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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. 

Locomotion is vital to the survival and fitness of animals and dominates daily energy budgets. The main energy consuming process of locomotion is the muscle activity needed to maintain stability or generate propulsive forces. In fish, the speed of swimming is thought to depend on the gait type, which may reflect an energetically efficient locomotory behavior. Bluegill Sunfish (Lepomis macrochirus) exhibit either steady or intermittent (burst-coast) gaits when swimming in the field, but whether these gaits differ in their energetic efficiency is unknown. We analyzed the electromyography (EMG) of oxidative muscle in Bluegill swimming at low velocities to determine if steady swimming is more or less energetically efficient than intermittent swimming. EMG data were acquired using bipolar fine wire electrodes implanted into oxidative musculature at 2/3 tail length. Steady swimming EMGs were recorded in a flume (fish treadmill) at incrementally increasing speeds relative to body length, until nonoxidative muscle was recruited. As speed increased, EMG intensity increased, which corresponds to increased muscle recruitment. Fish reached maximum EMG intensity (100% oxidative muscle capacity) between 1.75 - 2.25 BL/s. Intermittent swimming EMGs were recorded while the fish swam volitionally in a pool. The burst phase consisted of 2-3 tailbeats, followed by a coast phase duration of 1 second or less. Based on preliminary results, fish in the pool swam at an average of 62.1% (n = 10) of their maximum oxidative capacity. When intermittently swimming, muscle activity was 37.9% more efficient than steady swimming at similar speeds. This demonstrates that when swimming volitionally Bluegill choose the most energetically effective gait. However, further analysis is needed to determine how individual variation affects swimming performance. Continued comparison of these methods of locomotion will broaden the understanding of energy decisions that fish make. These results suggest that intermittent swimming is the more energetically efficient form of aquatic locomotion. This work is supported by NSF grant award number 2135851. 

P1-97: In early summer, nesting Bluegill Sunfish (Lepomis macrochirus) expend large amounts of energy building nests, spawning, protecting their offspring, and chasing away predators. The energetic demands of nesting are likely significant and may heavily influence their life history and reproductive success. However, the exact metabolic cost of nesting is difficult to quantify without precise information about the three dimensional position of the center of mass of nesting fish. Field observations of Bluegill Sunfish nesting in Lake Waban (Wellesley, MA) were obtained throughout June until early July by using underwater cameras, fitted with a temperature and light sensor, calibrated to allow three dimensional tracking. The positional data of nesting Bluegill Sunfish were analyzed to derive velocity and acceleration in order to calculate their metabolic rate. We chose to analyze repetitive nesting behaviors, such as rim circling and defensive chasing, due to their frequency of occurrence and consequentially high metabolic demand. Using metabolic rates calculated from Bluegill swimming in a flume, we found that rim circling is nearly 22.2 times more metabolically expensive than the average metabolic cost of swimming in a straight path for the same velocity and duration. Since rim circling is so metabolically expensive and since we estimate that rim circling occurs nearly 25,500 times during an 8 day nesting period, our results strongly suggest that the nesting cycle is one of the most critical periods in the life history of Bluegill Sunfish. The high energetic demands of nesting and the temporary bout of starvation while the fish occupies its nest results in a small margin of error for reproductive success. These conclusions deepen our understanding of male Bluegills’ true paternal investment and can serve to illuminate our understanding of their life history from a quantifiable perspective. 

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. 

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.