Advances in acoustic, archival, and satellite tagging of tunas

Spatial Dynamics and Fine-Scale Vertical Behaviour of Immature Eastern Australasian White Sharks (Carcharodon carcharias)

Knowledge of the 3-dimensional space use of large marine predators is central to our understanding of ecosystem dynamics and for the development of management recommendations. Horizontal movements of white sharks, Carcharodon carcharias, in eastern Australian and New Zealand waters have been relatively well studied, yet vertical habitat use is less well understood. We dual-tagged 27 immature white sharks with Pop-Up Satellite Archival Transmitting (PSAT) and acoustic tags in New South Wales coastal shelf waters. In addition, 19 of these individuals were also fitted with Smart Position or Temperature Transmitting (SPOT) tags. PSATs of 12 sharks provided useable data; four tags were recovered, providing highly detailed archival data recorded at 3-s intervals. Horizontal movements ranged from southern Queensland to southern Tasmania and New Zealand. Sharks made extensive use of the water column (0-632 m) and experienced a broad range of temperatures (7.8-28.9 °C). Archival records revealed pronounced diel-patterns in distinct fine-scale oscillatory behaviour, with sharks occupying relatively constant depths during the day and exhibiting pronounced yo-yo diving behaviour (vertical zig-zag swimming through the water column) during the night. Our findings provide valuable new insights into the 3-dimensional space use of Eastern Australasian (EA) white sharks and contribute to the growing body on the general ecology of immature white sharks.

Migration dynamics of juvenile southern bluefin tuna

Large scale migrations are a key component of the life history of many marine species. We quantified the annual migration cycle of juvenile southern bluefin tuna (Thunnus maccoyii; SBT) and spatiotemporal variability in this cycle, based on a multi-decadal electronic tagging dataset. Behaviour-switching models allowed for the identification of cohesive areas of residency and classified the temporal sequence of movements within a migration cycle from austral summer foraging grounds in the Great Australian Bight (GAB) to winter foraging grounds in the Indian Ocean and Tasman Sea and back to the GAB. Although specific regions within the Indian Ocean were frequented, individuals did not always return to the same area in consecutive years. Outward migrations from the GAB were typically longer than return migrations back to the GAB. The timing of individual arrivals to the GAB, which may be driven by seasonality in prey availability, was more cohesive than the timing of departures from the GAB, which may be subject to the physiological condition of SBT. A valuable fishery for SBT operates in the GAB, as do a number of scientific research programs designed to monitor SBT for management purposes; thus, understanding SBT migration to and from the area is of high importance to a number of stakeholders.

Evidence of separate subgroups of juvenile southern bluefin tuna

Archival tagging studies of southern bluefin tuna (SBT , Thunnus maccoyii) have revealed that juveniles residing in the Great Australian Bight (GAB) over the austral summer undertake seasonal cyclic migrations to the southeast Indian Ocean and the Tasman Sea during winter. However, there remains disagreement about the extent of mixing between juvenile SBT regularly caught by longline fleets south of Africa and those observed in the GAB. Some researchers have argued that archival tag recoveries indicate most juveniles reside in the GAB over the austral summer. Others have suggested that recoveries of conventional and archival tags are better explained by a juvenile population consisting of separate groups on the eastern and western sides of the Indian Ocean with limited intermixing. We present analyses of catch and tag recovery data and re-examine archival tagging studies. The evidence provided strongly favors the hypothesis of separate juvenile subgroups, or contingents, with limited intermixing. We draw some tentative conclusions about the nature of the putative contingents and discuss some implications of these findings for the interpretation of existing datasets and future research priorities. We also provide the first evidence that the migration choices of juveniles that summer in the GAB are influenced by fidelity to winter feeding grounds and suggest this helps explain the collapse of the surface fishery off New South Wales in the 1980s.

Quantifying energy intake in Pacific bluefin tuna (Thunnus orientalis) using the heat increment of feeding

Using implanted archival tags, we examined the effects of meal caloric value, food type (sardine or squid) and ambient temperature on the magnitude and duration of the heat increment of feeding in three captive juvenile Pacific bluefin tuna. The objective of our study was to develop a model that can be used to estimate energy intake in wild fish of similar body mass. Both the magnitude and duration of the heat increment of feeding (measured by visceral warming) showed a strong positive correlation with the caloric value of the ingested meal. Controlling for meal caloric value, the extent of visceral warming was significantly greater at lower ambient temperature. The extent of visceral warming was also significantly higher for squid meals compared with sardine meals. By using a hierarchical Bayesian model to analyze our data and treating individuals as random effects, we demonstrate how increases in visceral temperature can be used to estimate the energy intake of wild Pacific bluefin tuna of similar body mass to the individuals used in our study.

