 134 and 154° W in the region of the Society, Tuamotu and Marquesas Archipelagos in French Polynesia. Sites were selected on the basis of longline catches of adult tuna (a predator of larval reef fishes) and acoustic surveys of micronekton to a depth of 600 m. A total of 3369 larvae were collected by 93 net tows. Most were late-stage larvae representing 29 taxa, of which 26 were predominantly reef fishes. Samples were dominated by acanthurids (43%), pleuronectiform larvae, ostraciids, fistularids, balistids, holo- centrids, chaetodontids, mullids and carangids. Distance to reefs was the principal factor determining clustering patterns in classification and ordination analyses, and offshore samples (>150 km from reefs) were dominated by pelagic species, while collections nearer to reefs were dominated by coral reef species. There was a linear decline in richness of samples with increasing distance from reefs and an exponential decline in abundance, so that few reef fish larvae were collected >300 km from adult reef habitats. However, abundances of up to hundreds of reef fish larvae per hour of net tow were collected >100 km from the nearest reef. Our results show that relatively little connectivity is likely among reef systems separated by >300 km in French Polynesia, but substantial exchange of larvae within a restricted range of reef fish taxa might occur at smaller spatial scales.

To estimate the survival process of Pacific bluefin tuna Thunnus orientalis during the larval period, estimated growth histories were compared between larvae collected in late spring and juveniles collected in the boreal summer of 2004, which were considered to be survivors of the larval cohorts. Larval tuna (3.3 to 9.6 mm standard length, SL) were collected from mid-May to early June around the Ryukyu Islands, northwestern Pacific Ocean, and juvenile tuna were collected offshore of Kochi and Nagasaki prefectures in July-August. Preflexion, flexion and postflexion larvae were col- lected, and their ages ranged from 4 to 18 d. Back-calculated SLs by the biological intercept method showed that larval tuna in the postflexion phase were larger-at-age than preflexion and flexion larvae, suggesting that only larger and faster growing larvae were able to survive to the postflexion phase. The logarithms of otolith radii (ln OR: proportional to SL) of larvae with slower growth and development were smaller than the minimum ln OR of surviving juvenile tuna, which indicated the smallest possible size required for larvae to successfully recruit to the fishery. These results indicate that the survival of larvae of Pacific bluefin tuna depends largely on size and growth rates during early life history.

Spatial and temporal variability in mean age, breeding season and mean body length were examined for female short-beaked common dolphins Delphinus delphis in the eastern tropical Pacific (ETP). Specimens were obtained from central stock D. delphis incidentally killed in the tuna purse-seine fishery between 1973 and 1993. To examine spatial variability in life history parameters, oceanographic variables were used to characterize 2 primary habitats within the range of central stock animals: the Costa Rica Dome (CRD) and the surrounding areas of the CRD. Mean age of mature females did not vary through time or between habitats, and calving occurred throughout the year in both habitats. However, mean body length of D. delphis sampled 'on' the CRD was 10 cm shorter than those sampled 'off' the CRD during the years 1973-1974 but was not significantly different between the 2 habitats in the years 1988-1989. The difference between spatial groups in 1973-1974 is likely attributed to animals moving within the region in response to changes in their environment. That is, animal size and location is consistent with southern stock D. delphis moving into the higher quality habitat of the CRD area during the strong El Nino of 1972-1973 when preferred habitat was likely reduced in the area that the southern stock inhabits. Our evidence of animal movement suggests animals cross stock boundaries during periods of significant environmen- tal change. This result has implications for development and implementation of management plans and estimation of life history parameters for D. delphis inhabiting the ETP.

