Rates and consequences of relaying in Cassin's auklets Ptychoramphus aleuticus and rhinoceros auklets Cerorhinca monocerata breeding in a seasonal environment

We removed first eggs from early‐laying females to measure rates and consequences of relaying in Cassin's auklets Ptychoramphus aleuticus and rhinoceros auklets Cerorhinca monocerata at Triangle Island, British Columbia, Canada. Based on egg size and composition, the investment that Cassin's auklets made in first eggs was very close to that predicted from adult body mass, whereas rhinoceros auklets invested more. In both species, a high percentage of females relaid (90% of Cassin's and 87% of rhinoceros auklets). Breeding success declined weakly with later laying among control Cassin's auklet pairs, but pairs that we induced to relay bred more successfully than naturally late pairs, and similar to values predicted from laying dates of their first eggs. Their chicks also fledged heavier and younger than late control chicks, and similar to values in early control chicks, but followed the population‐wide seasonal decline in wing length at fledging. Nestling diets were dominated by Neocalanus copepods until late in the season, a sign that feeding conditions remained favourable until late. In contrast, rhinoceros auklet pairs induced to relay followed the population‐wide seasonal decline in breeding success, which was driven by a decline in hatching success. Pacific sandlance Ammodytes hexapterus, thought to be a preferred prey species, virtually disappeared from nestling diets in mid‐to‐late season, yet there was no seasonal decline in fledging mass. However, chicks from replacement eggs followed the declines among control chicks in both age and wing length at fledging. Despite the female having produced a replacement egg, and despite delayed breeding, there appeared to be little immediate consequence associated with relaying for Cassin's auklets, except for a tendency for their chicks to fledge with short wings. Consequences were more marked in rhinoceros auklets (greatly reduced hatching success, and having their chicks fledge with short wings), and this may have been due to the large investment made in eggs, and/or to delayed breeding. Results of this study show that attributes of Cassin's and rhinoceros auklets that lay at different times in the season can be important in driving seasonal declines in breeding performance, as found in studies on other Alcidae. They also show how decisions taken during the egg stage can have variable yet potentially important implications for fitness, even in relatively long‐lived species that lay single‐egg clutches.     

Kinematic compensation for wing loss in flying damselflies

Flying insects can tolerate substantial wing wear before their ability to fly is entirely compromised. In order to keep flying with damaged wings, the entire flight apparatus needs to adjust its action to compensate for the reduced aerodynamic force and to balance the asymmetries in area and shape of the damaged wings. While several studies have shown that damaged wings change their flapping kinematics in response to partial loss of wing area, it is unclear how, in insects with four separate wings, the remaining three wings compensate for the loss of a fourth wing. We used high-speed video of flying blue-tailed damselflies (Ischnura elegans) to identify the wingbeat kinematics of the two wing pairs and compared it to the flapping kinematics after one of the hindwings was artificially removed. The insects remained capable of flying and precise maneuvering using only three wings. To compensate for the reduction in lift, they increased flapping frequency by 18 ± 15.4% on average. To achieve steady straight flight, the remaining intact hindwing reduced its flapping amplitude while the forewings changed their stroke plane angle so that the forewing of the manipulated side flapped at a shallower stroke plane angle. In addition, the angular position of the stroke reversal points became asymmetrical. When the wingbeat amplitude and frequency of the three wings were used as input in a simple aerodynamic model, the estimation of total aerodynamic force was not significantly different (paired t-test, p = 0.73) from the force produced by the four wings during normal flight. Thus, the removal of one wing resulted in adjustments of the motions of the remaining three wings, exemplifying the precision and plasticity of coordination between the operational wings. Such coordination is vital for precise maneuvering during normal flight but it also provides the means to maintain flight when some of the wings are severely damaged.     

Stroke frequency, but not swimming speed, is related to body size in free-ranging seabirds, pinnipeds and cetaceans

It is obvious, at least qualitatively, that small animals move their locomotory apparatus faster than large animals: small insects move their wings invisibly fast, while large birds flap their wings slowly. However, quantitative observations have been difficult to obtain from free-ranging swimming animals. We surveyed the swimming behaviour of animals ranging from 0.5 kg seabirds to 30 000 kg sperm whales using animal-borne accelerometers. Dominant stroke cycle frequencies of swimming specialist seabirds and marine mammals were proportional to mass(-0.29) (R(2)= 0.99, n = 17 groups), while propulsive swimming speeds of 1-2 m s(-1) were independent of body size. This scaling relationship, obtained from breath-hold divers expected to swim optimally to conserve oxygen, does not agree with recent theoretical predictions for optimal swimming. Seabirds that use their wings for both swimming and flying stroked at a lower frequency than other swimming specialists of the same size, suggesting a morphological trade-off with wing size and stroke frequency representing a compromise. In contrast, foot-propelled diving birds such as shags had similar stroke frequencies as other swimming specialists. These results suggest that muscle characteristics may constrain swimming during cruising travel, with convergence among diving specialists in the proportions and contraction rates of propulsive muscles.     

