Significance: Swimming ability has contributed to the evolutionary success of fishes, and its mechanics have been studied extensively. Most fishes swim primarily through undulation of their body and caudal fin (BCF) and have been historically divided into four major kinematic modes based on their morphology. Here, we compare kinematics of BCF locomotion in 44 species. Contrary to expectations and despite considerable morphological diversity, fishes share major kinematic features during steady swimming and are placed on a continuum rather than in discrete categories. This suggests a unifying BCF mechanism to generate efficient aquatic propulsion. Our work reevaluates a well-established hypothesis in biomechanics, highlighting the importance of avoiding a priori partitioning of fishes into modes, to further our understanding of aquatic locomotion.

Quantifying the intricate relationship between locomotion, energy expenditure and environmental heterogeneity is pivotal for elucidating the ecological and evolutionary changes in locomotor performance in fishes. This Commentary synthesizes existing research to offer a perspective on how fishes actively exploit complex environments to enhance their locomotor efficiency. Contrary to conventional portrayals of fishes as passive responders to environmental stressors, empirical evidence supports the idea that fishes employ sophisticated strategies to navigate diverse hydrodynamic landscapes. Here, we show clever ways fishes bend the rules of a non-linear speed–energetics curve to save energy below and above optimal cruising speeds. The findings from these behavioral adjustments hold broader implications for understanding fish performance under dynamic environments and conserving fish populations.

Many aquatic animals swim by undulatory body movements and understanding the diversity of these movements could unlock the potential for designing better underwater robots. Here, we analyzed the steady swimming kinematics of a diverse group of fish species to investigate whether their undulatory movements can be represented using a series of interconnected multi-segment models, and if so, to identify the key factors driving the segment configuration of the models. Our results show that the steady swimming kinematics of fishes can be described successfully using parsimonious models, 83% of which had fewer than five segments. In these models, the anterior segments were significantly longer than the posterior segments, and there was a direct link between segment configuration and swimming kinematics, body shape, and Reynolds number. The models representing eel-like fishes with elongated bodies and fishes swimming at high Reynolds numbers had more segments and less segment length variability along the body than the models representing other fishes. These fishes recruited their anterior bodies to a greater extent, initiating the undulatory wave more anteriorly. Two shape parameters, related to axial and overall body thickness, predicted segment configuration with moderate to high success rate. We found that head morphology was a good predictor of its segment length. While there was a large variation in head segments, the length of tail segments was similar across all models. Given that fishes exhibited variable caudal fin shapes, the consistency of tail segments could be a result of an evolutionary constraint tuned for high propulsive efficiency. The bio-inspired multi-segment models presented in this study highlight the key bending points along the body and can be used to decide on the placement of actuators in fish-inspired robots, to model hydrodynamic forces in theoretical and computational studies, or for predicting muscle activation patterns during swimming.

The ability to move between habitats has important implications for fitness in many species. In‐stream barriers such as culverts can impede movements of riverine fishes and thus reduce connectivity between habitats. The ability of fish to overcome barriers is related to the features of the environment and the barrier itself, but also to physiological, morphological and behavioural traits of the fish. Among these, body shape varies among and within species, and influences swimming ability, a key component of passage performance through culverts. We conducted an experimental study on wild brook trout (Salvelinus fontinalis) to assess the effects of individual body shape on attempt rate and passage success through culverts on six streams. A more streamlined body shape was associated with an increased motivation to enter and ascend the culverts, and, to a lesser extent, with the probability of successful passage once an attempt was staged. Motivation and successful passage were also influenced by the density of conspecifics below the culvert, time of day, fish body size and water velocity. Policy implications. While fish body shape is expected to influence swimming performance, our research shows the most important effect of body shape to be on an individual's motivation to stage passage attempts at culverts. This study points to an important connection between behaviour and morphological traits that influence passage success and suggests that in‐stream barriers may be an important agent of selection on behaviour and morphology in wild fish populations.

River herring (Alosa spp.) are anadromous fish that enter North American Atlantic coastal rivers and lakes each spring to spawn. Anthropogenic structures such as dams and tide gates serve as physical obstacles that limit river herring access to spawning habitat. This study examined the physical and ecological components affecting herring passage through a tide gate by applying a time-to-event analysis framework to multiple movement behaviors derived from telemetry data. Herring had higher passage success early in the season (78%) than later (16%). Key behaviors that govern passage varied with diel period, tide, and flow direction through the gates. Furthermore, these behaviors shifted as the season progressed, consistent with the hypothesis that predator avoidance may be driving passage failure late in the spawning season.