Electronic tags have enhanced our understanding of the movements and behavior of pelagic animals by providing position information from the Argos system satellites or by geolocation estimates using light levels and/or sea surface temperatures (SSTs). The ability to geolocate animals that remain submerged is of great value to fisheries management, but the accuracy of these geoloca- tion estimates has to be validated on free-swimming animals. In this paper, we report double-tagging experiments on free-swimming salmon sharks Lamna ditropis and blue sharks Prionace glauca, tagged with satellite telemetry and pop-up satellite tags, which provide a direct comparison between Argos positions and geolocation estimates derived from light levels and SSTs. In addition, the Argos- based pop-up satellite tag endpoints and GPS-based recapture locations of Atlantic bluefin tunas Thunnus thynnus were compared with the last geolocation estimates from pop-up satellite and archival tags. In the double-tagging experiments, the root mean square errors of the light level lon- gitude estimates were 0.89 and 0.55°; while for SST latitude estimates, the root mean square errors were 1.47 and 1.16° for salmon sharks and blue sharks respectively. Geolocation estimates of Atlantic bluefin tuna, using archival data from surgically implanted archival tags or recovered pop-up satel- lite tags, had root mean square errors of 0.78 and 0.90° for light level longitude and SST latitude esti- mates, respectively. Using data transmitted by pop-up satellite tags deployed on Atlantic bluefin tunas, the light level longitude and SST latitude estimates had root mean square errors of 1.30 and 1.89°, respectively. In addition, a series of computer simulations were performed to examine which variables were most likely to influence the accuracy of SST latitude estimates. The simulations indi- cated that the difference between the SST measured by the electronic tag and the remotely sensed SST at a given location was the predominant influence on the accuracy of SST latitude estimates. These results demonstrate that tag-measured SSTs can be used in conjunction with light level data to significantly improve the geolocation estimates from electronic tags.

Analysis of data on the hydrodynamics of swimming by 100 species, ranging in body mass (M) from bacteria to blue whales, leads to a model of animal-induced turbulence in the ocean. Swimming speeds and Reynolds number (Re) are strongly correlated with body mass, both at typical cruising speeds and at escape speeds associated with predator-prey interactions. We find that animals operating at Re > 1000 typically form schools that are concentrated by many orders of magnitude above their average abundance. We calculate the rate of kinetic energy production by 11 representative species of schooling animals ranging in size from euphausiids to whales, and find it to be of the order of 10 -5 W kg -1 , regardless of animal size. Animal-induced turbulence is comparable in magnitude to rates of turbulent energy dissipation (e) that result from major storms. The horizontal length scale (10 to 1000 m) of energy production rate by animal schools is comparable to the observed fine-scale variability in e. We present detailed case studies of 4 species — Atlantic bluefin tuna, Norwegian herring, northern anchovy and Antarctic krill — all of which have schooling behavior that places them within the zone of maximum seasonal stratification where their energy production rate would be 3 to 4 orders of magnitude greater than the background average rate of turbulent energy dissipation. We conclude that schooling animals are an important source of fine-scale turbulent mixing in the ocean, especially in coastal regions during summer.

We present an analysis of the distribution of bluefin tuna Thunnus thynnus schools spotted during aerial surveys in the Gulf of Lions, in relation to oceanographic features. Bio-optical and thermal properties of the sea surface derived from high-resolution sensors (AVHRR and SeaWiFS) were studied on a daily basis, and an edge-detection technique was applied to detect frontal zones. Geostatistics and point-process analyses were used to evaluate the role of the environment in structuring the spatial pattern of bluefin tuna (BFT). The distribution of schools spotted was strongly non-stationary both in space and time; this is believed to be an effect of the survey design (transect sampling) and the influence of transient oceanographic structures (surface fronts and eddies). The empirical variograms indicated a spatial range of the BFT schools at around 40 km, with substantial daily variability. Ripley's K statistic, as well as autocorrelation plots, revealed that the fish schools were clustered over a wider range of scales (from 10 to 80 km), indicating more spatial structure than would be expected from a random process. Finally, BFT school distributions appeared well determined by the oceanic features, except at very small scales (40 km), where over-spreading was detected. Dynamical ecological processes, such as foraging, are likely to induce this complex spatial pattern. Possible reasons for the association of tuna with fronts are presented.

