Aug 30, 2016

Yellow Perch Management on the Winnebago System: 

Dynamics of a Boom-Bust Yellow Perch Fishery  

By: Ryan Koenigs - Senior Fisheries Biologist (WDNR) 

The Winnebago System is nationally known as a walleye factory, while also boasting one of North America’s largest lake sturgeon populations.  Comprising 17% of Wisconsin’s inland surface water the system is also home to other important recreational fisheries including northern pike, black crappie, white bass, and most notably yellow perch.  Lake Winnebago and the Upriver Lakes have maintained naturally reproducing yellow perch populations for decades, but historically there was limited sampling effort focused on yellow perch. The Oshkosh DNR fisheries team has been tracking yellow perch recruitment and adult abundance on Lake Winnebago via fall bottom trawling surveys since 1986, but unfortunately limited data exists for the Upriver Lakes.   

Bottom trawl survey results suggest that Lake Winnebago maintained a low density perch fishery from 1986-2000 due to relatively poor recruitment (0.31 young-of year/trawl).  However, improving water clarity and an increase in vegetative habitat contributed to a significant increase in recruitment from 2001-2011 (3.70 YOY/trawl).  Area of vegetative growth and cover peaked in 2010, but has since declined due to decreasing water clarity throughout Lake Winnebago. Similarly, yellow perch recruitment has significantly declined in recent years (0.66 YOY/trawl in 2012-2015).  Trends in recruitment are evident in adult catch rates as well with peak catch occurring from 2006-2011, but steadily declining in recent year (Figure 1).   

In addition to bottom trawl assessments, DNR fisheries staff have been intensively fyke netting yellow perch for the past five years (2012-2016) (Photo of fyke net).  Nets are typically set shortly after ice out to collect pre-spawn fish at various locations.  Multiple nets were set annually in the Fox River (2012-2016) in Oshkosh to collect reference data, while additional locations on Lake Winnebago and Lake Butte des Morts have been sampled intermittently.  On Lake Winnebago nets have been set in Asylum Bay (2012, 2015-2016) and Van Dyne Creek (2015), while nets were set in Lake Butte des Morts at Sunset Bay (2015-2016), Spring Brook (2016), Terrell’s Island (2016), Scott’s Bay (2015-2016), Preacher’s Bend (2016), and Nickel’s Marsh (2015-2016). 

Fyke net data support the trend of a significant decrease in yellow perch abundance since 2012 (Table 1).  Further, aging data demonstrate that the adult population of yellow perch is dominated by young fish; 2-3 year old fish comprised an average 88.1 and 86.6% of the adult female and male populations sampled in nets.  The strong contribution of young fish suggests that the Winnebago System is home to a boom-bust perch fishery that is heavily dependent on strong year classes.  Abundance, and in turn fishing success, is high 2-3 years following strong year classes, while abundance is lower when weaker year classes persist.  Unfortunately, both trawling and fyke net surveys are indicating that we are currently in a bust cycle resulting from poor recruitment in recent years. 

Variable recruitment resulted in changes in size structure observed during spring assessments.  Fish collected in 2012 were mostly fish less than 8” (78.9% females; 92.8% males), while fish larger than 8” comprised much higher percentages of samples from 2015 (67.3% females; 49.5% males) and 2016 (61.1% females; 40.7% males).  Anecdotal reports from anglers confirm this observation, stating they weren’t able to consistently catch large numbers of yellow perch in 2014 and 2015 but the fish were nice sized.             

Adult yellow perch abundance may currently be low, but there are still some quality size fish in the system and fish grow quite rapidly.  Most anglers report harvesting fish starting at 8” and growth data indicate fish reach this length during their third summer of growth.  Further, fish reach 10” at age 4 for females and at age 5 for males (Figure 2).  Therefore, fishing success can improve in short order following a strong year class.  On the flip side, 2-3 year old fish are the strongest contributors to the population, so a strong year class is likely unable to sustain quality fishing for more than 1-2 years.   

I’ve spent most of the article to this point explaining the “what,” but I’m sure many readers are now questioning the “why.”  Specifically why is the adult population dominated by young fish?  Why has abundance decreased and what has caused the decrease in recruitment? 

The first question is quite easy to answer.  Our age data demonstrate that yellow perch on the Winnebago System experience high mortality rates (63% average total annual mortality for females and 69.0% for males) meaning the majority of the fish don’t survive past age 3.  The question is whether the high mortality is attributable to fishing mortality (harvest) or natural mortality.  This can be a tough nut to crack, but the answer has important ramifications towards the effectiveness of any fishing regulation.  In essence, a reduced bag limit may not provide much benefit if the population experiences high natural mortality. 

