SUMMARY 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 ≈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 ≈2 FL s –1 (where FL =fork length), although speeds as high ≈4–7 FL s –1 were also noted.

SUMMARY A large swim tunnel respirometer was used to quantify the swimming energetics of the eastern Pacific bonito Sarda chiliensis (tribe Sardini) (45–50 cm fork length, FL ) at speeds between 50 and 120 cm s -1 and at 18±2°C. The bonito rate of oxygen uptake( V̇ O 2 )–speed function is U-shaped with a minimum V̇ O 2 at 60 cm s -1 , an exponential increase in V̇ O 2 with increased speed, and an elevated increase in V̇ O 2 at 50 cm s -1 where bonito swimming is unstable. The onset of unstable swimming occurs at speeds predicted by calculation of the minimum speed for bonito hydrostatic equilibrium (1.2 FL s -1 ). The optimum swimming speed ( U opt ) for the bonito at 18±2°C is approximately 70 cm s -1 (1.4 FL s -1 ) and the gross cost of transport at U opt is 0.27 J N -1 m -1 . The mean standard metabolic rate (SMR), determined by extrapolating swimming V̇ O 2 to zero speed, is 107±22 mg O 2 kg -1 h -1 . Plasma lactate determinations at different phases of the experiment showed that capture and handling increased anaerobic metabolism, but plasma lactate concentration returned to pre-experiment levels over the course of the swimming tests. When adjustments are made for differences in temperature,bonito net swimming costs are similar to those of similar-sized yellowfin tuna Thunnus albacares (tribe Thunnini), but the bonito has a significantly lower SMR. Because bonitos are the sister group to tunas, this finding suggests that the elevated SMR of the tunas is an autapomorphic trait of the Thunnini.

SUMMARY The swimming kinematics of the eastern Pacific bonito Sarda chiliensis at a range of sustained speeds were analyzed to test the hypothesis that the bonito's swimming mode differs from the thunniform locomotor mode of tunas. Eight bonito (fork length FL 47.5±2.1 cm, mass 1.25±0.15 kg) (mean ± s.d. ) swam at speeds of 50–130 cm s -1 at 18±2°C in the same temperature-controlled water tunnel that was used in previous studies of tunas. Kinematics variables, quantified from 60 Hz video recordings and analyzed using a computerized, two-dimensional motion analysis system, were compared with published data for similar sized tunas at comparable speeds. Bonito tailbeat frequency, tailbeat amplitude and stride length all increased significantly with speed. Neither yaw (6.0±0.6% FL ) nor propulsive wavelength (120±65% fish total length) varied with speed,and there were no mass or body-length effects on the kinematics variables for the size range of bonitos used. Relative to similar sized yellowfin( Thunnus albacares ) and skipjack ( Katsuwonus pelamis ) tunas at similar speeds, the bonito has a lower tailbeat frequency, a higher yaw and a greater stride length. The lateral displacement and bending angle of each intervertebral joint during a complete tailbeat cycle were determined for the bonito at a swimming speed of 90 cm s -1 . The pattern of mean maximum lateral displacement ( z max ) and mean maximum bending angle (β max ) along the body in the bonito differed from that of both chub mackerel Scomber japonicus and kawakawa tuna Euthynnus affinis ; z max was highest in the bonito. This study verifies that S. chiliensis is a carangiform swimmer and supports the hypothesis that the thunniform locomotor mode is a derived tuna characteristic associated with changes in this group's myotomal architecture. The finding that yaw and z max were greater in the bonito than in both mackerels and tunas suggests that swimming kinematics in the bonito is not intermediate between that of tunas and mackerels, as would be predicted on the basis of morphological characteristics.

