Prevalence, intensity and pathology of the nasal parasite Nasicola hogansi in Atlantic bluefin tuna (Thunnus thynnus)

Ectoparasitic flatworms of Nasicola (Monogenoidea: Capsalidae), which infect nasal epithelium of true tunas (Thunnus spp.), are not well studied, nor have their impacts on the host's olfactory organ been evaluated. Infections of Nasicola hogansi on Atlantic bluefin tuna, Thunnus thynnus, were investigated with emphasis on the relationship between infection prevalence, abundance and mean intensity with bluefin tuna size, sex, body condition and capture month, as well as histopathological effects. Commercially caught Atlantic bluefin tuna (n = 161, 185-305 cm curved fork length) from the Gulf of Maine were sampled during June through August 2009 for infections by N. hogansi. A total of 247 specimens of N. hogansi were collected, with a prevalence of 45.3%, mean abundance of 1.57 (CI: 1.21-2.03) and mean intensity of 3.45 (CI: 2.91-4.22). Neither fish sex nor landing month had a significant effect on parasite parameters. Larger and better-conditioned Atlantic bluefin tuna had a higher mean intensity of infection. Pathology associated with infection by N. hogansi included extensive necrosis, sloughing of the nasal epithelium and associated inflammation of underlying connective tissues. Further epidemiological and pathological study of this host-parasite system is warranted since impaired olfaction, if present, could adversely affect spawning and migration of this top ocean predator.

Declining Mercury Concentrations in Bluefin Tuna Reflect Reduced Emissions to the North Atlantic Ocean

Tunas are apex predators in marine food webs that can accumulate mercury (Hg) to high concentrations and provide more Hg (∼40%) to the U.S population than any other source. We measured Hg concentrations in 1292 Atlantic bluefin tuna (ABFT, Thunnus thynnus) captured in the Northwest Atlantic from 2004 to 2012. ABFT Hg concentrations and variability increased nonlinearly with length, weight, and age, ranging from 0.25 to 3.15 mg kg^-1, and declined significantly at a rate of 0.018 ± 0.003 mg kg^-1 per year or 19% over an 8-year period from the 1990s to the early 2000s. Notably, this decrease parallels comparably reduced anthropogenic Hg emission rates in North America and North Atlantic atmospheric Hg⁰ concentrations during this period, suggesting that recent efforts to decrease atmospheric Hg loading have rapidly propagated up marine food webs to a commercially important species. This is the first evidence to suggest that emission reduction efforts have resulted in lower Hg concentrations in large, long-lived fish.

Sailfish migrations connect productive coastal areas in the West Atlantic Ocean

Isla Mujeres, Mexico is home to one of the most well-known aggregations of sailfish. Despite its fisheries prominence, little is known about this sailfish assemblage, or its relationship to other aggregation sites in the western Atlantic. In January 2012, April 2013 and 2014, we deployed 34 popup satellite archival tags on sailfish in order to study their behavior, population connectivity and biophysical interactions. Sailfish were monitored for up to one year, and displayed (1) predominantly shelf associated activity (2) occupancy of the Yucatán Current near Isla Mujeres for up to five months and (3) subsequent dispersals from the Yucatán to productive coastal areas in the Gulf of Mexico, the Caribbean Sea and along the South American coast. Tagged sailfish occupied a median temperature of 26.4°C (interquartile range, IQR = 2.5 °C; range = 12.3-33.3 °C) and median depth of 4.4 m (IQR = 19 m; range = 0-452 m). Diel activity was present and individuals made distinctive descents before sunrise and sunset. Tracking missions of sufficient duration (~1 year) revealed previously undetected connectivity between western Atlantic sailfish fisheries and pelagic longline catches, and highlighted how fishery independent tagging can improve understanding of sailfish migrations and behavior for assessment and management.

Discovery of a spawning ground reveals diverse migration strategies in Atlantic bluefin tuna (Thunnus thynnus)

Atlantic bluefin tuna are a symbol of both the conflict between preservationist and utilitarian views of top ocean predators, and the struggle to reach international consensus on the management of migratory species. Currently, Atlantic bluefin tuna are managed as an early-maturing eastern stock, which spawns in the Mediterranean Sea, and a late-maturing western stock, which spawns in the Gulf of Mexico. However, electronic tagging studies show that many bluefin tuna, assumed to be of a mature size, do not visit either spawning ground during the spawning season. Whether these fish are spawning in an alternate location, skip-spawning, or not spawning until an older age affects how vulnerable this species is to anthropogenic stressors including exploitation. We use larval collections to demonstrate a bluefin tuna spawning ground in the Slope Sea, between the Gulf Stream and northeast United States continental shelf. We contend that western Atlantic bluefin tuna have a differential spawning migration, with larger individuals spawning in the Gulf of Mexico, and smaller individuals spawning in the Slope Sea. The current life history model, which assumes only Gulf of Mexico spawning, overestimates age at maturity for the western stock. Furthermore, individual tuna occupy both the Slope Sea and Mediterranean Sea in separate years, contrary to the prevailing view that individuals exhibit complete spawning-site fidelity. Overall, this complexity of spawning migrations questions whether there is complete independence in the dynamics of eastern and western Atlantic bluefin tuna and leads to lower estimates of the vulnerability of this species to exploitation and other anthropogenic stressors.

