Big Battles, Big Gonads: The Crazy World of the Bluegill Spawn

The bluegill is easy to take for granted: ubiquitous, easy to catch, quite likely swimming in a water body near you.

But, come spawning season, a bluegill colony may be one of the wilder scenes in nature: part barroom brawl, part cheesy ‘80s romantic comedy.

And this rowdy scene could actually have conservation implications.

The Biggest, Baddest Bluegill

Come spring, bluegills set up spawning beds – where the eggs are laid and protected. Central to spawning is what biologists call the parental male: a big bluegill that not only competes to reproduce with females, but also protects the eggs laid on the spawning bed.

Parental males clear the spawning beds of debris and guard them.

The bigger and badder the male, the better the location of the spawning bed. At the center of the colony, in the best position to fend off predators and other males, is the biggest and baddest bluegill of all.

“These males put all their energy into defending the nest and competing with the other male bluegills for the best females,” says Andrew Rypel, a research biologist with the Wisconsin Department of Natural Resources and bluegill expert. “All their energy goes into body growth. Being a parental male is a tough business, but his first goal is to attract a fantastic female mate. The female leaves after laying eggs, and the male has to guard the nest.” So far, this does not sound too outlandish: big males battle it out for the best females. The most fit and aggressive individuals are most attractive. The strongest pass on their genes. It’s a pretty standard wildlife narrative, right?

But here’s the wild card: Not all males participate in the festivities. As the jocks of the bluegill realm wage their battles, puny little specimens are watching on the sidelines. Biding their time.

The Little Fish with Very Big Gonads

“Bluegill males can have two completely different life histories,” says Rypel. “Some males opt out completely from growing big and guarding spawning beds. They become what we call sneaker males.”

Sneaker males are small. They do not look like they could win any battle. To an untrained eye, they may look like a different species.

Whereas the parental male puts all his energy into a big, strong body, the sneaker male puts all the energy into giant gonads.

“The sneaker male has stunted growth, but his testes are blown up like balloons,” says Rypel. “Whatever energy he acquires goes right into the testes.”

While the parental males are battling and protecting the spawning bed, the sneaker malesare roving around. And when the parental males are courting females, or fighting, the sneaker males, true to their name, sneak right onto the spawning bed.

“Because they have these huge testes, they have a lot of sperm,” says Rypel.

And so the sneaker male passes on his genes, too. It’s like that film where the quiet, wimpy guy wins out over the preening jock.

But there’s one more complication to the spring spawning bash – anglers.

The Big Fish That Didn’t Get Away

Bluegills are a favorite of anglers, especially in the spring, when they’re on the spawning beds. They’re easy to see, and easy to catch.

“Anglers aren’t interested in the sneaker males because they’re small,” says Rypel. “They’re targeting the biggest and most visible fish. And that means parental males.”

The parental males thus get hammered, leaving behind the small fish to do the breeding.

In many states, bluegill harvest limits are liberal. In Wisconsin, where Rypel works, the limit is 25 per day. What impact does removing all these parental males have on the population?

Many anglers think they know the answer: Not much. A common bit of fishing wisdom is that bluegills are prone to overpopulation, and harvesting as many as possible is necessary to prevent stunting.

But perhaps this ignores the complexity of bluegill social hierarchy.

And now new research conducted by Rypel casts doubt on that conventional wisdom. Heavy angling pressure may not reduce stunting at all. In fact, it may lead to decreases in fish size – the subject of tomorrow’s blog.