Telemetry tag effects on juvenile lingcod Ophiodon elongatus movement: a laboratory and field study

This study tested the behavioural effects of tagging subyearling and yearling lingcod Ophiodon elongatus with acoustic telemetry tags in laboratory tanks and in the natural environment (Puget Sound, WA). In the laboratory, tagged individuals showed less movement and feeding behaviour soon after tagging than untagged controls. The effect dissipated after c. 1 week, presumably as the tagged O. elongatus recovered from surgery or adjusted to the presence of the tags. This dissipation enabled a field study that compared early-tagged individuals with a long recovery period after tagging to recently-tagged individuals with a short recovery period after tagging. Consistent with findings from the laboratory experiment, recently tagged individuals showed less movement away from three release sites in Puget Sound than early-tagged individuals. Together, the laboratory and field results provide evidence of temporary tag effects on actual movement in the natural environment and provide a method for testing tag effects in the field. This study suggests that subyearling and yearling O. elongatus should be held for a recovery period before release. If holding after tagging is not an option, then movement data collected during the first week should be interpreted cautiously.

Effects of stochasticity in early life history on steepness and population growth rate estimates: an illustration on Atlantic bluefin tuna

The intrinsic population growth rate (r) of the surplus production function used in the biomass dynamic model and the steepness (h) of the stock-recruitment relationship used in age-structured population dynamics models are two key parameters in fish stock assessment. There is generally insufficient information in the data to estimate these parameters that thus have to be constrained. We developed methods to directly estimate the probability distributions of r and h for the Atlantic bluefin tuna (Thunnus thynnus, Scombridae), using all available biological and ecological information. We examined the existing literature to define appropriate probability distributions of key life history parameters associated with intrinsic growth rate and steepness, paying particular attention to the natural mortality for early life history stages. The estimated probability distribution of the population intrinsic growth rate was weakly informative, with an estimated mean r = 0.77 (±0.53) and an interquartile range of (0.34, 1.12). The estimated distribution of h was more informative, but also strongly asymmetric with an estimated mean h = 0.89 (±0.20) and a median of 0.99. We note that these two key demographic parameters strongly depend on the distribution of early life history mortality rate (M(0)), which is known to exhibit high year-to-year variations. This variability results in a widely spread distribution of M(0) that affects the distribution of the intrinsic population growth rate and further makes the spawning stock biomass an inadequate proxy to predict recruitment levels.

Temperature effects on Ca2+ cycling in scombrid cardiomyocytes: a phylogenetic comparison

Specialisations in excitation-contraction coupling may have played an important role in the evolution of endothermy and high cardiac performance in scombrid fishes. We examined aspects of Ca(2+) handling in cardiomyocytes from Pacific bonito (Sarda chiliensis), Pacific mackerel (Scomber japonicus), yellowfin tuna (Thunnus albacares) and Pacific bluefin tuna (Thunnus orientalis). The whole-cell voltage-clamp technique was used to measure the temperature sensitivity of the L-type Ca(2+) channel current (I(Ca)), density, and steady-state and maximal sarcoplasmic reticulum (SR) Ca(2+) content (ssSR(load) and maxSR(load)). Current-voltage relations, peak I(Ca) density and charge density of I(Ca) were greatest in mackerel and yellowfin at all temperatures tested. I(Ca) density and kinetics were temperature sensitive in all species studied, and the magnitude of this response was not related to the thermal preference of the species. SR(load) was greater in atrial than in ventricular myocytes in the Pacific bluefin tuna, and in species that are more cold tolerant (bluefin tuna and mackerel). I(Ca) and SR(load) were particularly small in bonito, suggesting the Na(+)/Ca(2+) exchanger plays a more pivotal role in Ca(2+) entry into cardiomyocytes of this species. Our comparative approach reveals that the SR of cold-tolerant scombrid fishes has a greater capacity for Ca(2+) storage. This specialisation may contribute to the temperature tolerance and thermal niche expansion of the bluefin tuna and mackerel.

