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Locomotion is an important behavior in the life history of animals and is characterized by discrete gaits, which may be adopted for optimal energetic efficiency, fatigue resistance, or maneuverability. We evaluated the kinematics and electromyography of Bluegill Sunfish (Lepomis macrochirus) swimming at different gaits to evaluate which factors might influence gait choice. When placed in the flume, Bluegill adopted a steady swimming gait until speeds reached 2.0 BL/s. When swimming volitionally, either in a laboratory pool or the field, Bluegill adopted an intermittent swimming gait (burst phase followed by a glide phase) and swam at average speeds of 1.0-1.3 BL/s. No statistical relationship was found between the kinematics of the burst and glide phases in either the lab or the field, so the phases were considered uncoupled. Furthermore, since the kinematics (tailbeat frequency, glide-duty factor) of lab and field volitional swimming were statistically identical, the EMGs of volition swimming in the lab likely reflect field effort. When relativized to volitional swimming speeds, the EMG intensities for both gaits were statistically identical. These results suggest that intermittent swimming may not reflect a strategy for energetic efficiency. Instead, the decoupling between the burst and glide phase may improve maneuverability, since 75% of 3D tracked intermittent swimming bouts (n=129) in the field involved a directional change. Although previous research suggests that intermittent swimming may also provide fatigue resistance, we hypothesize that intermittent swimming evolved in Bluegill as an adaptive gait for navigating their densely vegetated habitat. 

Locomotion that is driven by muscle activity dominates the daily energetic expenditure in most animals. In fish, routine propulsion when swimming at low, steady speeds and at various gaits is powered primarily by red, oxidative muscle. In Bluegill Sunfish (Lepomis macrochirus), swimming speed is thought to reflect the most energetically efficient gait type. Since field observations of Bluegill suggest that intermittent swimming is the preferred gait, we hypothesized that intermittent locomotion would be more energetically efficient than steady swimming. To test this hypothesis, we used electromyography to analyze muscle activation intensity of Bluegill swimming steadily in a flume and volitionally intermittently in a pool. In the flume, muscle activation intensity and tailbeat frequency increased as a function of speed. However, when swimming volitionally in the pool, muscle activation intensity varied relative to average velocity and tailbeat frequency was lower than in the flume at the same velocities. Although we expected muscle activation intensity to be higher when steady swimming at a given speed, ~48% of fish (n=11) had higher muscle activation intensities when swimming volitionally when compared at the same speed in the flume. Also, there was a positive relationship between speed and glide duration, but there was no relationship between speed and muscle activation intensity when swimming intermittently. Instead, intermittent swimming may lower fatigue and enhance maneuverability, rather than increase energetic efficiency. 

As generalists, Bluegill Sunfish (Lepomis macrochirus) feed in densely vegetated littoral and pelagic zones. Paradoxically, being a generalist requires that Bluegill adopt habitat-specific foraging strategies in order to successfully exploit local environments. To better understand their foraging behaviors, underwater cameras were deployed in different locations of Lake Waban, MA to reflect the diversity of local habitats within the lake. We identified three foraging strategies: hunting, grazing, and pelagic feeding. Each strategy is categorized as opportunistic or intentional and some are further subdivided into several modalities. Hunting occurs in shallow littoral zones, is intentional, often performed in groups, and is characterized by repeating cycles of burst-coast-stop-search until prey is visually detected. Grazing also occurs in shallow littoral zones, but is either intentional or opportunistic, and is characterized by three modalities depending on vegetation type. Active grazing involves biting and pulling on pondweed, whereas passive grazing involves hovering near milfoil and delicate suction feeding, and surface grazing involves searching beneath lily pads and explosive bouts of suction feeding. Pelagic feeding occurs in deep open water, is often opportunistic, may occur in groups, and is characterized by intermittent swimming from one morsel to the next. Some correlation exists between phenotype, age, and foraging strategy. For example, darker and deeper bodied adults engage in hunting, whereas lighter and fusiform Bluegill of all ages engage in pelagic feeding. These observations demonstrate the complex behaviors that characterize a paradigmatic generalist and illustrate the multitude of variables that impact their specific feeding strategies. 

Field observations of animal behavior are essential for guiding the interpretations of laboratory data in order to ensure that they coincide with biological reality. Knowing how an organism behaves in its natural environment is a necessary first step in bridging the gap between experimental data collected in the controlled, artificial environment of the lab and explaining the adaptive significance of measured traits. Field observations also challenge assumptions about behavioral definitions and the apparent discreteness of behaviors measured in the lab. As part of an ongoing study in the locomotor performance of Bluegill Sunfish (Lepomis macrochirus), we illustrate the role field observations play in contextualizing and expanding interpretations of experimental data and standard assumptions about Bluegill behavior. A comprehensive field study of Lake Waban (Wellesley, MA) and its inhabitants was carried out using underwater cameras, fish finding sonar, and temperature/luminosity loggers to develop a behavioral profile of Bluegill relative to their habitat and interspecific interactions. Although previous experimental work assumed Bluegill adopted locomotor strategies that maximized energy efficiency, field observations demonstrate that swimming performance is driven by a myriad of abiotic and biotic factors. These factors include the need to navigate complex habitats, to flee from predators, to adopt context-specific foraging strategies, to ward off rivals, and to coordinate social interactions. These observations add an extra dimension for understanding why Bluegill adopt particular swimming behaviors and how those behaviors might be adaptively significant at each stage of their life history.