SUMMARYThis study evaluated the link between swimming endurance and condition of Atlantic cod Gadus morhua that had been fed or starved during the 16 weeks preceding the tests, and assessed whether muscle metabolic capacities explain such links. The condition factor [(somatic mass × fork length-3)×100] of starved cod was 0.54±0.1 whereas that of fed cod was 0.81±0.1. In white and red muscle, we measured four glycolytic enzymes: phosphofructokinase (PFK), pyruvate kinase (PK), creatine kinase (CK) and lactate dehydrogenase (LDH), two mitochondrial enzymes:cytochrome c oxidase (CCO) and citrate synthase (CS), a biosynthetic enzyme, nucleoside diphosphate kinase (NDPK), glycogen and protein levels and water content. Muscle samples were taken at three positions along the length of the fish; starvation affected the metabolic capacities of white muscle more than those of red muscle. The levels of glycolytic enzymes and glycogen changed more in white than red muscle during starvation. Both in fed and starved cod, muscle metabolic capacities varied with position along the fish;starvation reduced this longitudinal variation more in white than red muscle. In white muscle of fed cod, the glycolytic enzyme levels increased from head to tail, while in starved cod this longitudinal variation disappeared. In red muscle mitochondrial enzyme levels were highest in the caudal sample, but fewer differences were found for glycolytic enzymes. Swimming endurance was markedly affected by fish condition, with starved fish swimming only 30% of the time (and distance) of fed fish. This endurance was closely linked with the number of burst—coast movements during the test and the activity of CCO and LDH in white muscle. The number of burst—coast movements was significantly linked with condition factor and PFK activity in caudal red muscle and gill arch mass. Our data indicated that cod use both glycolytic and oxidative capacities to support endurance swimming. Furthermore, swimming endurance is linked with the metabolic capacities of red and white muscle.

Endurance and swimming speed were measured in mackerel, herring and saithe when they were induced by the optomotor response to swim at prolonged speeds along a 28-m circular track throughstillwater ina 10-mdiametergantry tank. Themaximumsustainedswimmingspeed(U,,, was measurcd as body lengths per second (H.L.S -') for each species and for saithe of direrent size groups. Hcrring with Lfm% of 4.06 B . L .~ ' (25.3 cm, 13.5' C) were the fastest, mackerel U,, was 3 . 5 1 3 .~~ ' (33cm. I1.7"C) and saithe (14.4"C) showed a size effect where Urn, at 25cm was 3.5 R.L.S ' and at 50cm 2.2 B . L .~ I . When swimming at speeds higher that U,,, all three species showed reduced endurance as speed increased. How thecurved track reduces the swimming speed is discussed.

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Sustained swimming of bluefin tuna was analysed from video recordings made of a captive patrolling fish school [lengths (L) 1.7-3.3 m, body mass (M) 54433 kg]. Speeds ranged from 0.6 to 1.2 L s-' (86260 km day-') while stride length during steady speed swimming varied between 0.54 and 0.93 L. Maximum swimming speed was estimated by measuring twitch contraction of the anaerobic swimming muscle in pithed fish 5 min after death. Muscle contraction time increased from the shortest just behind the head (3G50ms at 20% L) to the longest at the tail peduncle (8G90 ms at 80% L) (all at 28" C). A fish (L= 2.26 m) with a muscle contraction time of 50 ms at 25% L can have a maximum tail beat frequency of 10 Hz and maximum swimming speed of 15 m s -I (54 km h-') with a stride length of 0.65 L. With a stride length of 1 L a speed of 22.6 m s-' (81.4 km h-') is possible. Power used at maximum speed was estimated for this fish at between IOand40 kW, withcorrespondingvaluesforthedragcoefficient at a Reynoldsnumber of 4.3 x 10' of 0,0007 and 0.0027.

Burst swimming speeds were measured in mackerel 0.2754.380 m long by filming newly caught fish, first released into a large shore-sited tank, using a high-speed cine camera and real time TV camera. The highest speed was 5.50 m s-or 18 body length per second (B.L. s -') in a 0.305 m long mackerel at 12" C. The recorded maximum tail beat frequency of 18 Hz agrees well with 19 Hz predicted from the measured contraction time of 0.026 s for the anterior lateral swimming muscle. The stride length was close to 1 B.L.; the power, calculated from the drag, was4.53 W, and, calculated from the muscle used, was 5.07 W; all suggesting that the mackerel is swimming close to its physiological limit.