Oceans apart? Short-term movements and behaviour of adult bull sharks Carcharhinus leucas in Atlantic and Pacific Oceans determined from pop-off satellite archival tagging

Adult bull sharks Carcharhinus leucas were monitored with electronic tags to investigate horizontal and vertical movements in the Atlantic and Pacific Oceans. In both locations, C. leucas showed some fidelity to specific coastal areas with only limited horizontal movements away from the tagging sites after tag attachment. Fish tagged in the Bahamas were detected mostly in the upper 20 m of the water column in water 25-26° C, whereas C. leucas tagged in Fiji spent most of their time below 20 m in water usually >26° C. The results highlight the importance of coastal inshore habitats for this species.

Transectional heat transfer in thermoregulating bigeye tuna (Thunnus obesus) – a 2D heat flux model

We developed a 2D heat flux model to elucidate routes and rates of heat transfer within bigeye tuna Thunnus obesus Lowe 1839 in both steady-state and time-dependent settings. In modeling the former situation, we adjusted the efficiencies of heat conservation in the red and the white muscle so as to make the output of the model agree as closely as possible with observed cross-sectional isotherms. In modeling the latter situation, we applied the heat exchanger efficiencies from the steady-state model to predict the distribution of temperature and heat fluxes in bigeye tuna during their extensive daily vertical excursions. The simulations yielded a close match to the data recorded in free-swimming fish and strongly point to the importance of the heat-producing and heat-conserving properties of the white muscle. The best correspondence between model output and observed data was obtained when the countercurrent heat exchangers in the blood flow pathways to the red and white muscle retained 99% and 96% (respectively) of the heat produced in these tissues. Our model confirms that the ability of bigeye tuna to maintain elevated muscle temperatures during their extensive daily vertical movements depends on their ability to rapidly modulate heating and cooling rates. This study shows that the differential cooling and heating rates could be fully accounted for by a mechanism where blood flow to the swimming muscles is either exclusively through the heat exchangers or completely shunted around them, depending on the ambient temperature relative to the body temperature. Our results therefore strongly suggest that such a mechanism is involved in the extensive physiological thermoregulatory abilities of endothermic bigeye tuna.

Habitat and behaviour of yellowfin tuna Thunnus albacares in the Gulf of Mexico determined using pop-up satellite archival tags

This study presents the first data on movement, habitat use and behaviour for yellowfin tuna Thunnus albacares in the Atlantic Basin. Six individuals were tracked in the Gulf of Mexico using pop-up satellite archival tags. Records up to 80 days in length were obtained, providing information on depth and temperature preferences as well as horizontal movements. Thunnus albacares in the Gulf of Mexico showed a strong preference for the mixed layer and thermocline, consistent with findings for this species in other ocean basins. Fish showed a diel pattern in depth distribution, remaining in surface and mixed layer waters at night and diving to deeper waters during the day. The vertical extent of T. albacares habitat appeared to be temperature limited, with fish generally avoiding waters that were >6 degrees C cooler than surface waters. The vertical and thermal habitat usage of T. albacares differs from that of bigeye Thunnus obesus and bluefin Thunnus thynnus, Thunnus orientalis and Thunnus maccoyii tunas. These results are consistent with the results of earlier studies conducted on T. albacares in other oceans.

Feeding ecology of wild migratory tunas revealed by archival tag records of visceral warming

