This ubiquitous panfish is a biological marvel, honed by selective forces to be a survivor, a reproducer, and more — an exciting gamefish and worthy adversary. And at no time is that truer than below the ice. Because the bluegill is so common and able to exist in a range of habitats, many scientific studies have been conducted with them. At least one scientist has labeled them the lab rat of the fish world. In his book, Bluegills, Biology and Behavior, Stephen Spotte of the Mote Marine Lab in Florida, has reviewed hundreds of these studies and presented fascinating facts about this favorite fish. The book is available from the American Fisheries Society, fisheries.org.
If you watch a 'gill closely (underwater or in an aquarium), you can see the marvelous way it uses its pectoral fins, located close to its center of mass, to scull back and forth. Watch it move forward, stop on a dime, turn around, or move in reverse. The fin rays pulse like a fan, but in complex movements that push water in uncountable ways. Countering movements of the left and right fins give remarkable maneuverability.
In addition to its fins, the round compressed shape of its body helps it make fast turns, stops, and other maneuvers needed to capture prey or escape predation, though at the sacrifice of speed. Less obvious but important to holding position is the fish's manipulation of the upper and lower lobes of its caudal fin, as well as the anal fin. Under control of the brain, fin rays control the position and shape of the fins. When swimming forward, the dorsal fin undulates continuously, creating thrust and a resulting vortex behind the fish.
These features make this species ideally suited to feeding on small, slow-moving objects at close range. Bluegills thrive among dense stands of aquatic plants, where they can pluck invertebrates from stalks or turn tail-up and feed on worms and small benthic creatures.
Hearing and Lateral Line: The bluegill's movements are directed by its brain, based on input from its complex sensory apparatus. They're considered hearing generalists, possessing no special adaptations to increase hearing range or acuteness, unlike hearing specialists like minnows, catfish, and characids. Yet, their sound detection is remarkable.
Minute hair cells in the inner ear and lateral line detect water displacement. At close range, neuromasts in the lateral line, running down the length of the fish, detect the velocity of water moving past, from underwater disturbances caused by other moving objects. The otoliths of the inner ear detect the fish's acceleration and help it maintain equilibrium. These related sensory systems work in concert so a bluegill is constantly aware of its position and speed, and that of stationary objects as well as other fish and creatures. They enable the fish to maintain its position in a group of fish, avoid obstacles, detect and capture prey, and avoid predators.
The lateral line's detection range is thought to extend no more than a few body lengths from the fish, being most effective within one body length. Using these senses, bluegills detect and presumably identify vortices created by the movement of passing fish. The wake left by a swimming fish or other aquatic animal slowly attenuates, but not without leaving clues about its owners location and distance.
Some scientists have labeled these invisible trails "fish footprints" or "fish fingerprints." A study found that the wakes of pumpkinseeds in still water remained up to five minutes. Other studies have shown that bluegills can detect and approach small preyfish by following the hydrodynamic trails they leave. When bluegills' lateral lines were blocked by a cobalt treatment, they lost this ability. Researchers have postulated the lateral-line sense of bluegills is sensitive enough to detect swimming insect larvae, even in complete darkness.
Vision: Scientists consider bluegills a primary sight feeder, relying on vision heavily though certainly not exclusively. Because the ability to focus is related to the diameter of the lens, visual ability of bluegills improves as the fish grows. Dr. Keith Jones, chief fish researcher at Pure Fishing, reports that bluegill vision is rated high, compared to most other species, and similar to the largemouth bass. "The area of highest concentration of cone cells, those that enable color vision and improve visual acuity is called the fovea," Jones says. "The location of the fovea tells us about the direction a species tends to feed in. For bluegills, the fovea is somewhat toward the upper part of the retina, suggesting that they see best when tilted downward slightly." This is a position they often feed in, especially when browsing among plants, and it would be beneficial for bottom-feeding as well.
