Winnebago Walleyes , Looking at Numbers
by Mike Arrowood

The question is always coming up about how the walleye population in Lake Winnebago is doing.  Those of us that have been around a while can remember how the population was back in the late 1980s.  N.E. Wisconsin had suffered through a series of very dry years that had resulted in virtually no water in the upriver spawning marshes during the annual spawning season.

In fact, that is the very reason that Walleyes For Tomorrow was formed.  It was assumed that something could be done to ensure that there were more consistent opportunities for walleyes to spawn.  Hence, the formation of WFT with the “Increased Production” operating format.

So, how has the walleye population done recently as compared to the “good old days” of the past?  How does one measure success?  More fishermen on the water?, more daily bag limits taken home?, bigger fish bagged?, no need for a minimum size limit?.  There are probably dozens of ways that fishermen determine for themselves how the walleye population is doing.

How do the biologists in Oshkosh assess how well the walleye population is doing?  The answer to that is, in many ways. Each segment of the walleye population must be consistently examined in a particular way to determine the health of the population as a whole.

The spring spawning run shocking and tagging program looks at the size distribution of the male and female spawners on an annual basis.  Thousands of fish are captured, measured for length, tagged and released.

Over time this effort yields an annual profile of the spawning stock in the System.  The profile is of the number of female and male walleye by inch class that is in the spawning stock that particular year. In subsequent years these inch class profiles can be used to determine how a particular year class moves through the population (by size in inches) and eventually disappears from the population completely.

A second sample of the spawning population is made by removing a number of dorsal spines from fish in each inch class of fish captured.  These spines are then sliced with a spine saw and the age of the fish is determined by counting the annual growth rings on the spine.  From that, an age vs. length comparison can be made to determine how well each particular year class is growing.

The Walleye Weekend and Otter Street tournaments provide a large number of mature fish for inspection on an annual basis.  All fish from the tournament are measured and their sex is determined.  The advantage of the tournaments is that the dead fish sampled are dissected to determine their state of sexual maturity.  The age and size of sexually mature females and males is determined in this manner. 

The size, length and age information from the spring shocking program when compared with the size, length and sexual maturity information from the tournaments provides an excellent opportunity to assess the state of the mature walleye population

Spawning success is always a topic for discussion.  Measuring the actual spawn (number of eggs deposited) is difficult. Measuring fry hatch is not a very practical thing to do on a system as large as the Winnebago System.  So, how is hatching success on an annual basis determined?

One method is by the late-summer population assessment trawling program managed by Kendall Kampke, the Winnebago System Biologist in Oshkosh.  The August, September and October trawling program was initiated in the late 1970’s to assess the over summer survival (the number) of fingerlings and young of the year (YOY) of all fish species in Lake Winnebago.

Young of the year (YOY) are the walleye that were hatched the previous spring and have survived their first summer of rapid growth, winter of difficult conditions and second summer of growth.  The number of YOY fish captured in the fall is the best measure of how the previous years spawn is contributing to the total walleye population of the lake.

The nuts and bolts of the trawling process are easy. The DNR uses a trawler based at Calumet Harbor to capture fingerling fish of all species on an annual basis.  The trawler is equipped with a large net to catch virtually every fish that the net encounters.  Large fish generally out-swim the net while smaller ones are unable to escape and are captured for identification and counting. 

46 GPS identified locations on the lake are sampled.  Each fall the trawler crew lowers the net and pulls in the same direction and speed from the sample site locations for five minutes.  The net is raised and the catch is emptied onto an assessment table on the trawler.  Every fish in the net is identified and logged as to species, age and/or size and number caught per species.  Sound easy?

There are around _____ species of fish found in Lake Winnebago.  This count is made for each of the 46 assessment trawl sites every year.  That is now for 30 years.    Over time the numbers turn into a catalog of fingerling and young of the year (YOY) population densities by species, by year, in Lake Winnebago.

So, how are the walleyes doing?  From 1980 till 1990 the net was dropped and pulled 460 times.  Of those 460 net pulls, only two (2) walleye fingerlings (spawned in April) were captured in the average net pull. 

From 1991 till 2000 the net was again dropped and pulled 460 times.  Things were looking up however.  Walleye fingerlings were more numerous.  Seven fingerlings were captured in the average net pull.

From 2001 till 2005 things got even better.  An average of nine (9) walleye fingerlings were caught per net pull.

So, what does this mean?   Over the course of 25 years walleye spring spawning activity success and fingerling survival till fall, as indicated by the number of fingerlings captured, had increased 400+%.

Literally hundreds of variables exist that can affect spring spawning success and fingerling survival until Fall.  Despite all these environmental hazards, over the course of 25 years, spawning success and fingerling survival has steadily increased.  Some might even say dramatically increased!

Will this increase in fingerlings result in more harvestable fish?  It is certainly hoped so. Again, there are hundreds of variables that exist to determine whether a fingerling survives to harvestable size.

