Bluegill are one of the largest, most popular and most widespread sunfish not only in Minnesota, but also in the nation. So, the bluegill has been studied more thoroughly and managed more intensively than other panfish. Consequently, the work discussed here has been aimed primarily at these popular fish, though many of the principles apply to other sunfish as well.
Because bluegill are terribly prolific, stocking is used only to introduce fish to a body of water or in highly used public fishing ponds. More often the problem is too many tiny bluegill and an absence of large sunfish, a condition particularly common in heavily fished lakes. The cause of this "stunting", as it's often called, is not truly known. It might be true stunting -- slow growth because of intense competition for food and an abundance of young fish. Or it may simply be a result of overfishing; that is, anglers simply remove most of the "keepers," leaving behind many small fish that further proliferate in the absence of larger sunfish.
The problem isn't discovering why bluegills are often stunted but what is the best mechanism to control their numbers so stunting also can be managed. For years fish managers believed that if a lake is filled with small panfish, the cause was intense competition for food. Without adequate prey, sunfish continue to reproduce but simply grow slower.
Stunting might be caused by an abundance of excellent spawning in conjunction with poor growing conditions, such as a lack of forage. Moreover, thick weed growth -- escape cover -- allows sunfish to escape predators and thus survive in greater numbers.
Another cause might be a lack of large game fish. Without a suitable predator to thin the ranks of small sunfish, the population remains too high for any individual fish to grow very fast.
These may be the causes of stunting. But how do fish managers correct it?
One approach is to remove small bluegill; but this sounds easier than it is. During the 1940s and `50s, fish managers tried seining bluegill to reduce their numbers but couldn't remove enough fish to make a difference. In a later experiment, managers used a herbicide to destroy weeds -- "escape cover" -- to make the bluegill more venerable to predators. This, too, was judged a failure.
In a more successful attempt to reduce the number of bluegill and increase their size, the DNR treated a lake with a toxicant in such a way to kill most small bluegill while sparing the larger fish. The experiment worked, but chemical treatment was expensive, and one of the chemicals used in the treatments is no longer made.
Another approach is to try to increase the predation on small sunfish. Anglers and fish managers long thought that an abundance of big northern pike would keep bluegill in check. Such is not the case, however. Pike eat tremendous numbers of fish, but mostly suckers, minnows and perch. Whether because of the bluegill's spiny rayed dish shape or its ability to take cover in thick weeds, pike do not eat enough bluegill to reduce their numbers.
Largemouth bass appear to be a different story. They feed heavily on bluegill and seem adept at ambushing sunfish in heavy weeds. Largemouth will feed on bluegill up to one-quarter their own length. Consequently, a promising way of controlling bluegill numbers is by maintaining great numbers of medium-sized bass. Bass appear to have two effects on bluegill. First, bass simply eat a lot of sunfish helps control their numbers. Second, bass force small, vulnerable sunfish into thick weeds; but the big sunfish -- too large to be eaten by the bass -- are free to forage wherever food is most plentiful. So the presence of bass reduces the competition between small sunfish and large, thus allowing the large sunfish to grow even larger and faster.
Researchers and managers have found that a minimum-size limit of about 16 inches on bass will direct fishing pressure toward the larger bass, leaving the more numerous 12 - to 16-inch bass in the lake to prey on small bluegill. A great number of small and medium-sized bass seems more effective in controlling bluegill than does a lesser number of large bass.
So, a possible way to produce large bluegill is to engineer an abundant population of small to medium- sized bass. Conversely, fishing regulations that produce large bass (such as catch-and-release requirements), are successful at producing large bass, but are not nearly as effective in controlling bluegill numbers.
Thus, anglers and managers may have to choose either big bluegill of big bass in a lake. This is yet another argument for diversity in fish management. Some lakes can be devoted to stunted bluegill and large bass; others can be managed for big bluegill and many small bass.
Yellow perch also appear to influence bluegill numbers and size. Whether through predation, competition for food or some other means, an abundance of yellow perch is often related to fewer but larger bluegill. Fisheries scientists are continuing to study this relationship.
Many fish managers are beginning to believe that overfishing -- not stunting -- is the reason some lakes do not produce the big bluegill they once did. In such cases, growth rates are normal and food is plentiful, but anglers simply catch and remove all the good-sized fish, leaving behind the smaller bluegill, which multiply without the controlling influence of the large fish.
If this is true, the solution seems to be more straightforward: more big sunfish must be left in the lake if people are going to continue to enjoy catching them. Again, a diverse approach seems most promising. A few "trophy bluegill" lakes could be managed with a restricted harvest of big sunfish -- perhaps even a catch-and-release requirement. Most lakes would continue to be managed with a liberal bag limit to provide panfish for the pan.
All talk of producing bigger sunfish makes sense only if they have the habitat necessary to spawn and grow. In general, sunfish need spawning areas with a firm bottom of sand, mud or gravel; beds of rooted aquatic weeds or other heavy cover to provide protection from predators; adequate dissolved oxygen; and reasonably clear water.
The state protects these kinds of habitat in a number of ways.
Shoreland zoning regulations restrict shoreline development that otherwise might pollute likes with septic-system drainage. This effluent contributes to eutrophication, which decreases water clarity and dissolved oxygen. The DNR requires landowners to obtain a permit before removing aquatic weeds that provide cover for spawning, nursery areas and feeding.
Various state laws prohibit or limit various kinds of industrial or sewage-plant pollution that deprive fish of oxygen or poison them outright. Other forms of pollution -- such as soil erosion, pesticide runoff from farmland, manure runoff from feedlots, and storm runoff from city streets and lawns -- also consume oxygen and contribute to algae blooms and other signs of eutrophication. These "nonpoint sources" of pollution are extremely tough to correct.
Winter aeration systems are a boon to shallow eutrophic lakes in southern Minnesota. In the past, these lakes would lose their fish to periodic winterkill. With aeration, however, these lakes produce fast- growing panfish that survive from year to year.
Greater opportunities to go fishing -- usually to go fishing for panfish -- result when the DNR installs a boat access, builds a fishing pier or clears brush on a bank to create a fishing site. The fishing piers are a particularly popular with youngsters, the elderly, the handicapped and other anglers who may not have boats or access to more remote waters. The DNR sometimes anchors brush bundles near fishing piers to attract panfish.
In another program aimed at anglers with limited mobility, the DNR has worked with communities to stock crappie and bluegill in about two dozen "kid's fishing ponds" in the Twin Cities and a few elsewhere in the state. Catchable-sized fish are stocked in the spring to provide fishing for the summer. Some of these ponds are aerated, but most winterkill during severe winters.
Panfish also are important additions to many of the lakes that are chemically "rehabilitated" to rid them of carp and other rough fish. Once fish managers remove the undesirable denizens, they stock bluegill, crappie, and various large game fish. Fish managers try to establish a predator such as largemouth bass that will prevent panfish from becoming too abundant immediately after reclamation.
Much yet must be learned about managing for sunfish and other panfish. Manipulating predators and forage species in an effort to produce better fishing is complicated and must be attempted largely by trial and error. Clearly, however these popular panfish must be manages as part of a larger fish community, rather than as individual species.