Bluegills: Biology and Behavior

5: Predation


Bluegills are most susceptible to predation as eggs and fry (Chapter 6), becoming less vulnerable as they grow. Their behavior shows it. Fear of predators, especially other fishes, diminishes upon reaching a certain size, enabling large specimens to wander freely among different habitats. A bluegill’s body depth, not its length, is the factor that determines whether it can be swallowed by another fish.529 In the bluegill, however, body length and depth show nearly perfect correlation (Figure 41), making them statistically redundant and therefore equally useful for predictive purposes.

The natural mortality of bluegills greater than 125 mm FL can be near 50%, little of it caused by predation.530 For example, predation on bluegills this size by spotted gars Lepisosteus oculatus in Indiana lakes was reported to be rare, although dead specimens less than 150 mm FL were sometimes eaten. A bowfin Amia calva of 740 mm TL and 4.3 kg from an Indiana lake contained two bluegills of 92 and 115 g (length unstated).531 Bowfins prefer soft-rayed fishes and crayfishes and are ineffective predators of bluegills.532

A predator’s gape limits the size of a bluegill it can swallow. Walleyes Sander vitreum can swallow bluegills of 127 mm TL,533 the limit for the largest northern pike is 160 mm TL, and for largemouth bass the maximum size is 150 mm TL534 or 160 mm TL,535 although other reports claim that larger sizes are vulnerable,536 including specimens to 192 mm TL.537 Centrarchids are not a preferred prey of pike.538 Bluegills, even small ones, are rejected because of their body morphology and evasiveness, and pike growth correlates negatively with the abundance of small bluegills.539 Information to calculate the gapes of largemouth bass based on their total lengths is given in Figure 42. A bluegill greater than 200 mm TL would likely be safe from all but the very largest piscine predators, the true “lunkers.” Even bluegills of 125 mm FL would constitute unlikely prey for all but the largest bass, and then the effort would be difficult.540 Estimating the vulnerability of bluegills of known size to predation by largemouth bass is a simple calculation. To swallow a bluegill of 200 mm TL, for example, would require a bass of 491 mm TL (Figure 43 and regression equation). In American anglers parlance this would be a 19-in fish weighing about 8 lb. Not many bass this big exist except in the southeast and certain California reservoirs. 541