Jul 10, 2015

Walleye Management On The Winnebago System

What You Should Know about the Fishery

By: Adam Nickel – Wisconsin DNR Senior Fisheries and Winnebago System Gamefish Biologist

Ranked as one of the top walleye fisheries in the nation, the Winnebago System walleye fishery is alive and well thanks to cooperative management among local DNR fisheries management staff, conservation groups, anglers, and area citizens.  The success of the fishery has led many Winnebago fishing enthusiasts to ask how the fishery can be maintained in order to ensure good walleye fishing into the future.  One of the most common questions asked is if a slot limit should be enacted to protect female walleye and provide greater trophy potential?  Other common questions also include if fishing closures should be implemented during the spring spawning run or if the bag limit should be lowered.  As a result, this article is intended to provide the facts regarding Winnebago system walleye management and insight to future walleye management on the system. 

The Oshkosh DNR fisheries staff relies heavily on three annual surveys to evaluate the walleye population: spring electrofishing stock assessments, bottom trawling assessments, and tournament monitoring.  The surveys are designed to evaluate growth, recruitment, and harvest dynamics of the walleye population to help guide management decisions, particularly regulation recommendations.  For starters, let’s discuss the growth characteristics of the Winnebago walleye population.

Each year otoliths (ear bones) are collected from a subsample of male walleye sampled during spring electrofishing surveys and female walleye sampled at major walleye tournaments.   Otoliths are the most accurate structure for estimating fish age as they develop annual growth rings, similar to estimating the age of a tree (Figure 1).  Growth analyses have indicated that Winnebago system walleye exhibit growth rates well above the state average. However, growth slows dramatically as fish reach older ages.  In fact, growth models indicate that the asymptotic length (length at which the growth of the average fish plateaus) is 20-21” for male walleye and 25-26” for female walleye (Figure 2).  Therefore, the system simply doesn’t have the growth potential to produce large numbers of 30+ inch walleye and instead produces a high number of “eater” sized fish, hence why many refer to Winnebago as a “walleye factory.”  Moreover, there are also good numbers of 20-26” fish to be found as well as a few trophies mixed in.   

Fishing enthusiasts are often surprised when I describe the growth characteristics of Winnebago walleye and naturally ask what are the driving factors of growth on the system?  There are many factors that may influence growth including genetics, water temperature regimes, and available forage.   One attribute that the Winnebago System lacks is deeper and cooler water during the summer months, meaning that walleye cannot seek out those areas to save energy when water temperatures rise.  Furthermore, although there are typically plenty of trout perch, gizzard shad, and other forage items available, other food sources, such as cisco, are not available in the Winnebago System.  Cisco are an important forage fish in deep water systems and may provide more bioenergetics value for species such as walleye.  Nonetheless, I could probably write a full article on growth characteristics alone, but now that we know some facts on growth let’s move on to talking about recruitment.

The Winnebago System boasts a naturally reproducing walleye population that has been producing strong year classes every 2-5 years since the 1990s. Oshkosh fisheries staff and a large host of dedicated volunteers conduct the Lake Winnebago bottom trawling assessment annually to assess year class strength of walleye and many other species.  The survey has been conducted since 1986 and in the last 10 years four strong walleye year classes (2005, 2008, 2011, and 2013) have been documented (Figure 3).  As a result, these strong year classes have contributed to a strong fishery and continue to produce spawning stock once maturity is reached. 

Female walleye in the Winnebago System become sexually mature between 4-6 years of age, with approximately 30% reaching maturity at age 4, 82% by age 5, and 99% by age 6.  For males, nearly 100% reach maturity by age 3.  The 2015 spring electrofishing survey illustrates that the size structure of male walleye is dominated by fish that are 15”-16” and 17”-18”, which can be attributed to the strong 2011 and 2008 year classes (Figure 4).  The strong 2008 year class is also well represented by 21”-22” females whereas the majority of the females from the 2011 year class will not reach maturity and make their first spawning run until 2016.   

So what are the magic ingredients for the system to pull off strong year classes?  Although many often wish for no more snow and few rain storms in the spring, walleye enthusiasts on the Winnebago System know that more water in the spring leads to greater inundation of walleye spawning marshes on the Wolf and upper Fox Rivers.  The higher water levels in the spawning marshes serves many functions including: opening up more available spawning habitat for walleye, providing sufficient flows to keep eggs well aerated and clean, and lastly to flush hatched walleye fry from the spawning marshes into the river channel to start their migration downstream to the lakes.  As a result, high water during the spring generally increases the probability of a strong walleye year class. For example, the high spring water levels in 2011 and 2013 correspond to the 6th and 3rd strongest walleye year classes observed since 1986.  However, as demonstrated in 2014, other factors, such as zooplankton availability shortly after hatching, may be just as important.  Therefore, Mother Nature is essentially in the driver’s seat when it comes to walleye year class strength on the system.

Another important factor in the walleye fishery is angler exploitation.  Since the walleye tagging program began in 1993, exploitation rates have been tracked annually.  The program relies upon anglers to report the catch of any tagged walleye to the Oshkosh DNR office.  Exploitation rates often vary on an annual basis due to year class strength and the condition of fish in relation to forage abundance.  Since 1993, annual male walleye exploitation rates have ranged from 4.2-23.0%, averaging 14.4% while annual female walleye exploitation rates have ranged from 5.4-32.9%, averaging 20.4% (Figure 5).  In most years, exploitation rates have stayed below the 30% mark that fisheries managers often consider the tipping point where harvest levels may not be sustainable.  Although female exploitation rates exceeded 30% in 1997, 1998, and 2009 the high exploitation rates were not sustained over an extended number of years.

