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luegill. America's favorite panfish.
Adored by many, abhorred by others.
Why such a stark attitude difference?
It almost always revolves around spawning,
or lack thereof. Bluegill are considered the
backbone of the food chain for largemouth
bass in the South and Midwest. Why? Because
they spawn often in warmer months. Bluegill
can keep pace with feisty hungry largemouth,
simply because there are more warmer months
than north of the Mason-Dixon Line. But, in the
north part of this country, biologists tend to turn
their noses up at bluegill for forage fish. Why?
Because they don't spawn early enough or
often enough to be an adequate forage species.
Not enough warm months. And, without much
competition for food and space, fast growing
young bluegill can become a dominant species
in any given pond or lake. At the same time,
anglers everywhere cherish the thought of a
one pound bluegill at the business end of light
tackle.
B
The bluegill is typically considered a
colonial, multiple-spawning fish. In general,
there is an expected, extended spawning
season where larval bluegills can be collected
throughout the summer in many geographic
locations. Generally, there will be peaks in
abundance of the 1/3 to 1/2 inch bluegill larvae
because most spawning is done in colonies and
thus is synchronized. About 95% of bluegill
offspring come from colonial spawning, and
only about 5% from solitary nesting males.
The issue here is the duration of bluegill
spawning in northern waters. Are there still
multiple spawns each summer? Heck, at some
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northern location, say southern Canada, won't
it be too cool to allow more than one spawn?
It's always something. Every time we think
something is simple, it turns out to be more
complex. That concept has especially proven
true for fish biology and behavior!
One study in Wisconsin lakes did indicate
variation in bluegill spawning duration, ranging
from as little as 31 days to as much as 112days
across three lakes. Understanding how these
differential durations in reproduction influence
bluegill spawning and eventual year-class
abundance could provide valuable information
into the population dynamics not only of
bluegill but also for their predators.
For some, but not all, fish populations, an
age-O fish going into the winter at a large size
will be more likely to survive the food-poor
winter period than a small age-O fish. Smaller
fish have lower energy reserves simply because
of their smaller size. Some fish species are
more likely to exhibit this overwinter pattern
than other fish species, and such patterns can
also vary by geographic location.
In fish populations that do experience
size-selective, overwinter mortality, both
when fish hatch and how fast fish grow are
important determinants for body size going
into the winter. Some studies have indicated
that later-hatched bluegills actually had higher
survival than their earlier-hatched counterparts.
However, little information was available on
bluegill reproductive biology in South Dakota
impoundments. Our study was developed to
provide initial insights in South Dakota.
After bluegills hatch and then absorb their
yolk sac, they suspend in the water column
across the entire pond or lake. We sampled
larval bluegills with a conical, fine-meshed
net pulled behind a work boat. A flow meter
is suspended in the mouth of the net, and we
can tell how much water we have filtered for
each sample. The more larval bluegill present,
the more we catch per volume of water towed.
The larvae are vulnerable to this net for only a
couple of weeks. After that, they have grown in
size and can swim to avoid the net. Because we
were looking for multiple spawns, we sampled
from June through September.
We worked at four impoundments to look for
differences among waters, and we also sampled
during the summers of 2005and 2006 to assess
potential differences between years. The water
bodies ranged from 100 to 800 acres in surface
area.
Both the peak larval bluegill abundances
(number per volume of water) and the date of
peak abundance were highly variable among
the four impoundments and between years.
However, only once out of eight possible
year-lake combinations did we see a "classic"
multiple-spawning series with three peaks in
larval bluegill abundance (Lake Mitchell, 2006,
Figure I). More typically, we saw a single peak
of larval abundance, sometimes with a small
second peak or at least an extended spawn with
low numbers of larvae produced. A typical
example is presented in Figure 2, showing
larval abundances over the summer of 2005 in
Lake Alvin.
As is typical with much of our research, we
did not get "all" the answers during this 2-year
September/October
2007
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Top-Figure 1. Trends over time for larval bluegill abundance (number per 100 cubic
meters of water sampled) in Lake Mitchell during the summer of 2006. Bottom-Figure
2. Trends over time for larval bluegill abundance (number per 100 cubic meters of
water sampled) in Lake Alvin during the summer of 2005.
study. However, what we learned is not just
"pie-in-the-sky" information. Because the
bluegill is important as both a sport fish and as
a prey fish for predators, understanding bluegill
reproduction is important.
For example, multiple studies have reported
that largemouth bass in northern climates
are vulnerable to overwinter mortality if
sufficient body size is not attained before the
onset of winter. From a fisheries management
standpoint, that means multiple-spawning
species such as bluegill may provide an
extended length range of prey for the.first-year
(age-O) largemouth bass, likely increasing the
growth and subsequent overwinter survival
for bass. During years such as 2006 in Lake
Mitchell, the age-O largemouth bass would
have had a continual supply of newly hatched
bluegill as a prey resource. They would have
had a chance to grow fast, assuming summer
water temperatures allowed them to do so. For
example, lots of food and average to aboveaverage summer temperatures likely would
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result in bigger age-Obass by late fall.
In contrast, during years such as 2005in Lake
Alvin, early-hatched.bluegills may outgrow the
gape size of age-O largemouth bass. That's a
key reason northern fisheries biologists thumb
their noses at bluegill as forage fish. Bass get
a fish prey source for a short time, but if no
more bluegill spawning occurs, they may have
to feed on less desirable prey such as aquatic
invertebrates. That means slower growth rates
which could lead to smaller size bass going into
their first winter.
Only time will tell just how important
this variability in bluegill spawning might
be. Meanwhile,it's fun to keep looking and
learning!
Kris Edwardsis a graduatestudentandDave
Willis a professorin theDepartmentof Wildlife
and Fisheries Sciences.South Dakota State
University.
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