Orientation behaviour of leatherback sea turtles within the North Atlantic subtropical gyre

Leatherback sea turtles (Dermochelys coriacea) travel thousands of kilometres between temperate feeding and tropical breeding/over-wintering grounds, with adult turtles able to pinpoint specific nesting beaches after multi-year absences. Their extensive migrations often occur in oceanic habitat where limited known sensory information is available to aid in orientation. Here, we examined the migratory orientation of adult male, adult female and subadult leatherbacks during their open-ocean movements within the North Atlantic subtropical gyre by analysing satellite-derived tracks from fifteen individuals over a 2-year period. To determine the turtles' true headings, we corrected the reconstructed tracks for current drift and found negligible differences between current-corrected and observed tracks within the gyre. Individual leatherback headings were remarkably consistent throughout the subtropical gyre, with turtles significantly oriented to the south-southeast. Adult leatherbacks of both sexes maintained similar mean headings and showed greater orientation precision overall. The consistent headings maintained by adult and subadult leatherbacks within the gyre suggest use of a common compass sense.

Atlantic bluefin tuna Thunnus thynnus feeding ecology in the northern Gulf of Mexico: a preliminary description of diet from the western Atlantic spawning grounds

A combination of stomach contents, nitrogen stable-isotope and tissue C:N values are presented to demonstrate feeding activity of Atlantic bluefin tuna Thunnus thynnus on the Gulf of Mexico (GOMEX) spawning grounds. Diets include teleosts, cephalopods, crustaceans and a pelagic tunicate (Pyrosoma atlanticum). Results reveal the need to classify the GOMEX as a T. thynnus feeding ground.

Near resonance acoustic scattering from organized schools of juvenile Atlantic bluefin tuna (Thunnus thynnus)

Schools of Atlantic bluefin tuna (Thunnus thynnus) can exhibit highly organized spatial structure within the school. This structure was quantified for dome shaped schools using both aerial imagery collected from a commercial spotter plane and 400 kHz multibeam echo sounder data collected on a fishing vessel in 2009 in Cape Cod Bay, MA. Observations from one school, containing an estimated 263 fish within an approximately ellipsoidal volume of 1900 m(3), were used to seed an acoustic model that estimated the school target strength at frequencies between 10 and 2000 Hz. The fish's swimbladder resonance was estimated to occur at approximately 50 Hz. The acoustic model examined single and multiple scattering solutions and also a completely incoherent summation of scattering responses from the fish. Three levels of structure within the school were examined, starting with fish locations that were constrained by the school boundaries but placed according to a Poisson process, then incorporating a constraint on the distance to the nearest neighbor, and finally adding a constraint on the bearing to the nearest neighbor. Results suggest that both multiple scattering and spatial organization within the school should be considered when estimating the target strength of schools similar to the ones considered here.

Behaviour and buoyancy regulation in the deepest-diving reptile: the leatherback turtle

In the face of the physical and physiological challenges of performing breath-hold deep dives, marine vertebrates have evolved different strategies. Although behavioural strategies in marine mammals and seabirds have been investigated in detail, little is known about the deepest-diving reptile – the leatherback turtle (Dermochelys coriacea). Here, we deployed tri-axial accelerometers on female leatherbacks nesting on St Croix, US Virgin Islands, to explore their diving strategy. Our results show a consistent behavioural pattern within dives among individuals, with an initial period of active swimming at relatively steep descent angles (∼–40 deg), with a stroke frequency of 0.32 Hz, followed by a gliding phase. The depth at which the gliding phase began increased with the maximum depth of the dives. In addition, descent body angles and vertical velocities were higher during deeper dives. Leatherbacks might thus regulate their inspired air-volume according to the intended dive depth, similar to hard-shelled turtles and penguins. During the ascent, turtles actively swam with a stroke frequency of 0.30 Hz but with a low vertical velocity (∼0.40 ms–1) and a low pitch angle (∼+26 deg). Turtles might avoid succumbing to decompression sickness (‘the bends’) by ascending slowly to the surface. In addition, we suggest that the low body temperature of this marine ectotherm compared with that of endotherms might help reduce the risk of bubble formation by increasing the solubility of nitrogen in the blood. This physiological advantage, coupled with several behavioural and physical adaptations, might explain the particular ecological niche the leatherback turtle occupies among marine reptiles.

