Bluegill Spawning

Part I: The extended spawning season

Bream, sunfish, bluegill — call them what you will. There are many species of sunfish, and bream include different sunfish among different anglers. This is about bluegill.

Most anglers know bluegill spawn throughout the summer in most lakes and ponds. There may be a peak in spawning activity in some waters, two peaks in others, or the bluegill just begin spawning when the water tops 72 degrees and keep spawning until autumn. Redear sunfish also spawn throughout the summer.

Fish do what they do because it conveys long-term survival advantage to the species. While most fish in the sunfish family — the black basses, crappies and several dozen sunfish species spawn once each year — bluegill have taken a different route to perpetuate the species.

Bluegill are what biologists call “fractional spawners” — only a portion of the eggs in the female are mature or “ripe” at any point in time. As summer progresses, more eggs ripen and, presumably, the female will spawn again. This seems like a good way to ensure that a new crop of young bluegill are produced every year.

For example, if environmental conditions turn bad early in the summer and jeopardize the survival of early-spawned fish, the offspring from later-spawned fish can fill in the gap.

Bluegill are often abundant. While the extended spawning season or protracted spawning sounds like an effective strategy for producing a lot of young, is this as effective as it seems?

Mortality of bluegill, as for many other freshwater fishes, is high during their first year of life. The small fish are vulnerable to myriad predators. For the young that survive the predator net, winter is an inescapable challenge. Predation persists. The water is cold and metabolism slows, but the fish still have to burn energy to stay alive.

If food is limited, the young bluegill have to rely heavily on their own energy reserves.

Young bluegill feed on zooplankton and bottom-dwelling insect larvae and other invertebrates. Zooplankton are scarce in the winter; the insect larvae are abundant but small. Bluegill have food, but not necessarily enough food for growth.

With insufficient food, the young bluegill metabolize their own tissue to provide life-giving energy.

Large bluegill have more energy to draw on than small bluegill. Thus, the larger, early-spawned young-of-the-year bluegill are more likely to survive than the smaller, later-spawned bluegill.

Life ends when fish run out of energy.

A recent study by Illinois Natural History Survey confirms the above scenario.

Larger (2- to 2 1/2-inch long) and small (1-inch long) young bluegill were stocked into tanks at water temperatures equivalent to winter temperatures in northern waters (39 degrees) and southern waters (49 degrees). Some fish were fed, some not.

The small, unfed bluegill in the warmer water began to die after 40 days in the tanks, and most were dead after 60 days.

Small, unfed bluegill in cold water and small fed bluegill in warmer water began to die after 60 days and most were dead by 100 days.

Small, fed bluegills lived the longest; 50 percent of these fish were still alive after 150 days when the trials ended.

The larger bluegill had much higher survival. The unfed bluegill in the warmer water began dying after 100 days, and 40 percent remained alive after 150 days.

Only a few of the larger, fed bluegill in the warm water or the fed or unfed bluegill in the colder water died.

With high proportions of the later-spawned and, therefore, small bluegill unable to survive the winter, whether in an ice-covered northern lake or a cool southern lake, the protracted spawning strategy seems like an evolutionary dead end.

While several explanations can account for the potential benefits of protracted spawning, all suffer from the fact that the smaller, later-spawned fish have little chance of surviving even a short cold period without abundant food.

I offer a different hypothesis: The small fish that are unlikely to survive the winter do provide forage for small predators like young bass or flathead catfish that are often abundant where bluegill live.

Might the small, late-spawned bluegill provide a food source for these predators, thereby increasing the likelihood of survival of the larger, earlier-spawned bluegill?

While scientists seek reasonable explanations, bluegill —with their extended spawning — will continue to be abundant and prolific sport and forage fishes.