Creel surveys and tagging programs are the two most commonly used methods to evaluate effects of harvest.  Given the size of the Winnebago System, a full-creel survey would be extremely costly.  However, the DNR did conduct a partial creel survey of the perch fishery from July-September of 2012.  A two man crew checked anglers at a single landing 3 days per week (2 weekdays and 1 weekend) over a 3 month period.  The crew rotated around 7 groups of high use boat landings on Lake Winnebago, being at each landing every 2.5 weeks.  In total, 35 shifts each lasting 6 hours (9:00 AM – 3:00 PM) were included in the survey.  During that time creel clerks interviewed 1,970 anglers (1,070 boats) who fished a total of 7,400 hours. These anglers reported a total harvest of 16,685 perch (8.5 perch/angler).  Sex data was collected from a sample of the harvested fish, of which 76.8% were female. 

The creel was not intended to estimate harvest, but the observed catch starts to make one think twice when considering a variety of factors including: sampling only occurred 3 days/week, clerks were only at a single landing per shift, clerks only creeled ½ of the fishing day, only Lake Winnebago was included and only for 3 months, etc.  When these factors are considered the data suggest that yellow perch harvest from the Winnebago System can be high.  However, we are not able to estimate exploitation rates, as we don’t have an estimated total harvest or population size, both of which would be very costly to estimate.  Therefore, the DNR started to mark perch with white anchor tags (similar to the yellow tags used to mark walleye) in the spring of 2015.  Each tag has a unique number identifying that fish and the address of the DNR Service Center in Oshkosh.  Anglers are encouraged to notify the DNR via email (DNRWinnebagoSystemTagReturns@Wisconsin.gov), phone (920-303-5429), or mail anytime they catch a tagged fish.  A total of 583 fish were marked in 2015 with an additional 421 fish marked in 2016.  Thus far we have had reports of 18 tagged fish harvested, of which 15 were marked in 2015 and 3 were marked in 2016. Further tagging and angler reporting will provide an estimate of harvest rates and help better estimate fishing and natural mortality.   

The cause of reduction in recruitment is more dynamic, but the loss of habitat is the overarching factor.  Yellow perch, and other panfish species, rely heavily on aquatic vegetation for spawning habitat, food production areas, and cover from predators.  Abundance of aquatic vegetation correlates to water clarity, and the increased turbidity (reduced water clarity) on Lake Winnebago has resulted in a consistent decline of rooted vegetation since 2010.  This decline is on the heels of a decade of improving water clarity that led to the increase in perch production from 2001-2011.  The Upriver Lakes have experienced a consistent decline in vegetation over the last 15 to 20 years, which is likely the driving factor behind declining perch populations in those lakes.  

Many anglers attribute the yellow perch decline to predation from double-crested cormorants and American white pelicans.  Pelicans are federally protected and there hasn’t been any diet research conducted in the Midwest.  However, multiple studies have reported that cormorants are natural predators that consume prey items that are readily available.  On the Winnebago System, we are now in the second year of a three year diet study to evaluate impacts of cormorant predation on sport and panfish populations.  In 2015 we analyzed gut contents of 353 cormorants (226 from Lake Butte des Morts and 127 from Lake Winnebago).  We observed 12 different fish species with gizzard shad (505), freshwater drum (229), white bass (18), emerald shiners (13), yellow perch (11), stickleback (6), and walleye (4) being the most commonly encountered prey items. These results confirm that cormorants are consuming prey items that are most readily available to them, freshwater drum and gizzard shad.  There has not been a strong hatch of yellow perch in recent years, so it’s not surprising to see few yellow perch in diets.  The next two years of sampling should shine more light on potential impacts of cormorant predation.                 

In summary, recent research has clearly revealed a few trends in the Winnebago System perch population and fishery.  First of all, the system exhibits a boom-bust fishery that heavily relies on strong recruitment to provide a quality fishery.  Further, adults experience high mortality and angler harvest has the potential to be high.  However, there are still some important questions that need to be further evaluated.  We need to continue to mark yellow perch to estimate exploitation rates and evaluate movement of fish within the system.  Exploitation data will help evaluate potential regulation options to ensure the best regulation is in place.  We also need to further study the impact of cormorant predation.  The most important factor though is 

Ryan Koenigs is a Senior Fisheries Biologist with the WDNR in Oshkosh.  Email Ryan.Koenigs@wisconsin.gov if you are interested in receiving reports pertaining to fisheries management on the Winnebago System.