Since 2010, male and female exploitation levels have stayed below 18%, including in 2014 when exploitation was estimated at 14.5% and 12.7%, respectively.  The lower exploitation rates over the last few years can likely be tied to forage base trends, specifically the strong hatches of gizzard shad in 2010 and 2012.  However, it should be noted that overall forage base numbers including trout perch and gizzard shad were lower in 2013 and 2014, thus overall fishing success may increase in 2015. In fact, the cumulative YOY catch of trout perch, freshwater drum, and gizzard shad averaged 119.84/trawl in 2014 which is the lowest cumulative average recorded since 1993. Nonetheless, tracking forage base numbers will be important in the coming years. 

Now that we have covered the growth, recruitment, and exploitation components we can finally move onto regulations.  Several different regulation packages can be applied to walleye fisheries including implementing various size limits, bag limits, and season closures.  Minimum length limits are designed to prevent overharvest of fish stocks and protect juvenile fish, giving them a better chance to reach maturity.  Therefore, length limits are most applicable to populations that demonstrate low recruitment or excessively high harvest rates.  Slot limits are designed to protect adult fish once they grow into the protected slot size, which should in theory bolster reproduction and increase an abundance of larger fish.  Bag limits are also a powerful management tool used to maintain exploitation rates at or below sustainable levels, while season closures are intended to eliminate exploitation during critically important time periods where overharvest would likely occur. 

You may be wondering how these regulation options would affect the Winnebago System walleye population.  The current walleye regulations for the Winnebago System are liberal with no closed seasons, a daily bag limit of 5, and no minimum length limit.  As outlined in the paragraphs above, our annual walleye assessments indicate that we currently have a healthy self-sustaining walleye population that displays good growth rates and adequate natural recruitment to sustain the fishery.  Overall, exploitation rates have also been kept in check and the population appears to be doing well with the current regulations in place.  Now, let’s go back to one of the most commonly asked questions I get from Winnebago walleye enthusiasts:  should a slot limit such as no harvest of walleye 20”-28” be enacted to protect female walleye and provide greater trophy potential? 

Remember that Winnebago walleye have good growth rates, but growth of the average fish plateaus at 20-21” for males and 25-26” for females.  Therefore, protecting these fish with a slot limit would likely not result in more trophy size fish.  In fact, most of the females would not grow out of the slot limit size.  Yes, there would likely be a few more trophy-sized fish out there, but the growth characteristics are simply not conducive to managing for a trophy fishery.  Also, remember that slot limits are intended to protect adult fish in populations that have low natural recruitment.  Conversely, the Winnebago System has one of the best naturally reproducing walleye fisheries in the country.  

What about spring fishing closures or lowered bag limits?  Thanks to the outstanding support for the walleye tag return program from anglers, the DNR has been able to track angler exploitation annually.  Walleye exploitation rates have stayed below 18% since 2010, which is well below the 30% mark that fishery managers often consider unsustainable.  Therefore, reducing fishing opportunities in the spring or lowering the bag limit would be unnecessary given the current harvest trends.  In addition, it is often thought that much of the exploitation occurs during the spring run on the Wolf and upper Fox Rivers. However, our tag return data indicates that the majority of harvest occurs in Lake Winnebago following the spring run.  In 2014, Lake Winnebago accounted for 51.5% of reported tag returns with the Wolf River 18.4%, Lake Poygan 10.2%, Hwy 116 Bridge 7.4%, Fox River 4.5%, Lake Butte des Morts 4.2%, and Lake Winneconne 3.8%. Therefore, a closure during the spring spawning run wouldn’t be the most suitable answer if the need to reduce exploitation arose.  Instead, an overall bag limit reduction would likely be the best way to reduce exploitation on the system. The tag return statistics are based on the assumption that 50% of anglers on the Winnebago System report their walleye tag returns. Nonetheless, continuing the tag return program will be vital for tracking exploitation and assessing the need for any future regulation changes.

  Ultimately, keeping tabs on the Winnebago system walleye population by conducting annual surveys will be crucial for future management.  It is also very important to note that these surveys would not be conducted annually without the help of local volunteers and conservation groups.  In recent years, the walleye population has demonstrated exceptional recruitment and exploitation has been kept in check without more restrictive regulations being needed.  As a result, strong year classes continue to drive the fishery and offer plentiful fishing opportunities for all types of anglers.  The key will be to continue evaluating recruitment, age and growth, exploitation, forage base trends, and other parameters to predict the management that is most suitable to ensure that a healthy walleye population persists for future generations.

 Thanks for taking the time to read this article and learn more about Winnebago walleye management.  If you would like to receive email updates regarding Winnebago fisheries assessments contact me and we can add you to our email distribution list.  Also, if you have walleye tag returns don’t forget to turn them in at DNRWinnebagoSystemTagReturns@Wisconsin.gov, mailing them to the Oshkosh DNR Office, or calling 920-303-5429.  Feel free to contact me by phone 920-424-3059 or email adam.nickel@wisconsin.gov with any questions or to just talk more about fish.