[h=1]Does Devils Lake Need Walleye Length Limits?[/h]
May 2015 Number 9 Volume LXXVII
Article By
Todd Caspers and Randy Hiltner
Walleye anglers care about the resource and often express concern when they believe their peers are keeping too many small or big fish.
These anglers often think a length limit will solve the problem, and sometimes they are correct. Length limits, if applied appropriately, can help improve or protect a fishery.
However, a length limit can harm the population it’s meant to help, and also needlessly restrict angler opportunities, if the regulation is inappropriate for the fishery.
We frequently get questions about Devils Lake, one of the region’s top walleye fisheries, related to whether a length limit would protect its walleye population?
Minimum length limits are likely to benefit fisheries that meet all of the following:
- Low reproductive or stocking success.
- Good growth.
- Low natural mortality.
- High angling mortality (fish dying from harvest or after release).
Maximum length limits (one fish longer than 20 inches, for example) are likely to benefit fisheries that meet all of the following criteria:
- Reproduction is limited by the number of adult fish.
- High angling mortality of large fish.
Harvest slot length limits must meet all of the requirements for a minimum length limit and a maximum length limit, since they are basically a combination of the two.
Protected slot length limits are likely to benefit fisheries that meet all of the following criteria:
- Good natural reproduction.
- Slow growth, especially for small fish.
- High natural mortality of small fish.
- High angling effort.
Does the Devils Lake walleye population meet the criteria for any of these length limits? Let’s investigate.
Minimum Length Limit
Currently, the Devils Lake walleye population does not meet many of the criteria necessary to benefit from a minimum length limit.
In 2008, walleye growth was similar to the North American average, but in 2014, growth was slower. The population has produced young fish consistently in recent years, and tagging studies and walleye aging data show that total mortality (natural and fishing mortality combined) is on the lower end of typical mortality rates in North America.
Additionally, with high numbers of 10- to 15-inch walleye in the lake, a minimum length limit would needlessly restrict harvest opportunities for anglers, and could further decrease growth due to increased competition if some fish were protected by a minimum size limit.
Maximum Length Limit and One-over Limits
Today, Devils Lake’s walleye population does not meet any of the criteria necessary to see a benefit of a maximum length limit.
Large walleye hatches of late indicate that current regulations are maintaining sufficient numbers of adults in the lake. Three of the four largest hatches, in fact, have been produced since 2008. While the percentage of adults longer than 15 inches in 2012 was relatively low at 24 percent, the second largest walleye hatch ever was recorded, indicating there are ample adults in the lake to produce a good hatch if conditions are favorable.
Recent creel surveys show that Devils Lake receives more than 1 million angler hours annually, which, considering the size of the big lake is only about seven angler hours per acre.
A one-over 20-inch limit would probably not have much impact, as few anglers harvest more than one large walleye per fishing trip. Moreover, creel surveys showed that less than 6 percent of walleye harvested were 20 inches or longer in 2007-08 and about 3 percent in 2013-14.
Anglers who fish during the spring spawning run in certain portions of the lake (upper basin coulees and current areas near bridges) tend to catch bigger fish. A 2010 creel survey during this time showed that 18.9 percent of harvested walleye were 20 inches or longer.
However, this percentage still equates to less than one in five fish (with the daily bag limit being five fish) 20 inches or longer, so most spring anglers probably do not harvest more than one walleye this size per trip. Therefore, a one fish over 20-inch limit would likely not prevent the harvest of many large fish, even during spawning when anglers are most likely to harvest larger fish. Even so, we will continue to use creel survey data to evaluate the effectiveness of a one-over 20-inch limit.
Additionally, recent walleye population modeling on Devils Lake by Jason Breggemann, as part of his doctorate research for South Dakota State University, indicates that a one-over 20-inch limit would do nothing to improve the size structure of the walleye population as a whole. This is because a one-over 20-inch limit will not increase the number of walleye that survive to attain 20 inches, and fish saved by such a regulation will not stockpile due to natural mortality.
Even if a regulation prohibited harvest of all walleye longer than 20 inches, only about 10,000 fish would have escaped harvest during the 2007 and 2008 seasons.
While this sounds like a lot of fish, recent tagging studies in North Dakota have demonstrated that relatively few released walleye will be caught again.
In Lake Audubon, for example, only 19 percent of tagged walleye that were caught and released were caught again. The percentage of tagged walleye released and caught again was only 9 percent in Devils Lake. Even if we assume a high recapture rate of 20 percent, then about 2,014 of the “saved” walleye would be caught again. The total angling effort in 2007-08 was more than 1 million hours, so the expectation is that one of the “saved” walleye might be caught again for every 508 hours of fishing effort. The average fishing trip is typically about five hours, so it would take an angler about 101 fishing trips to catch one additional walleye of 20 inches or longer.
