The Secret Life of Bluegill - South Dakota State University

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The Secret
Life of Bluegill
By Dr. Dave Willis
T
here just is no way to sugar-coat
this. The sex life of bluegill is Xrated. Rowdy males are the culprits.
Scientists call this topic area "behavioral
ecology." They typically discuss what
"strategies" are used by an organism to
increase its likelihood of passing genes
on to the next generation. Male bluegills
actually have alternative strategies. They
may be "parental males" or they may be
"cuckolders." Really, that's a word.
Bluegills are colony spawners. They
excavate saucer-shaped nests, usually in a
group which resembles similar size craters.
Once you've seen a spawning colony of 20
or 30 depressions in shallow water, you
always recognize them. Some colonies are
much larger. Comparatively, largemouth
and smallmouth bass nest are quite similar
in shape and appearance, but are somewhat
larger and solitary. Always solitary.
For those of you who happen to be
interested in birds, bluegill colonies are
very much like the "leks" or dancing
grounds for birds such as prairie chickens
or sharp-tailed grouse.
Most Pond Boss readers probably
recognize the most common reproductive
strategy used by male bluegills. They
are called "parental males." Adult male
bluegills develop a large black tab on the
back of their gill cover. Kayla Condello is
holding a big parental male bluegill on the
cover of the MarchiApril 2005 Pond Boss.
By the way, what a smile, eh? You'd think
her dad was a dentist.
Parental males normally grow to a large
size before maturing. The advantage to this
largeness is a result of fish physiology.
Fish consume a given amount of energy
(food) during a given year. While the fish
is still immature, a large portion of energy
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can be used for body growth. Once a fish
matures, some or even most energy will
be diverted to gonads. Males mature testes
that produce milt, and females mature
ovaries which produce eggs. At the onset
of maturity, fish growth generally slows
because of this diversion of energy. If you
want to grow big bluegills in your pond,
the later in life they mature, the better.
By putting off sexual maturation, bluegill
grow to much larger sizes.
Consequently, in a well balanced
bluegill population, parental males grow
to a large size before sexual maturity.
Bigger bluegill dominate nests within a
spawning colony. There is some evidence
of an actual hierarchy in nest sites, with the
most "dominant" males getting the most
desirable central nests and being most
likely to attract a female for spawning.
"Cuckold males" actually come in two
varieties. Here is where X-rating starts
to become obvious. The smaller type of
cuckold males is termed a "sneaker." When
a parental male is spawning with a female,
the sneaker darts into the nest, releases milt
in an attempt to fertilize some of the eggs,
and then gets out of there before the big
male can get after him. In videotapes I have
seen, these "darts" are really quick!
The other type of cuckold male is termed
a "female mimic." These males actually
adopt the coloration of a female bluegill.
When a parental male is spawning with a
female, the female mimic will approach
the female from the opposite side, and
release milt to fertilize some of the eggs.
The parental male does not chase away this
female mimic, likely because of the female
coloration pattern.
One of the experts in bluegill
reproductive behavior is Dr. Bryan Neff
at the University of Western Ontario
in London, Ontario. At his geographic
location, he finds sneaker males mature at
2-3 years of age, female mimics mature at
4-5 years of age, and the parental males do
not mature until age 7 or 8. These ages may
vary somewhat by geographic location, but
they give you a sense of the large difference
in age at maturity given the reproductive
strategy of males. Cuckolds do not live as
long as parental males, and never reach the
parental size and status. So, they may pass
on their genes at a young age, but they do
not live as long.
Fish truly have complex social behaviors,
and we are just now starting to understand
some of them. I suspect this is because fish
are in an environment in which they are
hard to see. Let me pass on one amazing
"factoid" on bluegill reproduction. It
appears male bluegills may be able to
differentiate between their own offspring
A parental male bluegill (left) and a
female mimic (right) spawn with a true
female (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 opercular 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 once
the fry hatch in the nest! Current thought
by researchers is that males can tell the
difference by their sense of smell. Some
males have been observed selectively
eating some of the fry in the nest, and we
assume these are the offspring from the
cuckold males! Now honestly, isn't that
amazing??
So, what are the management implications of this reproductive behavior in
bluegills? Well, Dr. Bruce Condello actually started this discussion for us in the
MarchiApril 2005 issue of Pond Boss. Our
initial concern within the fisheries profession was harvesting the big parental male
bluegills may create a genetic advantage
for the cuckolds. Could we shift more of
the population to cuckolds in the long-term
because of fishing pressure? Certainly we
don't want to create a population of small
bluegills in your pond.
Well, a lot of research has gone into
this topic, especially in the large, natural
lakes in northern Michigan, Minnesota,
and Wisconsin, and southern Ontario.
To date, evidence has been sparse, and
genetic changes over time have not been
documented. However, one research
biologist with the Minnesota Department
of Natural Resources found the length at
which his male bluegills matured actually
increased by about I inch (from 7 inches
to 8 inches) after he reduced harvest of
big bluegills. Unfortunately, he could only
show this response in two of four study
lakes, probably meaning other factors are
also involved.
What about ponds? Well, I suspect
there probably should not be as much
concern over genetically increasing the
abundance of cuckolds in ponds managed
with largemouth bass and bluegills. Can
you think of any advantage for a male
July/August 2005 / POND BOSS
bluegill to stay small and vulnerable
rather than growing large when it lives
in a pond full of effective predators
such as largemouth bass? I can't either.
However, as Dr. Condello indicated in
his article, harvest strategies could affect
size of parental males in your pond. If you
harvest excessive numbers of big males,
parentals will start to mature at a smaller
size. Remember, once they mature, growth
slows. All of a sudden, big males may only
be 6-7 inches, rather than 8-10 inches long!
In such a situation, you probably should cut
back 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 structure
of bluegills that you are producing.
So there you have it. X-rated bluegills
playing all kinds of reproductive games
in Bob Lusk's family-oriented magazine.
What is the world corning to? I'm going
fishing.
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 and
can be reached there.
Be wise...choose your terms of endearment carefully!
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