"Bluegills have two types of cones that peak in the range of green and red areas of the spectrum," Jones says. "This red-green combination is well matched to a daytime existence, enhancing contrast at dawn and dusk to increase feeding efficiency near the surface, while also increasing vision in deeper water during midday. Like bass, they seem to have little discrimination in the blue range. And they seem to have no capability to detect UV light. As for polarized light, early field studies suggested they were sensitive to it, but physiological studies have found no mechanism for this ability."
This cone pattern also helps them detect prey against the generally yellowish-green backgrounds they encounter in many waters far better than most freshwater species. This ability to heighten contrast is particularly helpful when feeding on nearly transparent zooplankton in open water.
Bluegills also excel in accommodation, meaning that they can extend or withdraw the lens of each eye to better focus on near objects. This movement is like the lens adjustment on an SLR camera, which focuses clearly on objects at varying distances as the lens is moved in and out. At close range, the lens protrudes toward the fish's snout, providing good discrimination of small nearby objects, such as zooplankton. These adaptations make the bluegill most adept at detecting small mobile prey in bright environments.
Their ability to find prey, particularly small ones, is reduced as turbidity increases and light levels decline. Field studies and lab experiments indicate that bluegills sometimes forage after dark, likely relying on their lateral line and inner ear systems.
Taste: According to Jones, bluegills have a sophisticated sense of taste, surprising given their habit of nibbling on the toes of bathers. Indeed, bits of dead skin might not be so distasteful as humans find them. "In our lab tests," he says, "bluegills are more sensitive to taste than crappies or largemouth bass. But in general taste preferences, they are similar to bass."
While some might find this surprising, there's considerable overlap in diet, especially when you consider fry and fingerling stages. "Overall, taste is phylogenetically conservative," he adds, "meaning that the genes that control taste do not vary greatly among closely related fishes, such as those in the centrarchid family. The sense of taste is under strong genetic control."
Jones' research has sought to define organic flavors and chemical classes, such as amino acids, that a species prefers. "Once we find the flavors they like, we can spike synthetic materials with high doses of those flavors and come up with baits that are preferred over food that they eat in nature." These findings have been used to create many of Berkley's successful panfish formulas in the Gulp! and PowerBait lines.
Bluegills are highly opportunistic feeders, especially when you consider the size of their mouth. Compared to the closely related pumpkinseed, their mouth is better suited to feeding on plankton. Pumpkinseeds have pharyngeal pads covered with flat, grinding teeth that enable them to crush and eat snails and mussels, but the pads of the bluegill are covered with fine needlelike teeth best suited to feed on small soft prey.
In addition to plants and many types of zooplankton, they consume great numbers of larval forms of aquatic insects, from gnats and flies to dragonflies and stoneflies. They also eat eggs of their own species and any others they can find, along with aquatic invertebrates including small crayfish and snails. Bluegills consume considerable amounts of aquatic plants in some waters, including pondweeds, cattails, elodea, and filamentous algae.
To ingest prey in open water, bluegills rely on suction feeding. To do this, a fish rapidly opens its mouth (less than 1/25 of a second), creating negative pressure as the mouth cavity expands, which causes a stream of water to rush into its mouth. If a prey item is caught in the stream of water, it's eaten. Observations even indicate that this inrush of water is capable of dislodging creatures clinging to sticks or plants. Studies comparing the feeding of bluegills and bass found bluegills about twice as efficient at capturing their target.
A study done during winter in Michigan found that bluegills reduced food intake considerably during winter. In early winter, they relied on mayfly nymphs, turning to Cladocerans, a type of zooplankton, in mid-winter, then insects at late ice. In mid-winter, bluegills ate more midge larvae that mayfly larvae. Overall feeding declined from early winter to mid-winter, rebounding in late February and March.
Next time 6-inchers surround your ice hole, don't disparage the little bait stealers, but watch their remarkable ability to sulk close to a lure, stare it down, then dismissively blow it off with a sudden blast of water. And sometimes, pulling a bait from a flock of little guys gets nearby bulls, lurking in the vegetation, to rise up and check out the action.