Kinematics of swimming of penguins at the Detroit Zoo

Kinematic parameters were examined in a study of the swimming abilities of seven species of penguins housed at the Detroit Zoo. Penguins produce thrust over both halves of the wing stroke cycle, as observed in fishes using the caudal or pectoral fins for locomotion, but not in other birds in level forward flight. Unpowered gliding phases between wing strokes were observed in all species at swimming speeds less than 1.25 m/sec, while Emperor, King and Adelie penguins interpose gliding phases over a broad range of speeds. Videotape records reveal that length-specific speed is correlated with increases in wingbeat frequency and, for most of the species examined, stride length. These findings are in contrast to those reported for other, flying birds, which maintain a relatively constant wingbeat frequency but vary stride length with forward speed, and for most fishes, which vary speed with tailbeat frequency but maintain a constant stride length. The results are somewhat comparable to those reported for Cymatogaster , a fish which uses the pectoral fins for locomotion. Drag coefficients of three gliding Emperor penguins were 2.1, 3.0 and 3.0 × 10-⁻³ at Reynolds numbers of 1.25, 1.62 and 1.76 × 10⁶, respectively.     

Seasonal changes of the at-sea distribution and food provisioning in rhinoceros auklets

Central-place foraging seabirds increase food-loads and decrease meal frequency when they forage in areas that are distant from the breeding colony. In 2001–2002, we studied the seasonal changes in at-sea distribution, food-load mass, meal frequency, and fledging mass in rhinoceros auklets (Cerorhinca monocerata), which forage in coastal waters during the day and feed their chicks at night. In both years, greater numbers of auklets were observed flying in northern waters that are more distant from the colony in June (65 km) and July (65–66 km) than in May (38–47 km). In July of both years, many auklets flew northward across the transect set 65–120 km north of the colony at sunrise; the birds returned south again at sunset, indicating that they foraged in waters outside the study area. This seasonal northward movement of the foraging area may reflect the migration of their main prey item, the Japanese anchovy (Engraulis japonicus), which move with the Tsushima Warm Current flowing from the southern Sea of Japan. Food-load mass did not increase seasonally. In both years, the estimated daily meal frequency was lower in July than in May or June, partly because of the increased foraging distance in July. Late-hatched chicks also displayed lighter fledging masses than early chicks in both years. We suggest that late breeders are required to forage at great distances for longer periods, which may result in decreased meal frequency and lighter fledging mass of their chicks.     

Discrimination of insect wingbeat-frequencies by the batRhinolophus ferrumequinum

Summary Five Greater Horseshoe bats,Rhinolophus ferrumequinum, were trained in a two-alternative forced-choice procedure to discriminate between artificial echoes of insects fluttering at different wingbeat rates. The stimuli were electronically produced phantom targets simulating fluttering insects with various wingbeat frequencies (Figs. 3, 4). Difference thresholds for wingbeat rates of 50 Hz and 100 Hz were determined. For an S+ of 50 Hz the difference threshold values lay between 2.8 and 4.6 Hz for individual bats; with an S+ of 100 Hz they increased to between 9.8 and 12.0 Hz (Figs. 5, 6, Table 1).Three bats, previously trained to discriminate between a S+ of 50 Hz and a S– with a lower wingbeat rate, were tested with higher frequency stimuli. When they had to decide between their old S+ of 50 Hz and either a 60 or 70 Hz echo two bats continued to select the 50 Hz stimulus while the third bat now preferred the faster fluttering insects (Table 2).During the discrimination task the echolocation behavior of the bats was monitored. When the phantom targets were presented all bats increased their duty-cycle of sound emission from about 40% to sometimes near 70%. They did so by either emitting longer echolocation calls or by increasing the sound repetition rate (Figs. 7, 8).The results show that Greater Horseshoe bats can determine the wingbeat rate of flying insects with an accuracy between 6 and 12%. Possible cues for flutter rate determination by cf-fm bats from natural and artificial insect echoes are discussed.Abbreviations DC duty-cycle - PD pulse duration - PI pulse interval - cf constantfrequency - fm frequency modulation     