SUMMARY Tunas (family Scombridae) and sharks in the family Lamnidae are highly convergent for features commonly related to efficient and high-performance(i.e. sustained, aerobic) swimming. High-performance swimming by fishes requires adaptations augmenting the delivery, transfer and utilization of O 2 by the red myotomal muscle (RM), which powers continuous swimming. Tuna swimming performance is enhanced by a unique anterior and centrally positioned RM (i.e. closer to the vertebral column) and by structural features (relatively small fiber diameter, high capillary density and greater myoglobin concentration) increasing O 2 flux from RM capillaries to the mitochondria. A study of the structural and biochemical features of the mako shark ( Isurus oxyrinchus ) RM was undertaken to enable performance-capacity comparisons of tuna and lamnid RM. Similar to tunas, mako RM is positioned centrally and more anterior in the body. Another lamnid, the salmon shark ( Lamna ditropis ), also has this RM distribution, as does the closely related common thresher shark ( Alopias vulpinus ; family Alopiidae). However, in both the leopard shark( Triakis semifasciata ) and the blue shark (Prionace glauca ),RM occupies the position where it is typically found in most fishes; more posterior and along the lateral edge of the body. Comparisons among sharks in this study revealed no differences in the total RM quantity (approximately 2–3% of body mass) and, irrespective of position within the body, RM scaling is isometric in all species. Sharks thus have less RM than do tunas(4–13% of body mass). Relative to published data on other shark species,mako RM appears to have a higher capillary density, a greater capillary-to-fiber ratio and a higher myoglobin concentration. However, mako RM fiber size does not differ from that reported for other shark species and the total volume of mitochondria in mako RM is similar to that reported for other sharks and for tunas. Lamnid RM properties thus suggest a higher O 2 flux capacity than in other sharks; however, lamnid RM aerobic capacity appears to be less than that of tuna RM.

SUMMARY The effects of a 6°C difference in water temperature on maximum sustained swimming speed, swimming energetics and swimming kinematics were measured in the chub mackerel Scomber japonicus (Teleostei:Scombridae), a primarily coastal, pelagic predator that inhabits subtropical and temperate transition waters of the Atlantic, Pacific and Indian Oceans. New data for chub mackerel acclimated to 18°C are compared with published data from our laboratory at 24°C. Twelve individuals acclimated to each of two temperatures (15.6-26.3 cm fork length, FL , and 34-179g at 18°C; 14.0-24.7 cm FL and 26-156g at 24°C) swam at a range of speeds in a temperature-controlled Brett-type respirometer, at the respective acclimation temperature. At a given fish size, the maximum speed that S. japonicus was able to maintain for a 30-min period, while swimming steadily using slow, oxidative locomotor muscle ( U max,c ),was significantly greater at 24 than at 18°C (52.5-97.5 cm s -1 at 18°C and 70-120 cm s -1 at 24°C). At a given speed and fish size, the rate of oxygen consumption( V̇ O 2 ) was significantly higher at 24 than at 18°C because of a higher net cost of transport (1073-4617 J km -1 kg -1 at 18°C and 2708-14895 J km -1 kg -1 at 24°C). Standard metabolic rate, calculated by extrapolating the log V̇ O 2 versus swimming speed relationship to zero speed, did not vary significantly with temperature or fish mass (126.4±67.2 mg O 2 h -1 kg -1 at 18°C and 143.2±80.3 mg O 2 h -1 kg -1 at 24°C; means ±S.D., N =12). Swimming kinematics was quantified from high-speed (120 Hz) video recordings analyzed with a computerized, two-dimensional motion-analysis system. At a given speed and fish size, there were no significant effects of temperature on tail-beat frequency, tail-beat amplitude or stride length, but propulsive wavelength increased significantly with temperature as a result of an increase in propulsive wave velocity. Thus, the main effects of temperature on chub mackerel swimming were increases in both U max,c and the net cost of swimming at 24°C. Like other fishes, S. japonicus apparently must recruit more slow,oxidative muscle fibers to swim at a given sustainable speed at the lower temperature because of the reduced power output. Thus, the 24°C mackerel reach a higher speed before they must recruit the fast, glycolytic fibers,thereby increasing U max,c at 24°C. By quantifying in vivo the effects of temperature on the swimming performance of an ectothermic species that is closely related to the endothermic tunas, this study also provides evidence that maintaining the temperature of the slow,oxidative locomotor muscle at 6°C or more above ambient water temperature in tunas should significantly increase sustainable swimming speeds, but also increase the energetic cost of swimming, unless cardiac output limits muscle performance.