The Secret Life of Bluegill

There just is no way to sugar-coatthis. The sex life of bluegill is X-

rated. Rowdy males are the culprits.Scientists call this topic area "behavioral

ecology." They typically discuss what"strategies" are used by an organism toincrease its likelihood of passing geneson to the next generation. Male bluegillsactually have alternative strategies. Theymay be "parental males" or they may be"cuckolders." Really, that's a word.

Bluegills are colony spawners. Theyexcavate saucer-shaped nests, usually in agroup which resembles similar size craters.Once you've seen a spawning colony of 20or 30 depressions in shallow water, youalways recognize them. Some colonies aremuch larger. Comparatively, largemouthand smallmouth bass nest are quite similarin shape and appearance, but are somewhatlarger and solitary.Always solitary.

For those of you who happen to beinterested in birds, bluegill colonies arevery much like the "leks" or dancinggrounds for birds such as prairie chickensor sharp-tailed grouse.

Most Pond Boss readers probablyrecognize the most common reproductivestrategy used by male bluegills. Theyare called "parental males." Adult malebluegills develop a large black tab on theback of their gill cover. Kayla Condello isholding a big parental male bluegill on thecover of the MarchiApril 2005 Pond Boss.By the way, what a smile, eh? You'd thinkher dad was a dentist.

Parental males normally grow to a largesize before maturing. The advantage to thislargeness is a result of fish physiology.Fish consume a given amount of energy(food) during a given year. While the fishis still immature, a large portion of energy

20

..

The SecretLife of Bluegill

By Dr. Dave Willis

can be used for body growth. Once a fishmatures, some or even most energy willbe diverted to gonads. Males mature testesthat produce milt, and females matureovaries which produce eggs. At the onsetof maturity, fish growth generally slowsbecause of this diversion of energy. If youwant to grow big bluegills in your pond,the later in life they mature, the better.By putting off sexual maturation, bluegillgrow to much larger sizes.

Consequently, in a well balancedbluegill population, parental males growto a large size before sexual maturity.Bigger bluegill dominate nests within aspawning colony. There is some evidenceof an actual hierarchy in nest sites, with themost "dominant" males getting the mostdesirable central nests and being mostlikely to attract a female for spawning.

"Cuckold males" actually come in twovarieties. Here is where X-rating startsto become obvious. The smaller type ofcuckold males is termed a "sneaker."Whena parental male is spawning with a female,the sneaker darts into the nest, releases miltin an attempt to fertilize some of the eggs,and then gets out of there before the bigmale can get after him. In videotapes I haveseen, these "darts" are really quick!

The other type of cuckold male is termeda "female mimic." These males actuallyadopt the coloration of a female bluegill.When a parental male is spawning with afemale, the female mimic will approachthe female from the opposite side, andrelease milt to fertilize some of the eggs.The parental male does not chase away thisfemale mimic, likely because of the femalecoloration pattern.

One of the experts in bluegillreproductive behavior is Dr. Bryan Neff

at the University of Western Ontarioin London, Ontario. At his geographiclocation, he finds sneaker males mature at2-3 years of age, female mimics mature at4-5 years of age, and the parental males donot mature until age 7 or 8. These ages mayvary somewhat by geographic location, butthey give you a sense of the large differencein age at maturity given the reproductivestrategy of males. Cuckolds do not live aslong as parental males, and never reach theparental size and status. So, they may passon their genes at a young age, but they donot live as long.

Fish truly have complex social behaviors,and we are just now starting to understandsome of them. I suspect this is because fishare in an environment in which they arehard to see. Let me pass on one amazing"factoid" on bluegill reproduction. Itappears male bluegills may be able todifferentiate between their own offspring

A parental male bluegill (left) and afemale mimic (right) spawn with a truefemale (center). Photo courtesy of Dr.

Bryan Neff.

POND BOSS / July/August 2005

Left: A large parental male bluegill. Note the dark opercular (gill flap) tab. Right: A large female bluegill. Note the smaller opercu-lar tab compared to the male. Also. females tend to have a more golden color on the breast. while the males are more coppery.

and offspring from cuckold males oncethe fry hatch in the nest! Current thoughtby researchers is that males can tell thedifference by their sense of smell. Somemales have been observed selectivelyeating some of the fry in the nest, and weassume these are the offspring from thecuckold males! Now honestly, isn't thatamazing??

So, what are the management implica-tions of this reproductive behavior inbluegills? Well, Dr. Bruce Condello ac-tually started this discussion for us in theMarchiApril 2005 issue of Pond Boss. Ourinitial concern within the fisheries profes-sion was harvesting the big parental malebluegills may create a genetic advantagefor the cuckolds. Could we shift more ofthe population to cuckolds in the long-termbecause of fishing pressure? Certainly wedon't want to create a population of smallbluegills in your pond.

Well, a lot of research has gone intothis topic, especially in the large, naturallakes in northern Michigan, Minnesota,and Wisconsin, and southern Ontario.To date, evidence has been sparse, andgenetic changes over time have not beendocumented. However, one researchbiologist with the Minnesota Departmentof Natural Resources found the length atwhich his male bluegills matured actuallyincreased by about I inch (from 7 inchesto 8 inches) after he reduced harvest ofbig bluegills. Unfortunately, he could onlyshow this response in two of four studylakes, probably meaning other factors arealso involved.

What about ponds? Well, I suspectthere probably should not be as muchconcern over genetically increasing theabundance of cuckolds in ponds managedwith largemouth bass and bluegills. Canyou think of any advantage for a male

July/August 2005 / POND BOSS

bluegill to stay small and vulnerablerather than growing large when it livesin a pond full of effective predatorssuch as largemouth bass? I can't either.However, as Dr. Condello indicated inhis article, harvest strategies could affectsize of parental males in your pond. If youharvest excessive numbers of big males,parentals will start to mature at a smallersize. Remember, once they mature, growthslows. All of a sudden, big males may onlybe 6-7 inches, rather than 8-10 inches long!In such a situation, you probably should cutback on your harvest of the biggest males.It may even help to harvest the 7-8 inch

males, and release 9 inch and longer males,depending, of course, on the size structureof bluegills that you are producing.

So there you have it. X-rated bluegillsplaying all kinds of reproductive gamesin Bob Lusk's family-oriented magazine.What is the world corning to? I'm goingfishing.

Dr. David W. Willisis a professor in the Department

of Wildlife and Fisheries Sciences, South Dakota

State University in Brookings. He frequents the

Pond Boss forum at www.pondboss.com andcan be reached there.

Be wise...choose your terms of endearment carefully!

21