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.