Lipid corrections in carbon and nitrogen stable isotope analyses: comparison of chemical extraction and modelling methods

1. Lipids have more negative delta(13)C values relative to other major biochemical compounds in plant and animal tissues. Although variable lipid content in biological tissues alters results and conclusions of delta(13)C analyses in aquatic food web and migration studies, no standard correction protocol exists. 2. We compared chemical extraction and mathematical correction methods for freshwater and marine fishes and aquatic invertebrates to better understand impacts of correction approaches on carbon (delta(13)C) and nitrogen (delta(15)N) stable isotope data. 3. Fish and aquatic invertebrate tissue delta(13)C values increased significantly following extraction for almost all species and tissue types relative to nonextracted samples. In contrast, delta(15)N was affected for muscle and whole body samples from only a few freshwater and marine species and had a limited effect for the entire data set. 4. Lipid normalization models, using C : N as a proxy for lipid content, predicted lipid-corrected delta(13)C for paired data sets more closely with parameters specific to the tissue type and species to which they were applied. 5. We present species- and tissue-specific models based on bulk C : N as a reliable alternative to chemical extraction corrections. By analysing a subset of samples before and after lipid extraction, models can be applied to the species and tissues of interest that will improve estimates of dietary sources using stable isotopes.

A comparison of carbon and nitrogen stable isotope ratios of fish tissues following lipid extractions with non-polar and traditional chloroform/methanol solvent systems

Stable isotope ratios act as chemical tracers of animal diet, and are used to study food web dynamics. Because carbon stable isotope values are influenced by tissue lipid content, a number of extraction methods have been used to remove lipid bias, but, in some species and tissues, extractions also alter nitrogen isotope values. We have analyzed delta(13)C and delta(15)N in Atlantic bluefin tuna liver and white muscle, and whole Atlantic herring, fish tissues covering a wide range of lipid content (bulk C:N 3.1-12.5). In order to compare delta(13)C and delta(15)N values from traditional chloroform/methanol extractions with non-polar solvent alternatives, we analyzed samples following (1) no treatment, (2) lipid removal using chloroform/methanol (2:1), and (3) Soxhlet extractions using chloroform, diethyl ether or hexane. Chloroform/methanol and chloroform extractions produced the lowest C:N values and highest delta(13)C values. In bluefin tuna, chloroform and hexane extractions significantly altered liver delta(15)N, and all methods significantly altered delta(15)N values in white muscle. Whole Atlantic herring delta(15)N was not altered by any extraction method, while the 2:1 chloroform/methanol extraction most completely removed fish tissue lipid components. Our results indicate that delta(15)N effects are not limited to common chloroform/methanol extractions and suggest that chloroform/methanol is the most effective extraction for delta(13)C correction. Given evidence for delta(15)N alteration among all tested methods, mathematical correction approaches should be further explored as an alternative to lipid correction.

Heart rates and diving behavior of leatherback sea turtles in the eastern pacific ocean

Heart rates and diving behavior of leatherback sea turtles (Dermochelys coriacea) were monitored at sea during the internesting interval. Instruments that recorded the electrocardiogram and the depth and duration of dives were deployed on six female leatherback turtles as they laid eggs at Playa Grande, Costa Rica. Turtles dived continually for the majority of the internesting interval and spent 57–68 % of the time at sea submerged. Mean dive depth was 19+/−1 m (mean +/− s.d.) and the mean dive duration was 7.4+/−0.6 min. Heart rate declined immediately upon submergence and continued to fall during descent. All turtles showed an increase in heart rate before surfacing. The mean heart rate during dives of 17.4+/−0.9 beats min-1 (mean +/− s.d.) was significantly lower than the mean heart rate at the surface of 24.9+/−1.3 beats min-1 (P<0.05). Instantaneous heart rates as low as 1.05 beats min-1 were recorded during a 34 min dive. The mean heart rate over the entire dive cycle (dive + succeeding surface interval; 19.4+/−1.3 beats min-1) was more similar to the heart rate during diving than to the heart rate at the surface. Although dive and surface heart rates were significantly different from each other, heart rates during diving were 70 % of heart rates at the surface, showing that leatherback turtles do not experience a dramatic bradycardia during routine diving.