A major factor many people are unaware of is winter mortality of walleye fingerlings and how it affects a particular year class size in subsequent years.  Fingerlings that go into the winter small and underweight tend to not survive the long frozen period of reduced food availability.

Whenever there is a huge hatch of walleye fry, literally millions of small walleye are foraging on prey that they can catch and swallow.  Competition for food is intense and the ability of the prey species to reproduce and remain abundant is both a challenge and of utmost importance for the health and growth of the walleye fingerlings.

When the prey species cannot keep up with the predation demands, the walleye fingerlings do not grow as fast or as fat.  Reduced size and weight in the fall are an indication of that and the over winter survival of that year class may not be very great.

Conversely, fingerlings that have experienced seven or eight months of abundant food will be larger, heavier and better suited to survive until Spring   In September and October of each year the fisheries staff at Oshkosh undertake a fall fingerling assessment.

The date of the survey is driven by water temperature.  When shoreline water temperatures fall to 50 degrees or so, walleye fingerlings congregate in shallow water.  The DNR crews use shock boats to sample the population of fingerlings at this time.

Like the trawler program, selected sites around the lake are sampled every year.  The fingerlings are captured, measured for length and weighed before being released.  These numbers when compared to the following year trawler young of the year catch is the best measure of over-winter survival by a given years walleye spawn.

The goal of WFT is and always has been to undertake projects on the Fox and Wolf Rivers that will result in spawning success every year, not just in years of excellent high water years.  The last fifteen years results look promising and WFT in cooperation with the DNR will continue undertaking projects improve spawning success.  That is a promise.

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Walleyes: From Egg to Minnow
By Mike Arrowood

In an attempt to have WFT members better understand the reproductive life cycle of walleyes this article will explain a few of the many aspects of walleye reproduction and growth.

Walleyes generally spawn when water temperatures are between 40 and 50 degrees.  The eggs in the female must be “ripe” or suitable for fertilization before they are released from the fish’s body to the water. This ripening is influenced by both water temperature and photoperiodism or the amount of daylight at the time of spawning.
When the female’s eggs are ripe she will move to a suitable site for spawning.  On the Wolf and Fox rivers those locations are usually flooded grass marshes.  In lakes, the location is generally shallow rocky or gravel shorelines.  Ripe or sexually mature males often congregate before the females arriave to begin the spawning process.

The females release their ripe eggs into the water and the males release milt that contains sperm.  Each egg must be found and penetrated by a sperm to be fertilized. All eggs have only one opening (the micropyle) in the outer membrane for the sperm to enter.  In the natural environment, this is a hit or miss proposition.  Eggs that are fertilized live and those that are not, die.

In the natural environment, it is just a matter of chance that an egg is fertilized.  A 5 lb. female walleye may release 200,000 or more eggs.  Generally, more than one male will attempt to fertilize eggs from each female.  Millions of sperm are released by the males when the spawning process takes place. This raises the odds that a large number of eggs will be fertilized.

In a WFT portable hatchery, two males are stripped of their milt and the eggs from one female are stripped.  The milt and eggs are mixed with water and stirred slowly for one minute.  This method normally results in 80-90 percent of the eggs being fertilized.

Those eggs that are fertilized undergo two rapid physiological changes.  The first is that the outside of the egg becomes very sticky.  This enables the egg to stick to the first solid object that it touches.  This anchors the egg so that waves or currents don’t wash it about while a young walleye is forming inside the egg.

The second change is that the eggshell starts to “water harden” or toughen.  Eggs are better suited to survive being washed about by waves or currents by “water hardening” or toughening of the eggshell.

In a WFT portable hatchery, the eggs must be kept from sticking together while they are in a hatching jar.  To prevent them from sticking, a “mud” of  bentonite clay is poured into a shallow pan with the fertilized eggs.  This mixture of eggs and watered down clay is then stirred for two minutes to ensure each egg is “coated” with a fine layer of clay particles. This prevents the eggs from sticking to each other.  The excess clay is rinsed from the eggs before they are placed in the hatching jars.

The reason for “claying the eggs is to keep each egg separate.  Eggs breathe or use oxygen while they are maturing.  When eggs stick together in clumps, the eggs on the inside of the clumps suffocate and die.  After the eggs die, they start to decay and a fungus starts to grow on the dead eggs. Eventually all eggs in the clump will die from the fungus growth.  It would be impossible to hatch eggs in a hatchery jar without claying them to prevent clumping and fungal growth.

Eggs that are naturally released and fertilized remain stuck to whatever they first touched.  The egg doubles or triples in size by the end of the second day.  Depending on water temperature, the fertilized eggs mature and a walleye larva develops in the egg over a two to three weeks period.  This is the most perilous time for fish eggs.  As they lay attached (stuck) to some solid object, they are eaten by any number of other animals such as fish, crayfish, snails, insects, birds etc.  There is also the possibility that water levels will drop and the eggs will be exposed to air and die.

