The survival advantage of this uncommon life-history strategy remains unexplained.
But there are other mysteries to reproduction of this feisty and fine-eating sport fish pursued by millions of anglers, young and old, every year.
Bluegills often nest in groups, or colonies, where several dozen or more large male bluegills build and guard the saucer-like nest. These are the "bream beds" avid bluegill anglers are constantly searching for.
Colonial nesting probably conveys survival advantage to the bluegill. The concentration of large male bluegill better keeps nest-robbing invaders out. The beds are probably located where environmental conditions are most favorable for successful spawning.
Fisheries researchers have noted that the largest males occupy the central portion of the colony, with smaller males nesting at the periphery.
The nests at the periphery provide less-desirable conditions, are more vulnerable to predators and are likely produce fewer young.
In addition, the females are attracted to the largest male bluegill.
Collectively, these factors could serve to regulate the number of bluegill produced.
Large numbers of large male bluegill delays the sexual maturation of young bluegill, but removal of large male bluegill triggers earlier maturation of smaller, younger bluegill. These small but sexually mature bluegill, in turn, invest more energy in the development of reproductive products and in the acts of nest building, spawning and nest guarding and, therefore, have less energy for growth.
Their growth rate slows and results in a population with smaller average size.
This creates an interesting management dilemma.
Often, populations of small, slow-growing bluegill are the result of too many bluegill competing for limited food, and the management solution is to reduce the numbers of bluegill.
For years, managers have encouraged high harvest to thin bluegill numbers to increase growth rate and the numbers of large bluegill. In some populations, it appears that maintaining good numbers of large bluegill by reducing the harvest of large fish can accomplish the same end result.
Possibly the most unusual aspect of bluegill reproduction is "sneaker" male bluegills. The "sneakers" are small male bluegill that do not build or guard nests or participate in the courtship of females. Rather, they wait outside the range of attack of the larger nest owner; when the female has been sufficiently stimulated by a nest-tending male and begins to release eggs, the sneaker dashes in and fertilizes the eggs.
While it is tempting to think that these are just young, oversexed bluegill that haven't mastered nest construction and the finer points of fish romance, these fish are different from the other male bluegill. The sneakers have slower growth rates, have a relatively high proportion of their body weight as testes, have a higher density of sperm in the milt and their sperm swim faster.
The slower growth rate may be a result of investing more energy into development of the testes. The larger testes relative to the size of the fish, greater sperm density and the faster sperm are mechanisms to increase the likelihood of the sneakers fertilizing some of the eggs released by the female.
Genetic analysis of the offspring from a nest ("paternity testing") prove that the sneakers are effective at fertilizing some of the eggs.
Clearly, there is a genetic basis for the sneaker males, but what triggers male bluegill to develop as sneakers is unresolved. The sneaker male condition is also known from several other fish in the sunfish family, including pumpkinseed and spotted sunfish.
Bluegill serve a dual role in Mississippi waters as both an important forage fish and a desirable sport fish. They receive little management attention.
The "conventional wisdom" that bluegill are abundant and highly productive and liberal harvest is desirable is sufficient in waters with relatively low fishing effort for bluegill. The complex reproductive tactics present management challenges in waters with high harvest of bluegill.