Habitat and the Importance of Structure

The largemouth bass and yellow perch populations crashed after researchers intentionally removed shoreline logs and brush from a central Wisconsin lake. Changes in fish assemblages have been linked to riparian development that alters littoral zones in Minnesota lakes. Outstanding largemouth bass fisheries - 30-plus fish days with many over 3 pounds - are only fond memories for several central Minnesota lakes where luxuriant deep-water beds of broad-leafed pondweed (cabbage) have since vanished. Almost half of U.S. reservoirs larger than 250 acres in the United Sates are rated moderately or highly impaired by fisheries managers, and loss or lack of structural habitat is a high-ranking reason. Fisheries from the upper Midwest to the southeast Atlantic and Gulf coasts are battle zones in weed wars between anglers and other aquatic resource users, and fish and anglers often lose these battles. There is no magic wand in the fisheries managers’ tool kit that can create or restore abundant fish populations - and the desirable fishing they provide - if habitat is lacking.

Habitat Basics

Habitat is where a fish lives and includes the chemical, physical, and biological components of their environment. “Chemical” is the water quality that includes everything from essential, life giving oxygen to toxic chemicals, a huge topic for another time.

The physical component of habitat is the geomorphology - the composition and shape - of the lake, reservoir, or river basin. Geomorphology includes whether the bottom is silt, sand, rock, rubble, or boulders; whether the basin is deep or shallow, flat-bottomed or irregular; and whether the basin is rather round with a fairly low shoreline length-to-surface area ratio or with many coves, bays, and islands with a high shoreline length-to-area ratio. Although not of geologic origin, I include other non-living hard objects in the water - standing timber and brush and man-placed structures like docks and shoreline protection―in the physical component.

The biological component is the living organisms that comprise the food web and that modify the fish’s environment. Predominant environmental modifiers are plants - the rooted macrophytes and planktonic algae that affect water clarity and how deep macrophytes grow.

Anglers have terms for the physical and biological components of habitat: “structure” and “cover.” These terms lack consistent use among anglers, so for the purpose of this discussion let’s define terms. “Structure” is the shape of the bottom - geomorphic features like humps, bumps, drop offs, creek channels, points, etc. - and objects above the bottom - aquatic plants, logs, brush piles, docks, etc. “Cover” is a biological consequence of structure that conveys concealment to both predators and prey. This discussion is about structure; more specifically, it is about the living and non-living above-bottom features.

The Ecology of Structure

Fish are the products of energy flow through the ecosystem. One energy pathway is from phytoplankton (algae suspended in the water) to zooplankton (small animals like microcrustaceans suspended in the water) and then to benthic invertebrates (bottom-dwelling animals like crustaceans, aquatic worms, and insect larvae) and plankton-eating (planktophagic) fishes like shads, many minnows, and the young of almost all fishes. The benthic invertebrates and planktophagic fish are forage for sport fishes. This is a common food web in the open water of lakes and reservoirs.

But there is another energy pathway in lakes and reservoirs that have abundant structure. The common attribute of structure, in addition to providing cover for many species of fish, is that it provides increased surface area, or substrate, for attachment of algae that grows on the structure (periphyton) and macroinvertebrates that feed on the periphyton. The macroinvertebrates are important energy sources for a variety of fishes. Many sunfishes and some minnows are particularly adapted for plucking these invertebrates off aquatic plants and hard structures; some of these fish are sport fish, and others are forage for sport fish. More structure - more substrate - leads to greater biomass of invertebrates and greater energy available to fish. And this structure-based energy pathway is in addition to the phytoplankton-to-zooplankton food web in the open water. It is not surprising that fish are attracted to structure that provides both cover and abundant energy resources.

Structure Management

Natural lakes and reservoirs (man-made impoundments) differ in many ways, but differences in structure are often the most striking. Natural lakes are old - most originated with the retreat of the last glaciers 12,000 to 14,000 years ago - and water levels are relatively stable. Time and stability, along with slow accumulation of sediments and nutrients, have led to establishment of rooted aquatic plants in shallow waters where light is sufficient for their growth. Thus, many lakes are structure rich. However, both the abundance the composition of the plant communities are changing.

Conversely, reservoirs are young, lack seed banks for aquatic plants, have bottom composition not favorable for growth of aquatic macrophytes, and many lack the stable water conditions necessary for aquatic plants. Although some shallow reservoirs have developed aquatic plants, which unfortunately often are non-native plants that can grow to nuisance levels, the primary structure in reservoirs is geologic features like rocks and trees, stumps, and brush inundated when the reservoir filled. Much of the above-bottom structure has decayed or been buried in sediment resulting in substantial loss of substrate for aquatic production and less fish- and angler-attracting structure.

