We tested the hypothesis that the energetics of swimming in a flume accurately represent the costs of various spontaneous movements using empirical relationships between fish swimming costs, weight, and speed for three swimming patterns: (1) ‘forced swimming’ corresponded to movements adopted by fish forced to swim against a unidirectional current of constant velocity; (2) ‘directed swimming’ was defined as quasi‐rectilinear movements executed at relatively constant speeds in a stationary body of water and (3) ‘routine swimming’ was characterized by marked changes in swimming direction and speed. Weight and speed explained between 76% (routine swimming) and 80% (forced swimming) of net swimming cost variability. Net costs associated with different swimming patterns were compared using ratios of model predictions (swimming cost ratio; SCR) for various weight and speed combinations. Routine swimming was the most expensive swimming pattern (SCR for routine and forced swimming =6.4 to 14.0) followed by directed (SCR for directed and forced swimming =0.9 to 2.8), and forced swimming. The magnitude of the difference between the net costs of forced and spontaneous swimming increases with movement complexity and decreases as fish weight increases.

We performed respirometry experiments to estimate the spontaneous swimming costs of brook trout (Salvelinus fontinalis) for 24 combinations of fish weight (3.5, 17, and 32 g), water temperature (4, 12, and 18°C), and respirometer size (27, 54, and 108 L). Fish swimming characteristics were estimated for each experiment using videocamera recordings and image analysis. Under our experimental conditions, average swimming characteristics of fish, such as swimming speed and turning and acceleration rates, varied from 2.5-to 29-fold. Our data, alone or combined with similar published results on brook trout weighing 1 g, indicated that fish weight was the only variable that could explain a statistically significant proportion of the variations of spontaneous swimming costs for that species (r 2 = 0.91). Our work confirms, with a wider range of experimental data, that spontaneous swimming costs of fish are 3-to 22-fold (8-fold average difference) more energy demanding than predicted by forced swimming models developed using fish swimming at constant speeds and directions in flumes.Résumé : Nous avons réalisé des expériences respirométriques visant à estimer les coûts de la nage spontanée de l'omble de fontaine (Salvelinus fontinalis) pour 24 combinaisons de masse des poissons (3,5, 17 et 32 g), de température de l'eau (4, 12 et 18°C) et de taille de respiromètres (27, 54 et 108 L). Les caractéristiques de nage des poissons ont été estimées, pour chaque expérience, à l'aide d'enregistrements par vidéocaméras et d'analyses d'images. Sous nos conditions expérimentales, les caractéristiques de nage moyennes dont la vitesse de nage, les taux de virage et d'accélération ont varié par un facteur de 2,5 à 29. Nos données, seules ou combinées avec des résultats similaires publiés pour des ombles de fontaine de 1 g, ont indiqué que la masse des poissons est la seule variable qui peut expliquer une proportion statistiquement significative des coûts de la nage spontanée pour cette espèce (r 2 = 0,91). Nos travaux confirment, avec une gamme plus étendue de données expérimentales, que les coûts de la nage spontanée des poissons sont 3 à 22 fois (8 fois en moyenne) plus élevés que ceux prédits par les modèles de la nage forcée développés avec des poissons nageant à des vitesses et dans des directions constantes.

We estimated the cost of spontaneous swimming and the swimming characteristics of juvenile brook trout for 21 combinations of water temperature (3.5, 6, 9, 12, 15, 18, and 20.7 °C) and respirometer volume (27, 54, and 108 L). Spontaneous swimming costs were estimated as the oxygen depletion in the respirometers corrected for biological oxygen demand of the water and standard metabolism of the fish. Spontaneous swimming costs varied 9-fold among our experiments. Swimming characteristics, such as the average and the variance of speed, acceleration, and turning rates, were determined using a pair of video cameras. Swimming characteristics varied 2- to 10-fold among experiments. Speed and turning rate tended to increase with water temperature up to 18 °C and decreased at 20.7 °C. Water temperature (r2 = 0.44) was the only variable that could explain a significant portion of the variations of spontaneous swimming costs between 3.7 and 20.7 °C. Variance of speed (partial R2 = 0.32) and the average turning rate (partial R2 = 0.34) explained 53% of the variation between 3.7 and 18 °C. Average speed never explained more than 35% of spontaneous swimming cost variation.

We tested the hypothesis that swimming characteristics of an active planktivorous fish, brook trout (Salvelinus fontinalis), are significantly influenced by the size of enclosure and the time of day. Swimming characteristics of 0+ brook trout kept in 1,- 8,- and 27-m3 enclosures were recorded with an underwater videocamera system between 06:00 and 20:00. Enclosure size had a significant influence on swimming characteristics. Median speed (range = 6.4–11.1 cm/s) and associated variance observed in the 27-m3 enclosure were approximately twice those estimated in the 1- and 8-m3 enclosures. Variance of acceleration rates varied three- to fourfold among enclosures. Median turning rates (range = 11.8–19.8°/s) and corresponding variances tended to decrease as enclosure size increased. Our analyses suggest that models of spontaneous swimming costs developed using respirometry experiments performed in small aquaria may not appropriately represent the complexity of swimming patterns, and consequently, the costs of spontaneous swimming in large enclosures or in the field.

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