An Analysis of the Fishery of Clear Lake, Iowa
James Wahl
A. History.
Bailey
and Harrison (1945) described the fish community of Clear Lake based on
collections made from 1941 through 1943.
Although no density estimates were made, relative abundance was assigned
to all species captured. Their work
offers the most complete historical record of fishes found in Clear Lake.
When
comparing the current fish community to that reported by Bailey and Harrison in
the early 1940’s there is one striking difference. Members of the Centrachid family were present in much greater
numbers historically than what is currently found. Largemouth bass were listed as very common and were considered to
be one of the dominant predators in the lake.
Bluegill were ranked as very abundant and along with bullhead were
considered to be the most abundant fish.
Crappie were listed as abundant and cited as one of the four most
abundant species in Clear Lake. All
three of these species are currently found in Clear Lake, however their
abundance would best be described as occasional.
The
disappearance of these species may be directly related to the loss of aquatic
vegetation in the lake. During and
prior to the 1940’s, Clear Lake supported extensive beds of both emergent and
submergent vegetation. Bass, bluegill
and crappie utilized these areas for spawning and nursery cover. As the vegetation declined, so did the
populations of these species, which were dependant upon this critical
habitat. Although emergent vegetation
(bulrushes) remains in the lake today its coverage has been greatly reduced and
submergent vegetation is virtually nonexistant.
Two
species that were historically abundant and remain in high densities today are
bullhead and carp. Bailey and Harrison
(1945) listed bullhead as very abundant, and carp as common. Biomass estimates completed by DNR fisheries
staff in 1999 and 2000 revealed bullhead density to be 150 to 300 lbs/acre, and
carp at 100 to 200 lbs/acre. Although
these species were abundant in the 1940’s, it is unlikely that they dominated
the total standing stock as they do today.
Despite
the historical presence of bullhead and carp, aquatic vegetation flourished in
Clear Lake. Apparently the density of
these bottom-feeding fishes was not great enough to have a severe impact on
vegetation. As water quality
deteriorated in Clear Lake and water clarity became reduced the vegetation
started to decline. As stated earlier,
the loss of vegetation severely impacted populations of bass, bluegill and
crappie. With a void created by their
absence, it is likely that bullhead and carp increased in numbers taking
advantage of the degraded environment, which they were better suited for.
B. Strategies for Improving Fish Community
and Habitat.
It
is obvious that there have been major changes in the Clear Lake fish community
over the past 50 to 70 years. These
changes have occurred because of a loss of habitat, which was impacted by
deteriorating water quality. The
challenge is to try and restore the Clear Lake fishery to resemble that found
in the 1940’s. To accomplish this two
critical areas will need improvement.
First, water quality needs improvement.
This will help reestablish aquatic vegetation, which so many fishes are
dependant upon. Second, populations of
bottom-feeding fishes, primarily carp and bullhead, will need to be reduced.
This will improve water clarity and also enhance aquatic vegetation. Doing one without the other may not bring
the desired results, so combining the two appears to be the best plan.
C. Ventura Marsh.
A
major goal for Ventura Marsh is to eliminate this area as a spawning and
nursery area for carp and bullhead. The
Iowa DNR (previously Iowa Conservation Commission) has attempted to keep carp
out of the marsh with a rod barrier and fish trap over the past 50 years. The fish trap has not been functional for
the past 30 years, however the barrier has been maintained and operated. Despite these efforts, carp and bullhead
have periodically become established in the marsh. Once adults are established, they frequently produce large year
classes of young carp and bullhead.
These small fish often migrate back into Clear Lake, thus increasing Clear
Lake’s carp and bullhead population.
It
is not realistic to think that we can keep carp and bullhead out of Ventura
Marsh. The close proximity of the
fishing jetty and the popularity of this area by anglers make it nearly
impossible to prevent movement of angler caught fish from one side to the
other. We can, however, manage the
marsh to create an environment that even carp find difficult to live in. This can be accomplished by creating
frequent winterkills and/or rotenone renovations.