1. Seasonal long-distance migrations are often expected to be related to resource distribution, and foraging theory predicts that animals should spend more time in areas with relatively richer resources. Yet for highly migratory marine species, data on feeding success are difficult to obtain. We analysed the temporal feeding patterns of wild juvenile southern bluefin tuna from visceral warming patterns recorded by archival tags implanted within the body cavity. 2. Data collected during 1998-2000 totalled 6221 days, with individual time series (n = 19) varying from 141 to 496 days. These data span an annual migration circuit including a coastal summer residency within Australian waters and subsequent migration into the temperate south Indian Ocean. 3. Individual fish recommenced feeding between 5 and 38 days after tagging, and feeding events (n = 5194) were subsequently identified on 76.3 +/- 5.8% of days giving a mean estimated daily intake of 0.75 +/- 0.05 kg. 4. The number of feeding events varied significantly with time of day with the greatest number occurring around dawn (58.2 +/- 8.0%). Night feeding, although rare (5.7 +/- 1.3%), was linked to the full moon quarter. Southern bluefin tuna foraged in ambient water temperatures ranging from 4.9 degrees C to 22.9 degrees C and depths ranging from the surface to 672 m, with different targeting strategies evident between seasons. 5. No clear relationship was found between feeding success and time spent within an area. This was primarily due to high individual variability, with both positive and negative relationships observed at all spatial scales examined (grid ranges of 2 x 2 degrees to 10 x 10 degrees ). Assuming feeding success is proportional to forage density, our data do not support the hypothesis that these predators concentrate their activity in areas of higher resource availability. 6. Multiple-day fasting periods were recorded by most individuals. The majority of these (87.8%) occurred during periods of apparent residency within warmer waters (sea surface temperature > 15 degrees C) at the northern edge of the observed migratory range. These previously undocumented nonfeeding periods may indicate alternative motivations for residency. 7. Our results demonstrate the importance of obtaining information on feeding when interpreting habitat utilization from individual animal tracks.

Differential heating and cooling rates in bigeye tuna (Thunnus obesus Lowe): a model of non-steady state heat exchange

We analyzed water temperature, visceral cavity temperature and depth data from archival tags retrieved from bigeye tuna (Thunnus obesus) at liberty in the central Pacific for up to 57 days using a mathematical model of heat exchange. Our model took into account the transfer of heat between the portions of the myotomes comprising red muscle fibers adjacent to the spinal column and served by vascular counter current heat exchanges (henceforth referred to as ;red muscle') and the water, as well as between the red muscle and the temperature sensor of the archival tags in the visceral cavity. Our model successfully predicted the recorded visceral cavity temperatures during vertical excursions provided that the rate constants for heat transfer between the ambient water and the red muscle during cooling (k(low)) and those during heating (k(high)) were very dissimilar. Least-squares fitting of k(low) and k(high) for the entire period that the fish were at liberty yielded values generally in the ranges 0.02-0.04 min(-1) and 0.2-0.6 min(-1) (respectively), with an average ratio k(high)/k(low) of approximately 12. Our results confirmed those from previous studies showing that bigeye tuna have extensive physiological thermoregulatory abilities probably exerted through changes of blood flow patterns that controlled the efficiency of vascular countercurrent heat exchanges. There was a small but significant negative correlation between k(low) and size, whereas there was no correlation between k(high) and size. The maximum swimming speeds during vertical excursions (calculated from the pressure data) occurred midway during ascents and averaged approximately 2 FL s(-1) (where FL=fork length), although speeds as high approximately 4-7 FL s(-1) were also noted.

Bioenergetics and diving activity of internesting leatherback turtlesDermochelys coriaceaat Parque Nacional Marino Las Baulas, Costa Rica

Physiology, environment and life history demands interact to influence marine turtle bioenergetics and activity. However, metabolism and diving behavior of free-swimming marine turtles have not been measured simultaneously. Using doubly labeled water, we obtained the first field metabolic rates (FMRs; 0.20–0.74 W kg–1) and water fluxes (16–30% TBW day–1, where TBW=total body water)for free-ranging marine turtles and combined these data with dive information from electronic archival tags to investigate the bioenergetics and diving activity of reproductive adult female leatherback turtles Dermochelys coriacea. Mean dive durations (7.8±2.4 min (±1 s.d.), bottom times (2.7±0.8 min), and percentage of time spent in water temperatures (Tw) ≤24°C(9.5±5.7%) increased with increasing mean maximum dive depths(22.6±7.1 m; all P≤0.001). The FMRs increased with longer mean dive durations, bottom times and surface intervals and increased time spent in Tw≤24°C (all r2≥0.99). This suggests that low FMRs and activity levels, combined with shuttling between different water temperatures, could allow leatherbacks to avoid overheating while in warm tropical waters. Additionally, internesting leatherback dive durations were consistently shorter than aerobic dive limits calculated from our FMRs (11.7–44.3 min). Our results indicate that internesting female leatherbacks maintained low FMRs and activity levels, thereby spending relatively little energy while active at sea. Future studies should incorporate data on metabolic rate, dive patterns, water temperatures, and body temperatures to develop further the relationship between physiological and life history demands and marine turtle bioenergetics and activity.

