Bluegills: Biology and Behavior

5: Predation

doi: https://doi.org/10.47886/9781888569933.ch5

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