Staff
with the Iowa DNR tested this approach in 1999 and 2000. During the summer of 1999, Ventura Marsh was
treated with rotenone, a fish toxicant, to reduce/eliminate bottom feeding
fishes. Prior to the renovation, the
marsh was lowered 0.8 feet. Rotenone
was applied at a rate of 4 ppm.
Although water levels were lowered in the marsh, water still remained in
much of the cattail vegetation. This
area was very difficult to treat, even with an aerial application. A follow-up netting survey revealed only a
33% reduction in the carp population.
Shortly
after the 1999 waterfowl season, stop log boards were removed from Ventura
Marsh lowering the water level 1.7 feet below crest. Under normal conditions the marsh can only be lowered one foot,
however low water levels in Clear Lake allowed for an additional ¾ foot. The goal was to induce a natural
winterkill. Unfortunately the winter
was very mild and only a slight kill occurred reducing the carp population by
50%. This kill was also enhanced by the
addition of rotenone under the ice while carp were congregated in front of the
old fish trap.
During
the summer of 2000 a second aerial rotenone application was planned. Two major changes were made on this attempt
compared to the 1999 spraying. First,
the DNR wildlife section pumped water out of the marsh utilizing a crissifoli
pump to a level of 2 feet below crest.
This was critical to the success of the project because it eliminated
all the water within the cattails, thus removing escape areas that are
extremely difficult to treat. The
second was the rate of application was increased to 8 ppm. The result was a 99% reduction in carp. Unfortunately the renovation was conducted
too early in the summer and the few adult carp that remained were still gravid. These fish successfully spawned and produced
enough young to begin filling the void created by the renovation.
Future
management of Ventura Marsh should incorporate late fall/early winter drawdown
to induce winterkill and periodic aerial rotenone applications. To assist in this effort a new control
structure should be considered that would improve water level manipulation and
also fish barrier capabilities. An
electric pump should be installed that would allow for significant water level
reduction in the marsh. This would
enable sufficient water level removal even when high water existed in Clear
Lake. The need to remove water from the
vegetation in the marsh is critical and pumping is the only technique that will
work since there is only a one foot head difference between the marsh and the
lake.
D. Mechanical
Removal of Carp and Bullhead.
Clear
Lake has a long history of rough fish removal.
Eight hundred thirty-nine thousand pounds of carp were reported removed
between 1929 and 1943. An additional
733,000 pounds were removed from 1949 through 1973. During these years, the State of Iowa had “rough fish crews”
which conducted carp removal on Clear Lake as well as many other lakes. Beginning in 1980, contract commercial
fishermen harvested carp and from that time until 1999 they removed 593,000
pounds from Clear Lake. A total of over
2.2 million pounds of carp have been removed from Clear Lake over the past 70
years.
Although
past removal of carp from Clear Lake appears impressive, it has not been
adequate to have a major impact on the fish community or water quality. Currently contract commercial fishermen have
been taking the surplus and not making a substantial dent in the
population. To increase the harvest a
monetary incentive could be considered.
The fishermen would continue to receive payment for the sale of fish,
but they would also receive an additional payment (so many cents per pound)
from the DNR.
Standing
stock estimates conducted by the Iowa DNR showed that carp biomass ranged from
110 to 240 lbs/acre during 1999 and 2000.
If standing stock estimates were continued in the future, the DNR could
target a pre-determined poundage of carp to be removed and budget for that
total. For example, if the standing
stock was 100 lbs./acre, we could request a 50% removal or 50 lbs/acre. Fifty lbs/acre would equal about 180,000 pounds. If we paid 10 cents/lb, then $18,000 would
need to be budgeted for carp removal.
Bullhead
are not currently available to harvest under the present contract commercial
fishing program. Some neighboring
states do allow for the commercial harvest of bullheads. This could be considered for Clear
Lake. Population estimates of bullhead
during 1999 and 2000 estimated a population of 1.5 to 3 million bullheads in
Clear Lake.
Despite
this dense population, only 36,000 bullheads were harvested by sport anglers
during those two years combined. These
fish were considered to be angler acceptable size averaging 9 inches long and
0.4 pounds.
A
review of creel surveys on Clear Lake shows a downward trend in bullhead
harvest over the past 15 years. Two
hundred thousand bullhead were taken by anglers in both 1986 and 1987, but have
never approached these levels since.
