October 05, 2015
By Dr. Hal Schramm
The best way to keep bluegill numbers in check? Maintain a high density of bluegill predators in the pond. Typically that predator is largemouth bass. I've repeatedly seen anglers unhooking palm-size bluegills and throwing them on the bank in efforts to reduce the abundance of small, slow-growing bluegills. Good intentions, bad strategy. First, how much fun is it to catch 4-inch bluegills? Second, even the most zealous and dedicated angler can't keep up with the sunfish removal capability of a largemouth bass.
A largemouth bass consumes its weight in bluegills each month when the water temperature is near 75°F. A 4-inch bluegill weighs less than an ounce, a 5-inch bluegill weighs about 1.4 ounces. From spring through fall, a single 1-pound largemouth bass eats at least two dozen 4-inch bluegills or about a dozen 5-inchers each month. Not only are largemouth bass superior bluegill removing machines, they crop 'gills at a much smaller size than do anglers. This is important because a 3- or 4-inch bluegill eats the same food as — and therefore competes with — a half-pound or larger 'gill.
Minnesota DNR fishery biologist Pete Jacobson suggests an opposite strategy may produce quality bluegills that panfish piscatores seek. He found average length of bluegills increased in three of four lakes where the sunfish daily creel limit was reduced from the statewide limit of 30 sunfish per day to 10 per day. During the same period, average length decreased in four similar lakes where the bag limit remained 30 sunfish per day.
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Nothing mysterious here, or so it seems — harvest fewer fish, more survive to grow large, and the average length of bluegill increases. Good thinking, but to grow larger, bluegills need food. With lower harvest, more bluegills would survive to share limited food resources, and growth rate should slow. But Jacobson found growth rate increased in the reduced-harvest lakes, and the greatest increase in growth rate occurred in the reduced harvest lakes that had the greatest proportion of large bluegills.
The likely key to why reduced harvest resulted in larger bluegills was not that they survived to live longer and grow larger, but because they didn't begin reproducing until they reached a larger size. Sexually mature fish channel a lot of energy into developing gonads and building and guarding nests, which leaves less energy for body (somatic) growth. By delaying sexual maturity until reaching a larger size, the fish can grow faster because energy is not shunted to reproduction. Jacobson found that average length at maturity of male bluegill increased from 6 inches before the 10-fish regulation to 6½ to 7 inches four years after the regulation was implemented. During the same time period, average length at maturity stayed at 6 inches in the reference lakes with 30-fish limits.
Does that small difference in length at maturity matter? Yes. Faster growth in the regulation lakes translated into bluegills that were 7 to 8 inches long at age-7 compared to 6 to 6½ inches at age-7 in the 30-fish limit lakes. While the 1- to 1½-inch difference in growth may not sound like much, it equates to a bluegill that weighs twice as much at age-7 in the 10-fish-limit lakes as in the 30-fish-limit lakes.
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Reduced harvest, delayed maturation, faster growth, bigger bluegills. But something is missing: reduced harvest does not directly affect sexual maturation. Research at the Illinois Natural History Survey provides the missing link. Small (5- to 6-inch) male bluegills stocked into newly filled research ponds spawned. In identical ponds where large male bluegills (1 to 2 inches longer than the small bluegills) were stocked with small ones, almost all large males spawned, but only 6 of 208 small bluegills nested. In other words, large male bluegills suppressed spawning of their smaller brethren.
Reduced reproduction by smaller male bluegills can result from three different processes. First, bluegills nest in colonies of several dozen to several hundred tightly packed, saucer-like beds. Large males occupy the best spawning sites near the center of the colony. Smaller males are relegated to sub-optimal spawning sites at the periphery of the colony where they and their offspring may be more vulnerable to predation. Second, smaller males are less attractive to females than large males and, therefore, may not spawn. Third, the presence of large males suppresses gonad development of the smaller males. The Illinois researchers found that the testes weight relative to the body weight of small bluegills was significantly lower when large male bluegills were present. This suppression of gonad development, which has also been observed in other fishes, is attributed to "social influences" and likely mediated by hormones, but the exact mechanism for delaying maturation is yet to be identified.
The "social influence" effect works both ways. When large male bluegills are present, sexual maturation is delayed. But removing large male bluegills triggers maturity at a smaller size. This might be the case in high-harvest bluegill fisheries. With bluegill density reduced by harvest, surviving fish should grow faster. But if high harvest selectively depletes larger male bluegills, smaller bluegills mature earlier, more energy goes to growing gonads, and growth rate slows. End result — a lot of small, slow-growing bluegills, and a lot of reproduction to perpetuate the problem.
Jacobson's findings were encouraging but not perfect — bluegill size structure increased in only three of four reduced-harvest lakes. A question is whether highly restricted harvest creates large size structure of bluegill populations in other lakes. This management strategy has been expanded to other lakes in Minnesota, including a few with the creel limit reduced to only five bluegills, so we wait to see.
Another question is whether the reduced-harvest strategy tested in natural lakes with diverse fish communities would work in small impoundments with simple fish communities? Dr. Derek Aday, fishery professor at North Carolina State University, has done extensive research on the effects of bluegill size structure on energy allocation and growth. He asserts that the large size-structure/delayed maturation/faster growth management strategy should be effective in small impoundments but emphasizes that rigorous evaluation is needed. Testing the effectiveness of limited bluegill harvest will be a tough sell in small impoundments where high harvest — by anglers and bass — is the prevailing mindset.
Spawn Early, Die Young
Sexual maturity of male bluegill is affected by population size structure and abundance, a process that biologists call phenotypic plasticity — the variation in anatomy, physiology, or behavior exhibited by individuals with similar genetic structure. Intensive study of bluegill spawning behavior revealed that not all sexually mature male bluegills build nests, attract and court females, and then patiently and aggressively guard the eggs and fry.