Yolk carotenoids and stable isotopes reveal links among environment, foraging behavior and seabird breeding success

Nutrients that are limited in availability, such as carotenoids, are potentially involved in trade-offs between homeostasis and reproduction. Despite their importance, factors that affect the capacity of female birds to meet their carotenoid requirements are poorly understood. We used δ¹⁵N stable isotope analysis to relate foraging behavior to yolk carotenoid deposition in two seabirds, Cassin’s auklet (Ptychoramphus aleuticus) and rhinoceros auklet (Cerorhinca monocerata), during each of five years. As expected from their narrower trophic range, Cassin’s auklets produced yolks with fewer carotenoid types than did rhinoceros auklets (one vs. three). Cassin’s auklets also fed on a lower trophic level diet richer in carotenoids, yet had lower total yolk carotenoid levels, which suggests a role for species-specific adaptations for carotenoid uptake and utilization. Within both species, lower trophic-level feeding was linked to higher yolk carotenoid levels, but through different mechanisms. In Cassin’s auklets, it was due to a population-wide response to environmental variation: in warm-water years, all females fed at a low trophic level and produced carotenoid-rich yolks. In rhinoceros auklets, it was due to individual differences similarly expressed in all years: females fed across a wide trophic range, and those that fed at a low trophic level produced carotenoid-rich yolks. Rhinoceros auklets bred more successfully in years when their yolks were rich in carotenoids, probably due to a correlated response to stronger marine primary production. Our results are novel because they link avian yolk carotenoid deposition to behavioral and environmental variations.     

Aerobic swimming performance of juvenile Schizothorax chongi (Pisces, Cyprinidae) in the Yalong River, southwestern China

Schizothorax chongi (locally known as Xilian Yu), a fish species commonly found in Yalong River, has been declining quickly in recent years. One of the important factors, among many, is the interruption of the free flowing river by dams. To obtain data that can be applied to the design of a fishway for S. chongi and other species in the community, a laboratory study of juvenile S. chongi’s swimming energetics and kinematics was conducted in a flume-type respirometer equipped with a high speed video camera system to record swimming behavior. The aerobic metabolic rate, tail beat frequency (TBF) and tail beat amplitude (TBA) were measured during steady swimming at varying flow rates for fish of similar mass. A power function accurately describes the relationship between oxygen consumption rate (MO₂) and swimming speed (U). The estimated standard metabolic rate (SMR) calculated from the power function was 445.34 mg O₂ kg⁻¹ h⁻¹, similar to the experimental result of 431.5 mg O₂ kg⁻¹ h⁻¹. The relationship between cost of transport (COT) and U was, characteristically, inverse bell-shaped, with COT_min = 44.6 J kg⁻¹ m⁻¹ at U _opt = 5.5 body lengths per second (bl s⁻¹). There was a significant positive linear correlation between TBF and U. The slope of the correlation (0.33) was lower than for many other species, implying that S. chongi swim efficiently. The TBA, ranging from 0.15 to 0.2 bl, was found to be independent of U. Kinematic analyses indicates that S. chongi primarily depends on the caudal fin to generate forward thrust and employs three velocity-dependent swimming gaits. This investigation provides data on the swimming ability of S. chongi that will add to the basic science required for fishway design.     

Foraging behaviour of chick-rearing rhinoceros auklets Cerorhinca monocerata at Teuri Island, Japan, determined by acceleration-depth recording micro data loggers

Flight and diving activity of rhinoceros auklets Cerorhinca monocerata breeding on Teuri Island, Japan, were monitored during the summers 1999 and 2000 using miniaturized time-depth and acceleration recorders. Birds made 14.5 dive bouts per day of on average 15.4 min duration, which consisted of on average 16.2 dives of 12.1 m depth and 42.7 s duration. Birds made 13.8±7.3 flight bouts per day, which lasted on average 11.5±4.5 min. Daily total flight duration was 2.7±1.7 h (range 54 s–5.1 h) and the mean potential foraging range was estimated to be 87 km (maximum 164 km). Most birds stayed at the colony or rested on the water surface during the night. Rhinoceros auklets dived more actively in early morning and in late afternoon than during mid-day. Compared to results from studies of time allocation in other alcids species, rhinoceros auklets spent longer time flying (3.3 hd⁻¹) and resting on water (13.1 hd⁻¹), and less time diving (3.1 hd⁻¹) and staying at the colony (4.4 hd⁻¹). These foraging patterns are probably related to the nest attendance pattern of rhinoceros auklets, i.e. leaving the colony early in the morning, staying at sea all the day and returning to the colony in the evening to provision their chicks.     