Angler attitudes have changed over the past two decades. Twenty to thirty percent of Clear Lake
fishermen were specifically targeting bullheads during the mid-1980’s, while in
recent years less than 5% have targeted this species. It is unlikely that angler harvest will have any impact on
reducing bullhead numbers in the future.
It may also be socially acceptable to allow for a commercial harvest
since so few anglers desire to catch these fish.
If
a commercial harvest were allowed, several questions remain. Are Clear Lake bullheads large enough to
have a market value? Are there
commercial fishermen with the appropriate equipment to harvest fish of this
size? Are there any fishermen in the
area with an interest in catching bullhead?
E. Biological
Control.
Flathead
catfish appear to be the best predator for controlling undesirable
species. Flatheads have been used
successfully in Minnesota and Iowa on small lakes to reduce overabundant
bullhead. A small number of flatheads
were stocked in the fall of 2000 in Clear Lake. Additional fish are scheduled to be released this summer. A stocking strategy needs to be developed
and refined as work continues with this species. Besides being a very effective predator, they will also provide a
unique opportunity to catch a trophy-sized fish in the future.
Other
predators that might be considered include largemouth bass and walleye. Although largemouth bass are an effective
predator of bullhead, previous stockings have not done well in Clear Lake. Walleye will also readily consume bullhead,
however large numbers of walleye are already stocked. Walleye density could be improved through the use of large
fingerlings (>8 inches), in years when fry stockings produce a weak year
class.
Any
significant reduction in bullhead or carp populations, whether it be through
mechanical removal or biological control, must be accompanied with a strategy
to fill the void created with desirable sportfish. Sufficient predators must be available to control increased
bullhead and carp reproduction. In
addition, adequate panfish brood stock must be available to fill the void
created.
F. Creating
Habitat for Centrarchids.
Historically Ventura Marsh was open to free movement
of fishes from Clear Lake. It was
considered to be a prime spawning and nursery area for many sportfish. The placement of a rod fish barrier now
prevents movement of adult fish into the marsh. It has been suggested that the barrier should be removed and once
again allow free movement of all species.
Although some desirable gamefish would use the marsh, we feel this
management practice would do more harm than good. Carp and bullhead would likely utilize the area and dominate over
bass, bluegill and crappie.
Two artificial canals are presently found on Clear
Lake. These areas currently provide
some of the best spawning habitat for Centrarchids. Crappies congregate heavily in the canals during the spring and
bass and bluegill do as well, although to a lesser degree. Constructing additional canals might enhance
the needed habitat to increase populations of these species. On the negative side, the natural shoreline
must be broken to create a canal. This
change may outweigh any benefits derived from increasing spawning habitat.
Another option would be to connect several small
wetlands that currently exist to the lake.
Although these areas would provide much needed nursery cover, the same
problems that were discussed with opening up Ventura Marsh would occur in these
small wetlands. Carp and bullhead would
likely benefit the most and negate any value the area would have for desired
sportfish.
The
construction of breakwaters may have the greatest potential to improve
fisheries habitat and improve water quality at the same time. Early findings by Iowa State University
researchers has shown that wind resuspension is a major problem for Clear Lake
water quality. Breakwaters placed
parallel to existing bulrush beds would protect them from the pounding forces
of the wind. These areas would then
grow more vigorously and provide quiet water that would enhance the growth of
submergent vegetation within the bulrush.
Potential
sites for breakwaters include: State
Dock, Baptist Camp, McIntosh Woods State Park, Farmers Beach, Lekwa Marsh. All of these sites are either publicly owned
or undeveloped shorelines, which would improve the likelihood of public
acceptance. Placing these structures in
5 to 6 feet of water would dampen the wind resuspension of nutrients, reduce
wind disturbance to nearshore vegetation, reduce turbidity, and create
excellent fish habitat. Constructing
these structures from the shoreline out in a T or L configuration would allow
shore anglers to access the main arm of the breakwater. The riprapped portion of the breakwater
would attract small and large fishes and the quiet water on the backside would
provide quality spawning and nursery habitat with the mixed growth of submergent
and emergent vegetation.