Our 10 yr study is the first to associate seabirds with oceanic boundaries, or fronts, in tropical oceans. It addresses the semi-permanent Equatorial Front, hereafter 'front', a vertical bound- ary lying between the South Equatorial Current (SEC) and North Equatorial Countercurrent (NECC) in the eastern tropical Pacific (ETP). We also consider the subsurface, horizontal thermal boundary created by the thermocline. Our study area included the front and the north and south boundaries of the SEC and NECC respectively, between 100 and 145°W. Our data were based on 45 north-south crossings of this area. The intensity of the front differed with season and phase of the El Nino South- ern Oscillation (ENSO). Densities of planktivorous seabirds, higher at the front than in the SEC or NECC, increased with front intensity (i.e., during autumn/La Nina). Densities of piscivorous seabirds were unaffected by the front, but were related to depth and intensity of the thermocline. Piscivore densities were highest in the NECC, and higher during spring than autumn. Furthermore, they were higher in the western part of the study area, i.e., when and where the thermocline was deepest and most stratified. We suggest that the difference in patterns observed for the tropical planktivorous ver- sus piscivorous seabirds reflects processes that affect the distribution and availability of their respec- tive prey. Zooplankton are concentrated in the surface layer by physical processes that structure the front. Prey fish, on the other hand, are made available by foraging tuna, which drive them to the ocean surface. As a result, the distribution of the piscivorous seabirds is closely tied to that of school- ing surface-feeding tunas, that are most abundant in the NECC. Other studies have shown that tunas spend most of their time near the thermocline. We suggest that this, along with the patterns observed among the piscivorous seabirds, indicate that the tuna's prey aggregate near the thermocline. Specif- ically, most tropical squid and small fishes perform vertical movements to the surface at night. A deeper thermocline, such as that of the NECC, is likely to be attractive to these animals for hiding during the day (when the tuna and birds forage) because of the lack of light at depth, and because of the enriched O2 content of the cold-water layer lying immediately below the warmer surface layer. Hence, by searching on a horizontal plane near the boundary created by a deep, well stratified ther- mal structure, schooling tuna, and thus piscivorous seabirds, are likely to find concentrations of prey more readily than equally abundant prey dispersed throughout a less stratified water column.

Otolith chemistry of juvenile Pacific bluefin tuna Thunnus orientalis was measured to assess differences in composition among 3 nursery areas in the North Pacific Ocean: East China Sea, Sea of Japan and the Pacific Ocean off Shikoku. Six elements (Li, Mg, Ca, Mn, Sr and Ba) were mea- sured in whole otoliths using solution-based inductively coupled plasma mass spectrometry. Univari- ate contrasts of T. orientalis otoliths collected in 1994 and 1995 indicated that concentrations of 5 ele- ments (Li, Mg, Ca, Mn, Sr) differed among nurseries. Concentrations of Ca and Sr were significantly higher in the Pacific Ocean than in either marginal sea (East China Sea or Sea of Japan) nursery, while concentrations of Li, Mg and Mn were higher in fishes inhabiting marginal seas. Discriminant analysis showed clear separation of elemental fingerprints between Pacific Ocean and marginal sea nurseries, and to a lesser degree separation between the 2 marginal sea groups. Temporal stability of the elemental fingerprint was examined over a 3 yr period (1995 to 1997) in the East China Sea. Significant interannual trends were observed for 3 elements (Mg, Mn and Ba); however, elemental fingerprints of T. orientalis from the Pacific Ocean nursery were markedly different from all year- classes in the East China Sea.

Immature Pacific bluefin tuna Thunnus thynnus orientalis, marked with an archival tag, were released near Tsushima Island in the eastern East China Sea (58 individuals on 7 to 14 Decem- ber 1995, 47 individuals on 29 November 1996). Of these, 15 were recovered by the following sum- mer period. From the tags retrieved, we analyzed time-series data on swimming depth and ambient temperature recorded every 128 s. Our main objectives were to determine diurnal and seasonal changes in the swimming depth of bluefin tuna and examine the effect of ambient temperature on their vertical distribution and movement. In winter, the bluefin swam within the surface mixed layer at shallower depths during the night than during the day; swimming depth displayed diel periodicity. Some individuals displaying marked diel periodicity tended to migrate further offshore (direction southwest), where greater temperature gradients in the vertical water column were observed. This indicates that diel vertical migration of bluefin tuna is a direct response to changes in the thermo- cline. However, as the thermocline developed further in summer, the bluefin spent most of their time at the surface, suggesting that bluefin avoid rapid temperature change at the thermocline by making behavioral thermoregulation. During the summer, however, the fish made frequent dives through the thermocline during the day, and diel periodicity of swimming depth was more prominent than that in winter. The purpose of this movement is considered to be foraging. In conclusion, spatial and sea- sonal changes of the vertical structure of ambient water temperature have a great effect on the ver- tical distribution and movement of the bluefin tuna.