Tuna comparative physiology

Thunniform swimming, the capacity to conserve metabolic heat in red muscle and other body regions (regional endothermy), an elevated metabolic rate and other physiological rate functions, and a frequency-modulated cardiac output distinguish tunas from most other fishes. These specializations support continuous, relatively fast swimming by tunas and minimize thermal barriers to habitat exploitation, permitting niche expansion into high latitudes and to ocean depths heretofore regarded as beyond their range.

Field physiology: physiological insights from animals in nature

Whereas comparative physiology documents the range of physiological variation across a range of organisms, field physiology provides insight into the actual mechanisms an organism employs to maintain homeostasis in its everyday life. This requires an understanding of an organism's natural history and is prerequisite to developing hypotheses about physiological mechanisms. This review focuses on a few areas of field physiology that exemplify how the underlying physiology could not have been understood without appropriate field measurements. The examples we have chosen highlight the methods and inference afforded by an application of this physiological analysis to organismal function in nature, often in extreme environments. The specific areas examined are diving physiology, the thermal physiology of large endothermic fishes, reproductive physiology of air breathing vertebrates, and endocrine physiology of reproductive homeostasis. These areas form a bridge from physiological ecology to evolutionary ecology. All our examples revolve around the central issue of physiological limits as they apply to organismal homeostasis. We view this theme as the cornerstone of physiological analysis and supply a number of paradigms on homeostasis that have been tested in the context of field physiology.

Comparative studies of high performance swimming in sharks II. Metabolic biochemistry of locomotor and myocardial muscle in endothermic and ectothermic sharks

Metabolic enzyme activities in red (RM) and white (WM) myotomal muscle and in the heart ventricle (HV) were compared in two lamnid sharks (shortfin mako and salmon shark), the common thresher shark and several other actively swimming shark species. The metabolic enzymes measured were citrate synthase (CS), an index of aerobic capacity, and lactate dehydrogenase (LDH), an index of anaerobic capacity. WM creatine phosphokinase (CPK) activity, an index of rapid ATP production during burst swimming, was also quantified. Enzyme activities in RM, WM and HV were similar in the two lamnid species. Interspecific comparisons of enzyme activities at a common reference temperature (20 degrees C) show no significant differences in RM CS activity but higher CS activity in the WM and HV of the lamnid sharks compared with the other species. For the other enzymes, activities in lamnids overlapped with those of other shark species. Comparison of the HV spongy and compact myocardial layers in mako, salmon and thresher sharks reveals a significantly greater spongy CS activity in all three species but no differences in LDH activity. Adjustment of enzyme activities to in vivo RM and WM temperatures in the endothermic lamnids elevates CS and LDH in both tissues relative to the ectothermic sharks. Thus, through its enhancement of both RM and WM enzyme activity, endothermy may be an important determinant of energy supply for sustained and burst swimming in the lamnids. Although lamnid WM is differentially warmed as a result of RM endothermy, regional differences in WM CS and LDH activities and thermal sensitivities (Q(10) values) were not found. The general pattern of the endothermic myotomal and ectothermic HV muscle metabolic enzyme activities in the endothermic lamnids relative to other active, ectothermic sharks parallels the general pattern demonstrated for the endothermic tunas relative to their ectothermic sister species. However, the activities of all enzymes measured are lower in lamnids than in tunas. Relative to lamnids, the presence of lower WM enzyme activities in the thresher shark (which is in the same order as the lamnids, has an RM morphology similar to that of the mako and salmon sharks and may be endothermic) suggests that other factors, such as behavior and swimming pattern, also affect shark myotomal organization and metabolic function.

Migratory movements, depth preferences, and thermal biology of Atlantic bluefin tuna

The deployment of electronic data storage tags that are surgically implanted or satellite-linked provides marine researchers with new ways to examine the movements, environmental preferences, and physiology of pelagic vertebrates. We report the results obtained from tagging of Atlantic bluefin tuna with implantable archival and pop-up satellite archival tags. The electronic tagging data provide insights into the seasonal movements and environmental preferences of this species. Bluefin tuna dive to depths of >1000 meters and maintain a warm body temperature. Western-tagged bluefin tuna make trans-Atlantic migrations and they frequent spawning grounds in the Gulf of Mexico and eastern Mediterranean. These data are critical for the future management and conservation of bluefin tuna in the Atlantic.