Acoustic irradiation produced by flying moths (Lepidoptera,Noctuidae)

Characteristics of acoustic waves accompanying the flight of noctuid moths (Noctuidae) were measured. The low-frequency part of the spectrum is formed of a series of up to 17 harmonics of the wingbeat frequency (30–50 Hz) with a general tendency toward the decrease in the spectral density and the increase in the sound frequency. The root-mean-square level of the sound pressure from flapping wings was found to be 70–78 dB SPL. Besides low-frequency components, the flight of moths was accompanied by short ultrasonic pulses, which appeared with every wingbeat. Most of the spectral energy was concentrated within a range of 7–150 kHz with the main peaks at 60–110 kHz. The short-term pulses were divided into two or more subpulses with different spectra. The high-frequency pulses were produced at two phases of the wingbeat cycle: during the pronation of the wings at the highest point and at the beginning of their upward movement from the lowest point. In most of the specimens tested, the peak amplitude of sounds varied from 55 to 65 dB SPL at a distance of 6 cm from the insect body. However, in nine noctuid species, no high-frequency acoustic components were recorded. In these experiments, the acoustic flow from the flying moth within a frequency range of 2 to 20 kHz did not exceed the self-noise level of the microphone amplifier (RMS 18 dB SPL). Probable mechanisms of the high frequency acoustic emission during flight, the effect of these sounds on the auditory sensitivity of moths, and the possibility of their self-revealing to insectivorous bats are discussed. In addition, spectral characteristics of the moth echolocation clicks were more precisely determined within the higher frequency range (>100 kHz).     

In Vivo Time-Resolved Microtomography Reveals the Mechanics of the Blowfly Flight Motor

Dipteran flies are amongst the smallest and most agile of flying animals. Their wings are driven indirectly by large power muscles, which cause cyclical deformations of the thorax that are amplified through the intricate wing hinge. Asymmetric flight manoeuvres are controlled by 13 pairs of steering muscles acting directly on the wing articulations. Collectively the steering muscles account for <3% of total flight muscle mass, raising the question of how they can modulate the vastly greater output of the power muscles during manoeuvres. Here we present the results of a synchrotron-based study performing micrometre-resolution, time-resolved microtomography on the 145 Hz wingbeat of blowflies. These data represent the first four-dimensional visualizations of an organism''s internal movements on sub-millisecond and micrometre scales. This technique allows us to visualize and measure the three-dimensional movements of five of the largest steering muscles, and to place these in the context of the deforming thoracic mechanism that the muscles actuate. Our visualizations show that the steering muscles operate through a diverse range of nonlinear mechanisms, revealing several unexpected features that could not have been identified using any other technique. The tendons of some steering muscles buckle on every wingbeat to accommodate high amplitude movements of the wing hinge. Other steering muscles absorb kinetic energy from an oscillating control linkage, which rotates at low wingbeat amplitude but translates at high wingbeat amplitude. Kinetic energy is distributed differently in these two modes of oscillation, which may play a role in asymmetric power management during flight control. Structural flexibility is known to be important to the aerodynamic efficiency of insect wings, and to the function of their indirect power muscles. We show that it is integral also to the operation of the steering muscles, and so to the functional flexibility of the insect flight motor.     

Effects of acute temperature change on the metabolism and swimming ability of juvenile sterlet sturgeon (Acipenser ruthenus,Linnaeus 1758)

The objective of this study was to provide information on changes in the metabolism and swimming ability of juvenile sterlet sturgeon, Acipenser ruthenus, caused by acutely low or high temperatures. Changes in critical swimming speed (U_crit), oxygen consumption rate (MO₂), tail beat frequency (TBF) and tail beat amplitude (TBA) were observed with a Steffensen‐type swimming respirometer, an oxygen electrode and a camera at different swimming speeds at three temperatures: 5°C, 15°C, and 25°C. Fish tested at 5°C and 25°C were maintained at 15°C (near optimal) for one week to simulate conditions below a dam. The U_crit value decreased significantly during acute temperature changes at 5°C and 25°C; U_crit was highest near the optimal temperature. Oxygen consumption rate (MO₂) increased with the swimming speed at 15°C; however, at 25°C and 5°C, the MO₂ decreased with the swimming speed. Both TBA and TBF decreased at 5°C and 25°C compared to values at 15°C. The slopes of the regression lines (TBF/U) at 5°C and 25°C seemed lower compared to 15°C.     