Physiologic and clinicopathologic effects of crude oil on loggerhead sea turtles

The physiologic and clinicopathologic effects of weathered South Louisiana crude oil exposure were studied in the laboratory in juvenile loggerhead sea turtles. Sea turtles ingested oil incidentally, and oil was observed clinging to the nares, eyes, and upper esophagus, and was found in the feces. Oiled turtles had up to a four-fold increase in white blood cell counts, a 50% reduction in red blood cell counts, and red blood cell polychromasia. Most serum blood chemistries (e.g., BUN, protein) were within normal ranges, although glucose returned more slowly to baseline values than in the controls. Gross and histologic changes were present in the skin and mucosal surfaces of oiled turtles, including acute inflammatory cell infiltrates, dysplasia of epidermal epithelium, and a loss of cellular architectural organization of hte skin layers. The cellular changes in the epidermis are of particular concern because they may increase susceptibility to infection. Although many of the observed physiological insults resolved with a 21-day recovery period, the long-term biological effects of oil on sea turtles remain completely unknown.

Serotonin is necessary for short-term modulation of the exercise ventilatory response

The exercise ventilatory response is augmented during conditions of increased respiratory dead space (delta Vd), a phenomenon that we refer to as short term modulation (STM). To test the hypothesis that serotonin is necessary in the mechanism underlying STM, experiments were conducted on ten awake goats. Ventilation, CO2 production and PaCO2 were measured at rest and during treadmill exercise (4 km/h, 5% grade), with and without delta Vd (0.25 L), before and after systemic administration of the serotonin receptor antagonist, methysergide maleate (n = 6; 1 mg/kg, i.v.), or the tryptophan hydroxylase inhibitor, p-chlorophenylalanine (PCPA; n = 4; 100 mg/kg, i.v.). Pre-methysergide: (1) PaCO2 decreased from rest to exercise to a similar degree with (-1.9 mmHg) and without (-1.8 mmHg) delta Vd; (2) the exercise ventilatory response increased 59% +/- 13% (P < 0.01) with delta Vd, accounting for similar exercise PaCO2 regulation and demonstrating STM; and (3) effects of delta Vd on exercise tidal volume and frequency responses were inconsistent. Post-methysergide: (1) there were no significant effects on ventilation or PaCO2 at rest or during exercise in control (mask) conditions; (2) the exercise ventilatory response was unaffected by delta Vd, thereby allowing PaCO2 to increase 4.1 +/- 3.0 mmHg from rest to exercise (P < 0.05); and (3) with delta Vd during exercise, the tidal volume response was increased, but was offset by a decreased frequency response. Following PCPA (16-24 h): (1) hyperventilation was evident at rest and during exercise; (2) the exercise ventilatory response was augmented, indicating STM; and (3) the exercise ventilatory response with delta Vd was not affected further, allowing PaCO2 to increase from rest to exercise and indicating an inability to elicit further STM. These data suggest that serotonin is necessary for short term modulation of the exercise ventilatory response with increased respiratory dead space, although the location of relevant serotonin receptors is not yet clear.

Oxygen stores and aerobic metabolism in the leatherback sea turtle

The leatherback sea turtle, Dermochelys coriacea, is a large, deep-diving species that has a blood oxygen carrying capacity twice that of smaller, shallow-diving sea turtles. In this study we measured lung volume (by argon dilution) and blood volume (by dilution of Evans' blue dye) in leatherbacks to estimate partitioning of oxygen stores and their potential contribution to aerobic metabolism during diving. Blood volume (77 mL∙kg−1) was slightly higher, yet lung volume was considerably smaller (64 mL∙kg−1), than in other sea turtles, so that potential oxygen stores were almost equally divided between the lung (12 mL∙kg−1) and the blood and tissues (15 mL∙kg−1). At a body temperature of 32–34 °C and high heart rates (43–48/min), oxygen consumption of beached and netted leatherbacks was 1.1 mL∙min−1∙kg−1. The respiratory quotient exceeded unity, suggesting that the turtles were repaying an oxygen debt incurred in the netting procedure. Estimates of the probable utilization of oxygen stores and possible maximum and minimum oxygen uptakes were used to obtain a range of dive times (5–70 min) that can be supported aerobically.

Respiratory mechanics of the loggerhead sea turtle, Caretta caretta

Respiratory mechanics were evaluated in excised lungs and in spontaneously breathing loggerhead sea turtles (Caretta caretta). Respiratory compliance curves reflect the pressure volume characteristics of the body wall. Compliance values are comparable to those of reptiles having simpler lung structure. Maximum flow rates in excised lungs (18-25 ml.sec-1.kg-1) were only slightly below the range reported for marine mammals. During spontaneous tidal breathing expiratory flow rates (11.8 ml.sec-1.kg-1) were lower than maximum values. In the sea turtle, respiratory adaptations including reduced airway resistance and muscular contribution to breathing are similar to marine mammals. These mechanical specializations shorten breathing time, which contributes to diving performance.