Walleye being measured

Walleye Regulation Fact Sheets

Assessing a Minimum Length Walleye Restriction | Assessing a "One-Over" Walleye Length Restriction


Assessing a Minimum Length Walleye Restriction

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Ample water and aggressive fish stocking produced numerous "new" walleye fisheries across North Dakota in the last decade. These new lakes are productive, and have provided fantastic fishing opportunities.

With an increase in walleye anglers, concerns by some regarding the potential for overharvest and requests for special regulations to curtail harvest are more common. On many new waters, a minimum length limit to allow little fish to grow before being harvested is the most often-requested regulation.

North Dakota Game and Fish Department biologists responsible for stocking, managing and developing these new waters are monitoring their progress since before the first walleye were stocked. They gather information on lake water quality, forage abundance, walleye growth, survival, stocking success, and in some instances angler use and preferences. This collection of information allows biologists to determine stocking rates and evaluate whether any special regulations would improve the walleye populations, either biologically through improved growth or survival, or perceptually by providing a size structure anglers find more desirable.

Biological Factors

Biologically, a minimum length limit would benefit a walleye population that meets four specific criteria:

  • Population exhibits low reproductive or stocking success,
  • Fish exhibit good growth,
  • It's subject to high fishing mortality
  • Natural mortality is low

If all criteria aren't met, a minimum size limit would not improve the population. Alternately, if a minimum size limit is applied to a population with abundant small fish that are slow growing, protecting them from harvest would create a stockpiling effect that may result in a stunted population. Also, if natural mortality is high, protected fish would die of natural causes before reaching a desirable size.

We seldom see poor stocking success that would require protection of smaller fish on most North Dakota waters. Especially on newer lakes, we have established good year-classes through stocking. In most cases where we see a paucity of small fish, it's the result of other factors, such as predation or lack of forage. So, the first requirement for a successful minimum length limit is seldom met.

On traditional lakes and reservoirs across North Dakota, walleye growth tends to be good, satisfying the second requirement. On new lakes, above average growth is common. Most new waters are targeted for stocking because biologists have noted an abundance of fathead minnows. Newly stocked walleye survive and grow fast on this abundant forage, commonly reaching 14-15 inches after only two summers. During this developmental period, a minimum length limit is unnecessary for two reasons. First, when forage is abundant, fish are generally hard to catch and little harvest of small fish occurs. Second, fish grow so rapidly they quickly outgrow the small size range.

High fishing mortality is the most important criteria to meet for any harvest regulation. If anglers aren't having an impact on the population, there's no need to restrict harvest, and doing so will do nothing to improve the population.

Biologists use fish population information to monitor total mortality, which combines both fishing mortality and natural mortality. As long as total mortality is in a sustainable range, we can be certain that neither fishing nor natural mortality are excessive. Natural mortality is usually low in most new walleye lakes where forage is abundant. Similarly, when forage is abundant and fish are hard to catch, fishing mortality can be low, too. When forage declines and a good bite occurs, anglers will harvest a substantial number of walleye and fishing mortality can be high for a period. In cases like this, high fishing mortality is necessary to reduce the population size and bring it into balance with the forage base.

Marvin Miller Lake in Logan County is a good example, where it took 10 years of aggressive stocking before the forage base declined. The first reports of walleye being harvested came in 2012 and fishing has remained relatively good since. Recently, anglers expressed concern that harvest may be too high. However, netting data from Marvin Miller shows the forage is still low, and individual fish have been skinny in recent years. When forage is low, the alternative to high fishing mortality would be high natural mortality. We'd much rather see those walleye go home with anglers than die of starvation.

When we evaluate the biology of our new walleye fisheries, as outlined above, harvest of small fish isn't limiting the quality of fishing in any of the waters. We find that most cases do not meet the criteria for a special regulation, and that minimum length limits would not have much success at enhancing fishing in those lakes.

Social Factors

Some anglers, including those familiar with the biology of these lakes, maintain that a minimum length limit should be implemented because it's their belief that small fish should be allowed to get bigger. Although some anglers do harvest small fish, there's no biological evidence to support this on our productive walleye fisheries. To implement a minimum length limit where it isn't needed, particularly where it wouldn't improve the population, is simply imposing some anglers' personal beliefs on other anglers. Moreover, doing so gives anglers the false impression that the regulation is in place because it's enhancing the population, when in fact it isn't.

Conclusion

The Department's philosophy regarding regulations is to implement them where they have a reasonable chance of improving the population for anglers, and to make those decisions based on the science and biology of the fishery. Statewide minimum length limits would certainly be inappropriate, considering the varying conditions on walleye fisheries across North Dakota. Minimum length limits are considered on a per-lake or regional basis, and implemented when evidence suggests a walleye population, and ultimately anglers, can benefit from having the regulation in place.