The critical swimming speed of Iberian barbel Barbus bocagei in relation to size and sex

The swimming capacity of Barbus bocagei was measured with the critical swimming speed (U_crit) standard test in a modified Bla?ka‐type swim tunnel. Sixty B. bocagei were tested and they exhibited a mean ±s .d . U_crit of 0·81 ± 0·11 m s^?1 or 3·1 ± 0·86 total lengths per second (L_T s^?1). Sex had no effect on U_crit but significant differences were found between the swimming performance of fish with distinct sizes.     

Swimming performance of 12 Schizothoracinae species from five rivers

A series of stepped velocity tests were carried out in a Brett‐type swimming respirometer and the overall range in swimming performance for 12 Schizothoracinae species was measured. The relative critical swimming speed U_crit and burst speed U_burst decreased with body length, while absolute U_crit and U_burst increased with body length. U_crit increased with temperature up to approximately 15^° C and then decreased. Species with a high U_crit also displayed a higher U_burst.     

Phylogenetic clustering of wingbeat frequency and flight‐associated morphometrics across insect orders

Wingbeat frequency in insects is an important variable in aerodynamic and energetic analyses of insect flight and often is studied on a family‐ or species‐level basis. Meta‐analyses of these studies report order‐level patterns suggesting that flight strategy is moderately well conserved phylogenetically. Studies incorporated into these meta‐analyses, however, use variable methodologies across different temperatures, which may confound results and phylogenetic patterns. In the present study, a high‐speed camera is used to measure wingbeat frequency in a wide variety of species (n = 102) under controlled conditions aiming (i) to determine the validity of previous meta‐analyses showing phylogenetic clustering of flight strategy and (ii) to identify new evolutionary patterns between wingbeat frequency, body mass, wing area, wing length and wing loading at the order level. All flight‐associated morphometrics significantly affect wingbeat frequency. Linear models show that wing area explains the most amount of variation in wingbeat frequency (r² = 0.59, P ≤ 0.001), whereas body mass explains the least (r² = 0.09, P ≤ 0.01). A multiple regression model incorporating both body mass and wing area is the best overall predictor of wingbeat frequency (r² = 0.84, P ≤ 0.001). Order‐level phylogenetic patterns across relationships are consistent with previous studies. Thus, the present study provides experimental validation of previous meta‐analyses and provides new insights into phylogenetically conserved flight strategies across insect orders.     

Wing kinematics of avian flight across speeds

To test whether wing shape affects the kinematics of wing motion during bird flight, we recorded high-speed video (250 Hz) of four species flying in a variable-speed wind tunnel. The birds flew at intervals of 2 m s⁻¹, ranging from 1 m s⁻¹ up to their respective maximum flight speed, which varied from 14 to 17 m s⁻¹ depending on the species. Kinematic data obtained from two synchronized, high-speed video cameras were analyzed using 3D reconstruction. Three species with relatively pointed, high-aspect ratio wings changed wingbeat styles according to flight speed (budgerigar, Melopsittacus undulatus ; cockatiel, Nymphicus hollandicus ; ringed turtle dove, Streptopelia risoria ). These species used a wing-tip reversal upstroke, characterized by supination of the distal wing at mid-upstroke, at equivalent airspeeds ≤7 to 9 m s⁻¹. In faster flight, they used a swept-wing upstroke, without distal wing supination. At mid-upstroke at any speed, wingspan in these species was greater than wrist span. In contrast, at all steady flight speeds, the black-billed magpie Pica hudsonia with relatively broad, low-aspect ratio wings, used a flexed-wing, feathered upstroke in which wrist spans were equal to or greater than wingspans. Our results demonstrate that wing kinematics vary gradually as a function of flight speed, and that the patterns of variation are strongly influenced by external wing shape.     