Mercury, cadmium, zinc and copper concentrations were analysed in the liver and kidney tissues of 14 species of albatross and petrel. These birds were obtained as by-catch of the long-line tuna fishing industry in New Zealand waters, and provided a unique opportunity to compare heavy metal accumulation in a group of closely related species. Mercury levels in the liver of the wandering and royal albatrosses were among the highest recorded for free-living birds. In multiple regression analyses, much of the inter-specific variation in cadmium and mercury levels was related to the importance of Crustacea in the diet, to phylogeny, or to the duration of the moult cycle. Species in which crustacea constituted >33 % of the diet had significantly lower cadmium concentrations in liver tissues, and mercury concentrations in both liver and kidney tissues, than those in which birds consumed mainly or entirely squid and fish. This accords reasonably well with information on relative mercury and cadmium content of prey species. After accounting for dietary variation, Procellariidae (petrels, shearwaters and prions) and Hydrobatidae (storm petrels) still exhibited higher cadmium concentrations in the liver than Diomedeidae (albatrosses). In addition, albatrosses which took more than a year to moult accumulated higher mercury concentrations in their livers, probably because of a restricted ability to excrete mercury into growing feathers.

Tuna distribution may be directly linked to food availability, the density of which can be assessed by echo-sounding. A study was performed in the French Polynesian EEZ (Economic Exclusive Zone) between 1995 and 1997 with a 38 kHz echo sounder working down to 500 m depth. With the settings used, acoustic back-scattered energy may be considered as representative of micronektonic fish biomass and provided information on horizontal and vertical micronekton variabilities and the structure of the echograms. Three different zones were defined by multivariate analysis. The classical biomass decreasing gradient from the equator to higher latitudes did not appear. Instead, the maximum micronekton abundance was found between Marquesas Archipelagos and a west-northwest/east-southeast oriented line stretching between 11 and 14°S, i.e. in a weak convergence, favourable to micronekton development due to the concentration of lower trophic levels with no oxygen limitation in the deep layers. Two zones with very different hydrological features, but with comparable micronektonic abundances, surround the richest micronekton zone. To the north, waters are enriched by the equatorial upwelling, but intense organic matter remineralisation limits oxygen availability under the thermocline. To the south, waters are influenced by the great southern gyre and present oligotrophic characteristics less favourable to micronekton development. The present results suggest that prediction of tuna forage distribution should take not only trophic parameters into account, but also environmental ones. They also suggest that echo-sounding data should be used more extensively in the validation of models predicting tuna forage.

A total of 52 sailfish Istiophorus platypterus from Queensland (Australia) coastal waters were examined for parasites which might provide information on the relationships and movements of fish from different areas. Sailfish from 4 locations between Cape Moreton in southern Queensland and Dunk Island in the north were dissected from 1987 to 1989. Of the 36 parasite taxa which were identified from sailfish, 22 were new host records. Both ordination and classification strategies applied to a combination of 8 long-lived and 8 short-lived parasite taxa indicated different histories of movement for fish from northern and southern locations. The distributions of 2 trypanorhynchs, Callitetrarhynchus gracilis and Otobothrium d~psacum, a copepod, Pennella jnstructa, and a sanguinicolid, Card~cola grandis, were primarily responsible for discriminating these groups of fish. Analyses of the parasite data from adjacent fishing seasons at Cape Moreton (summer) and Cape Bowling Green (winter). which controlled for apparent interannual variability in parasite abundance, produced clear evidence of discreet subpopulations of sailfish from these locations. Sa~lfish from the Whitsunday Islands were all mature and, in instances, in spawning condition, whereas fish from other areas were either immature or non-active. Some of the Whitsunday fish had parasite faunas similar to those from northern fishing grounds while others were more similar to Cape Moreton fish. These combined data suggest that fish from the northern and southern grounds may mix in the reef waters of the Whitsundays when mature The results of analyses undertaken in t h ~ s tudy and a concurrent study (Speare 1994, Aust. J mar. Freshwat. Res. 45: 535-549) indicate the utility of parasites to discriminate between billfishes with different histories of movement.