Water temperatures of around 50 degrees will result in the eggs hatching in about three weeks.  55-degree water will speed egg hatching to about two weeks.

When the egg hatches and the young walleye is released, it is not really a “fry”.  This is a generic name used when discussing a newly hatched fish.  What really emerges from the egg is walleye larvae.

Why isn’t it a fry?  For one thing, there is still a large part of the egg yolk sac attached to the abdomen of the young fish. Secondly, the young fish hasn’t developed a bony mouth structure at this point in its life.  For the sake of this article, the larval stage of young walleyes will by called fry.

Young fish that have just exited the egg have a swim bladder as do all fish.  This bladder is inflated soon after the fry exits the egg.  The fry swims to the surface and gulps air to inflate the swim bladder. The inflated bladder is not large enough to “float” the small fish however.  The yolk sac has nearly as much mass as the fry itself and results in negative buoyancy, even with the swim bladder inflated.  Yolk sacs provides the necessary nutrients for the fry to continue to grow the over the next three to four days.  As the nutrients in the sac feed the fry, the bony mouthparts are forming.

Eggs that are laid on spawning marshes must have a water flow to “wash” the fry from the marsh.  Eggs that are laid in the shallows of lakes must have currents to “wash” the fry from the spawning site to disperse them over a wide area.  This dispersal results in fry being able to find adequate food sources and to limit predation.

It is during this time that the fry is extremely vulnerable to predation.  These fry cannot swim horizontally or sideways because of the weight of the yolk sac.   They cannot swim to safety when a predator approaches.  At this stage of the fish’s life it is “lunch” for virtually any predator from water beetle to bluegill.

They can however swim vertically.  For this reason they are called “swim up fry”.  After hatching from the egg, the fry fills its swim bladder and swims to the surface.  There, the currents will wash it to deeper and safer water.  The fry repeatedly swims up, sinks, swims up and sinks as currents carry it from the spawning area.

On a river system, when the fry is washed from the spawning marsh it enters the main river channel and moves downriver in this swim up and sink motion.  This lasts until the egg sac is absorbed.  Three to four days of swimming up and down as it floats downriver is an ordeal all young river walleyes must endure. The fry are dispersed over the entire river/lake system in this manner.

In a WFT portable hatchery, the eggs hatch and the fry leave the hatching jars via the overflow spout.  The fry are then held in holding tanks outside the hatchery. Each day’s hatch is kept in a separate tank. The fry are held in the holding tanks for three to four day to allow them to absorb their yolk sacks and physically mature without the threat of predation.

On the fourth day, the fry are transported to the lake where they are to be released.  They may be released in shallow and heavily vegetated areas or in a mid lake and open water area.  The open water release method allows currents to naturally disperse the fry over a large area for wider distribution.  The shallow water method allows the fry to hide in vegetation from potential predators.  Either way, the fry must now have an adequate food source or they will not survive.

With the yolk sac now absorbed the fry is called a “horizontal swimmer” and is every much a predator as when it is mature. The swim bladder enables the fish to stabilize its self and actually propel itself through the water in pursuit of prey.

What does it eat?  Zooplankton or minute aquatic animals of many kinds make up a walleye fry’s first food. Zooplankton are minute animals while phytoplankton are minute plants. Generally speaking, zooplankton are vegetarians and eat phytoplankton.  The phytoplankton are single cell plants that live on nutrients that they absorb from the water.

Daphnia is one particular kind of zooplankton that is easily seen by the human eye. They are more commonly called water fleas and are a preferred and important food for young walleye. The majority of the zooplankton walleye fry consume is microscopic however.

Water bodies that do not have a surge in the zooplankton population when young walleyes are at this stage of fry development have a hard time supporting a population of walleye.  Adult fish may spawn and the eggs may hatch but without adequate food                 (zooplankton), the fry will starve to death in a very short time.  Likewise, fry stocked from a hatchery must have a robust zooplankton population at the release site to survive.

In about 15 days after hatching, a young walleye is ½ inches long and scales start to form.  In another 20-30 days the young walleye is 1 - 2 inches long and has all the visible features of an adult walleye.  This even includes the white tip on the tail.

Until the walleye fingerling is 1½  inches or so it can survive and grow well on a diet of zooplankton.  In order to grow rapidly a young walleye must have a high protein diet.  From this size on, the fingerlings must find prey of larger size to support their rapid growth. Their diet begins to shift to other species of fish, leaches, insects and invertebrates supply high quality protein for rapid growth. 

With an adequate supply of high quality food the young walleye will reach 2 - 2 1/2 “ in three months.  After being spawned in May many young of the year walleye fingerlings will reach 4 – 6 inches by September

The next stages of a walleyes life will be covered in a future article.

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