Effects of temperature,swimming speed and body mass on standard and active metabolic rate in vendace (<Emphasis Type="Italic">Coregonus albula</Emphasis>)

This study gives an integrated analysis of the effects of temperature, swimming speed and body mass on standard metabolism and aerobic swimming performance in vendace (Coregonus albula (L.)). The metabolic rate was investigated at 4, 8 and 15°C using one flow-through respirometer and two intermittent-flow swim tunnels. We found that the standard metabolic rate (SMR), which increased significantly with temperature, accounted for up to 2/3 of the total swimming costs at optimum speed (U _opt), although mean U _opt was high, ranging from 2.0 to 2.8 body lengths per second. Net swimming costs increased with swimming speed, but showed no clear trend with temperature. The influence of body mass on the metabolic rate varied with temperature and activity level resulting in scaling exponents (b) of 0.71–0.94. A multivariate regression analysis was performed to integrate the effects of temperature, speed and mass (AMR = 0.82M ^0.93 exp(0.07T) + 0.43M ^0.93 U ^2.03). The regression analysis showed that temperature affects standard but not net active metabolic costs in this species. Further, we conclude that a low speed exponent, high optimum speeds and high ratios of standard to activity costs suggest a remarkably efficient swimming performance in vendace.     

Discrimination of wingbeat motion by bats,correlated with echolocation sound pattern

Summary Bats of the species Rhinolophus rouxi, Hipposideros lankadiva and Eptesicus fuscus were trained to discriminate between two simultaneously presented artificial insect wingbeat targets moving at different wingbeat rates. During the discrimination trials, R. rouxi, H. lankadiva and E. fuscus emitted long-CF/FM, short-CF/FM and FM echolocation sounds respectively. R. rouxi, H. lankadiva and E. fuscus were able to discriminate a difference in wingbeat rate of 2.7 Hz, 9.2 Hz and 15.8 Hz, respectively, between two simultaneously presented targets at an absolute wingbeat rate of 60 Hz, using a criterion of 75% correct responses.The performance of the different bat species is correlated with the echolocation signal design used by each species, particularly with the presence and relative duration of a narrowband component preceding a broadband FM component. These results provide behavioral evidence supporting the hypothesis that bats that use CF/FM echolocation sounds have adaptations for the perception of insect wingbeat motion and that long-CF/FM species are more specialized for this task than short-CF/FM species.Abbreviations CF constant frequency - FM frequency modulation     

Heart rates of Atlantic salmon Salmo salar during a critical swim speed test and subsequent recovery

In this study, heart rate (HR) bio-loggers were implanted in the abdominal cavity of 12 post-smolt Atlantic salmon Salmo salar weighing 1024 ± 31 g and acclimated to 12°C sea water. One week after the surgical procedure, a critical swim speed (U_crit) test was performed on tagged and untagged conspecifics, whereafter tagged fish were maintained in their holding tanks for another week. The U_crit was statistically similar between tagged and untagged fish (2.67 ± 0.04 and 2.74 ± 0.05 body lengths s⁻¹, respectively) showing that the bio-logger did not compromise the swimming performance. In the pre-swim week, a diurnal cycle was apparent with HR peaking at 65 beats min⁻¹ during the day and approaching 40 beats min⁻¹ at night. In the U_crit test, HR increased approximately exponentially with swimming speed until a plateau was reached at the final speed before fatigue with a maximum of 85.2 ± 0.7 beats min⁻¹. During subsequent recovery tagged fish could be divided into a surviving group (N = 8) and a moribund group (N = 4). In surviving fish HR had fully recovered to pre-swim levels after 24 h, including reestablishment of a diurnal HR cycle. In moribund fish HR never recovered and remained elevated at c. 80 beats min⁻¹ for 4 days, whereafter they started dying. We did not identify a proximal cause of death in moribund fish, but possible explanations are discussed. Tail beat frequency (TBF) was also measured and showed a more consistent response to increased swimming speeds. As such, when exploring correlations between HR, TBF and metabolic rates at different swimming speeds, TBF provides better predictions. On the contrary, HR measurements in free swimming fish over extended periods of time are useful for other purposes such as assessing the accumulative burden of various stressors and recovery trajectories from exhaustive exercise.     

Critical swimming speed of yellow‐ and silver‐phase European eel (Anguilla anguilla,L.)

The prime objective of this study was to evaluate differences between the swimming performance of two distinct life stages of European eels. The critical swimming speed (U_crit) of 29 yellow‐ and 33 silver‐phase eels was evaluated in a swim tunnel. Silver‐phase eels showed a better swimming performance (U_crit = 0.66 ms^?1) than yellow individuals (U_crit = 0.43 ms^?1). Male and female silver eels reached an identical U_crit despite their different sizes, which may be a strategy to increase the synchronization of arrival at the spawning grounds.