Thirty years ago, Dr. Mart Gross, working in Lake Opinicon, Ontario, identified bluegill phenotypes that he named "sneakers" and "satellites" and referred to both as cuckholders. Both phenotypes are males that mature at young ages — age-2 or age-3 — compared to typical males ("parentals") that mature at age-7. Sneakers are small males that dash into a parental's nest and fertilize the eggs as the female spawns. Satellites are older and larger males, colored to mimic females, that more boldly enter a parental male's nest and fertilize the eggs. Both exit after their sneaky deed and leave parenting chores to the parental male.
As would be predicted by the mature early-grow slowly model, both phenotypes have slow growth. Gross' data also suggests that while sneakers may transition to satellites, neither transition to "normal" parentals, and the cuckholders may die young.
Further research reveals these seemingly deviant phenotypes are reproductive machines. The testes of sneakers and satellites are a significantly greater proportion of the fish's body weight than are the testes of a parental bluegill. Further, the milt of the cuckholders has a higher concentration of sperm, and the individual sperm cells have longer flagella ("tails"), higher concentration of energy compounds, and swim faster, giving the cuckholders a competitive advantage in the race to an unfertilized egg.
What triggers the cuckholder phenotype — the development, physiology, and behavior — awaits explanation. The cuckholders have been found to have higher concentrations of sex hormones, which could be triggered by stress hormones released in response to high population density.
And there's one more twist: bluegill cuckholdry may be restricted to northern populations, suggests Dr. Derek Aday. He has examined the gonads of thousands of bluegills from southern waters as part of his maturity-energy allocation research and has seen few examples of small, sneaker males with mature testes. The relative absence of cuckholders in southern waters may be a consequence of the longer growing season that results in fast growth to maturity of all male bluegills. "When male bluegills mature in their first or second year of life, there may be no opportunity for development of sneaker phenotypes," reasons Aday.
To catch big fish you need to fish where big fish live. Returning to waters where you have caught big bluegills works, usually. Chasing off to a new water on a "hot tip" from a friend works, sometimes. Based on research findings of Minnesota DNR biologist Pete Jacobson, a trip to lakes with low sunfish creel limits should consistently yield big 'gills. You'll also probably find reduced competition by other sunfish anglers, at least for a while, who tend to be consumption oriented and discouraged by the low harvest limits.
Ponds and small impoundments can produce exceptional fishing for large bluegills. With so many to choose from, where do you start? A little "reverse reasoning" may work. Instead of tuning in to reports of big bluegills (which are reports of how the fishing was), listen closely to bass anglers who are complaining about "a lot of small bass." These are the lakes that offer high probability of bluegill populations with a lot of large fish. Then target lakes without gizzard shad. If it's a private pond, always ask permission before accessing.
Show-Me State Bluegill
Tobacco Hills Lake in northwestern Missouri has provided exceptional fishing for magnum 'gills for at least 20 years. Missouri Department of Conservation fishery biologist Jake Allman gave me the stats on this unique, 17-acre lake located 20 miles from metro Kansas City. The lake is fertile but clear. Tobacco Hills is managed with a 10-fish daily creel limit and an 8-inch minimum length limit. The lake receives high fishing effort (300 angler hours per acre per year), and high angler success results in high bluegill harvest. Despite the high harvest, electrofishing catch rates of bluegills 8 inches and longer rank among the highest in the state.
Tobacco Hills might be the perfect storm of bluegill fisheries. The fish community is simple — bluegill, largemouth bass, and a few crappies. The largemouth bass have a small size structure and are abundant. They severely crop the bluegills, which results in fast growth to large size for those that escape predators. The persistent scarcity of crappies, which commonly overpopulate small impoundments, is additional evidence of heavy bass predation. This small, clear lake has a narrow fringe of pondweed that provides refuge to small bluegills, so there's a steady supply of small bluegills to move up the size ladder. Tobacco Hills Lake bluegills reach 8 inches in only 5 years, 3 years sooner than the average bluegill in Missouri.
Allman also emphasized that this impoundment does not have gizzard shad. How gizzard shad suppress bluegills has eluded researchers for years. Possibly it is because bass selectively feed on shad, which allows bluegills to overpopulate, which in turn results in slow growth. Nevertheless, the reality is that bluegills don't grow big in small impoundments where gizzard shad are present. Allman reported that bluegills don't exceed 7½ inches in other lakes in his Northwest Missouri district that have shad.
The unanswered question at Tobacco Hills Lake is whether the exceptionally large size structure of the bluegill population follows the Illinois Natural History Survey model of large bluegills suppressing maturation of small bluegills. Or maybe it's a combination of high bass predation and the "social influence" process that results in exceptionally fast growth.
Despite the ubiquity of bluegill and their popularity as a sport fish, they have received little active management. The management strategy in ponds has been to maintain high predation through a lot of hungry bass. Good "bream" fishing in thousands of ponds where quality bluegills are the management target is evidence that the strategy works. But the downside is that it is accomplished by giving up quality bass fishing.
Active management by reducing harvest holds promise to provide quality bluegill fishing without compromising the other fisheries available to anglers. Clearly, more evaluations are needed to test the effect of the reduced harvest regime, and also to understand why a specific management strategy is effective (or ineffective). These studies are difficult to conduct in public waters because they require intense creel surveys to measure angler effort, catch, harvest, and compliance with regulations. –Â
*Dr. Hal Schramm, Starkeville, Mississippi, is an avid angler, fishery biologist, and freelance writer. He frequently contributes to In-Fisherman publications on science topics.