Repeating these calculations using data from the 2013-14 creel survey (12,687 saved fish and about 1.3 million hours of angler effort) would result in an angler catching an additional walleye of 20 inches or longer about once in 105 fishing trips.
These small gains in catches of large walleye are not worth restricting angler opportunity to harvest large fish. Additionally, many anglers who fish Devils Lake do not necessarily want more large walleyes. The 2013-14 creel survey showed that about 83 percent of anglers said that they would rather keep five 15-inch walleyes than two 20-inch fish.
Since the Devils Lake walleye population does not meet the criteria for either a minimum or a maximum length limit, a harvest slot limit would not be appropriate or effective.
Currently, the Devils Lake walleye population meets some of the criteria necessary to see benefits from a protected slot length limit.
Reproduction has been good in recent years. The walleye hatch has exceeded the long-term average in five of the past eight years, and the two largest hatches were produced in the past six years.
Fish in 2008 were growing at roughly the same rate as the North American average, but growth has since slowed. Despite a slowed growth rate, stunting of small walleye is not a problem today, it’s just that fish are growing at a slower pace.
Another criterion needed for a successful slot limit is high angling effort. Past creel surveys indicate that Devils Lake receives more than 1 million angler hours annually, or over 10 hours per acre in 2007-08, which is relatively high for a large lake.
However, the 2013-14 creel survey still showed more that 1 million hours of angler effort, but due to the larger size of the lake, there were about seven hours of angler effort per acre.
One of the criteria for a successful slot length limit is that small fish have high natural mortality. It appears that mortality rates among younger walleye could be higher than for the entire population, but these possibly higher rates are still on the lower end of the normal range for populations across North America.
Also, Devils Lake has produced good numbers of 15- to 20-inch walleye recently without any length limits. This indicates that fish can grow into the 15- to 20-inch range without any length limits.
For example, test netting showed that numbers of 15- to 20-inch walleye were very high from 2006 to 2010 before dropping back to more typical levels. A portion of the decline in fish this size is likely due to the rarity of 9- to 11-year-old fish (due to weak hatches from 2003-05) and also to possible emigration to connected waters.
As one of the best walleye lakes in the region, biologists often get questions about whether a length limit would be appropriate on Devils Lake.
Numbers of 15- to 20-inch walleye should begin to increase in the next few years as fish from the abundant 2009 year-class continue to grow.
From a biological standpoint, the Devils Lake walleye population does not need a slot length limit right now. However, recent walleye population modeling did indicate that a slot length limit could potentially produce more large walleye in the lake.
For example, a protected slot limit of 16-20 inches could result in about 5-10 percent more adult walleyes that are 20 inches or longer. A protected slot limit of 18-22 inches could result in about a 3-5 percent increase.
However, as with most things in life, there is no “free lunch.” In the case of the 16- to 20-inch slot limit, the down-side is no harvest of those 16- to- 20-inch fish, which are highly desirable fish for anglers. During the 2013-14 creel survey, about 14.5 percent of walleye harvested were between 16-20 inches, and 5.5 percent between 18-22 inches. These are significant amounts of harvest for anglers to give up.
For example, walleyes of that size were more abundant in Devils Lake in 2007-08 and the creel survey that year found that about 41.5 percent of walleye harvested were between 16-20 inches, and 19 percent between 18-22 inches. Also, as mentioned before, the 2013-14 creel survey indicated that about 83 percent of Devils Lake anglers prefer to harvest a greater number of medium-sized walleye as opposed to fewer, larger fish.
In effect, using a protected slot limit just to slightly improve the number of large walleye would be forcing anglers to give up a larger number of fish they want, in order to produce a smaller number of fish that most people don’t want as much.
With more large walleye in the lake because of a slot limit, there is another “lunch” to consider.
Smaller walleye seem to be perfectly happy eating scuds, but larger fish typically want a bigger and better “lunch.” If the number of larger walleye were to increase, this could impact recruitment of walleye and other fish such as yellow perch because of increased predation.
The current walleye population is closest to meeting the criteria for a slot length limit. However, the mortality criterion is not being met, and others such as growth and fishing effort are questionable.
A slot length limit could become a valid management strategy for Devils Lake walleye in the future, especially if growth continues to slow and natural mortality increases. However, current conditions do not warrant putting a slot length limit in place as modeling indicates that anglers would have to give up a large amount of harvest to produce a relatively small increase in the number of larger fish. Additionally, a slot limit would have negative impacts to other areas of the fishery.
We have seen that the current walleye population is not really suited to any of the length limits. However, the North Dakota Game and Fish Department will continue to monitor the fishery, and if conditions change will likely implement whatever regulation is deemed necessary to protect this valuable resource.