Walleye being measured

Assessing a "One-Over" Walleye Length Restriction

Printable Fact Sheet

Many anglers wonder if fish size restrictions would make fishing better on their favorite water. The most common suggestion is either a water-specific or statewide one-over regulation, allowing anglers to keep one walleye over a certain length in their daily limit.

Game and Fish Department fisheries biologists have made a concerted effort the past few years to review all possible length regulation restrictions, and routinely reassess when conditions change. From the extra effort to conduct these evaluations, biologists have not found any instances where data supports a length restriction to improve existing walleye populations.

And yet, some anglers still suggest that additional conservation measures are needed because too many large walleye are being harvested, particularly on Devils Lake and the Missouri River. Some of these anglers request that larger fish be protected to enhance the opportunity for themselves and others to catch large fish. Although not a biological reason for the well-being of the population, this social aspect is also considered when setting regulations.

Biological

From a biological standpoint, maximum length limits (one-over is a derivative of a maximum length limit) are likely to benefit fisheries where fish recruitment is limited by the number of brood fish, and angling mortality of large fish is high. Therefore, a one-over regulation might help in a lake where angling exploitation is reducing the number of spawning-age fish, and inhibiting natural reproduction.

In North Dakota, only Devils Lake and the Missouri River/ Lake Oahe could have this concern as they are the only major walleye fisheries that currently rely almost entirely on natural reproduction. All other waters, including Lake Sakakawea, are supplemented or maintained through stocking.

A typical walleye population is considered sustainable with total mortality rates up to 40-55 percent, according to scientific literature from across North America. Rates above 55 percent become problematic.

At Devils Lake, a recent tagging study estimated angler exploitation in 2007-08 at about 25 percent each year, while the average total mortality from 2008-14 was estimated at 38 percent.

On the Missouri River/Lake Oahe, total annual mortality from 2012-14 was 46 percent, which is higher than Devils Lake, but still within the sustainable range. A large portion of the mortality since 2012 was due to natural causes from the lack of forage after the 2011 flood. Angler harvest of larger fish has been relatively low in recent years. From May 15 through June 30, 2015, creel clerks measured roughly 1,800 harvested walleye on the Missouri River and Lake Oahe, with only 1 percent 20 inches or longer.

Since mortality is not excessively high, it's also not surprising that reproduction is not being affected. Anglers expressed concern over the harvest of large pre-spawn walleye from Devils Lake's tributary coulees in spring 2009. Later that year, Game and Fish biologists recorded the highest walleye reproduction ever documented on Devils Lake, without stocking any walleyes in the lake that year. Even during years of heavy stocking, subsequent analysis showed that stocked fish only contributed about 25 percent of the total young-of-year catch those years. This strongly indicates that natural reproduction was a far larger contributor than stocking, even with the spring harvest.

Similarly, the Missouri River and Lake Oahe have not been stocked with walleye since 1981, despite traditionally good pre-spawn fishing every spring.

Social

Beyond biological considerations, some anglers feel a one-over length limit will extend the big fish resource, allowing anglers to catch released fish another time. Tagging studies on North Dakota walleye populations have shown that less than one of every five walleye released is caught again.

Biologists have evaluated the potential effectiveness of a possible one walleye over 20 inches regulation on various waters using creel survey data collected since 2009. The 2009 Missouri River and Lake Oahe walleye population a higher proportion of 20-inch-plus walleye than any fishery in the state has seen for years. From April through August 2009, anglers on the Missouri River and Lake Oahe harvested about 300,000 walleye. Of that total, about 37,000 (12.4 percent) were longer than 20 inches and 8,000 were longer than 22 inches.

Based on the proportion of anglers who harvested more than one fish longer than 20 inches during that survey, if a one-over 20 inches regulation had been in place in 2009, anglers would have had to release about 6,600 fish. Of these, anglers could expect to re-catch about 1,200, while 5,400 would die naturally. As a point of reference, during this same time anglers voluntarily released more than 77,000 walleye, many of which were longer than 20 inches.

In more recent surveys:

  • During the 2013 Devils Lake open water survey, a one-over 20 inches regulation would have mandated the release of 1,300 walleye out of 382,700 harvested, with 240 expected to be caught again. With an estimated 886,000 angler hours in 2013, it would take anglers 3,700 collective hours to catch one of those released walleye.
  • During the 2015 open water survey on the Missouri River, Lake Oahe, Lake Audubon and Lake Sakakawea:
    • None of the surveyed anglers on the Missouri River or Lake Oahe harvested more than one walleye 20 inches or longer.
    • From May 15 through August 15 on Lake Audubon, four walleye out of nearly 1,500 measured would have been released under a one over 20 inches regulation.
    • On Lake Sakakawea during June, only five out of 1,021 measured walleye would have been released under a one over 20 inches regulation.

Conclusion

Game and Fish is fortunate to have sufficient long-term information to help effectively manage the state's fisheries. At current fishing effort and exploitation rates, Game and Fish biologists are confident that a one-over regulation would serve no biological or social purpose.