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Numerous state fisheries agencies, the U.S. Army Corps of Engineers, and state and local angler organizations, often working in partnerships, have programs to replenish the vanishing structure in aging reservoirs. The projects range in scope from a 6-year, $4 million project led by Missouri Department of Conservation that installed more than 2,000 structures composed of brush, rocks, or stumps in 43,100-acre Table Rock Lake, Missouri, to the efforts of numerous angler groups pooling their time and resources to place Christmas trees in nearby reservoirs. Projects conducted by state fisheries agencies report GPS coordinates of the added structure to anglers via the agency’s website. A 2017 book Reservoir Fish Habitat Management authored by Dr. Steve Miranda and published by the Reservoir Fisheries Habitat Partnership provides succinct guidance for reservoir habitat restoration.

Installed brush and small-wood structures, although beneficial, have short lifespans. Many companies now manufacture plastic fish structures, and there are numerous DIY fish structures. These structures are far more durable than natural brush and more easily installed by anglers lacking access to a habitat barge and other necessary heavy equipment for collecting and moving structure materials. Cost of these synthetic-material structures exceed the cost of natural materials, but these costs are counterbalanced by less frequent addition of the long-lasting synthetic structures.

Aquatic macrophytes provide abundant substrate to boost biological production. Unlike non-living structure. they are self-maintaining once established, filter sediment from the water, stabilize the bottom to prevent resuspension of sediment, and protect shorelines from erosion. Thus, establishing native aquatic plants is a desirable alternative for improving fish habitat in the many reservoirs that have sufficiently stable water levels to support the growth of aquatic plants.

Extensive research during the last three decades has greatly advanced managers’ ability to establish native aquatic macrophytes. Success is enhanced by first selecting native aquatic plants that grow in the locale and have proven successful. Next, a source of propagules - a mature plant or seeds, root crowns, turions, or tubers - is needed. Propagules can be transplanted from nearby wetlands or may be available commercially, but often the propagules have to be produced in a nursery from locally adapted plants. The propagules are then planted at the start of the growing season to establish founder colonies along protected shorelines. Water depth is critical, and fenced exclosures must be installed around the founder colonies to exclude terrestrial and aquatic herbivores. The plants spread from the established founder colonies.

While extensive research has advanced the success of establishing aquatic plants, achieving consensus to establish plants is often the greater challenge. Based on a nationwide survey of fisheries managers, the reservoirs that could benefit from more macrophytes far outnumber the reservoirs that have excessive macrophytes, although this varies geographically. The need for macrophytes is less and the problems of excessive macrophytes is greater in the southeastern U.S. where weather is warmer, reservoirs are shallower, and water levels are more stable. Nevertheless, fishing is just one use of lakes and reservoirs. Riparian property owners and many aquatic recreationists do not want any aquatic vegetation. Many do not understand the beneficial roles of macrophytes, including the clear water that makes the lake attractive and maintains their property values. But, possibly more importantly, some fear that the nuisance growths often attained by non-native plants like hydrilla and Eurasian water milfoil are characteristic of all aquatic plants. This problem can only be solved by educating all stakeholders and then developing mutually acceptable management strategies.

Any individuals or angler groups intending to add structure of any type to a lake or reservoir should consult with their state fisheries biologist for permission, advice, and possibly assistance.

Attraction or Production

Structure attracts fish. Angler testimonies and numerous evaluations by state fisheries agencies support the conclusion that adding structure in reservoirs increases angler catch rates. In natural lakes, “weed beds” are where the action is. But unresolved is whether added structure increases fish production, the rate at which fish biomass is added to the fish populations, more than harvest reduces biomass. Conceivably, adding a small amount of structure could have a significant fish-concentrating effect and result in harvest of more fish biomass than produced by the added substrate. It is also possible to have too much structure, a problem common to aquatic macrophytes in shallow lakes and reservoirs, that results in altered predator-prey dynamics, undesirable fish population size structures, and poor fishing. So the question is: how much structure needs to be added to get a noticeable increase in fish production without adverse effects?

Measuring fish production requires a huge amount of effort and is unlikely to be accomplished for recreational fisheries. Observational studies in Florida, Illinois, Mississippi, and Texas suggest that largemouth bass production, recruitment, and standing crop may peak at 20% to 40% areal coverage of macrophytes. Addition of manufactured fish structures to achieve 20% areal coverage of the littoral zone increased the abundance of bluegill and the growth rate of largemouth bass - two measures suggestive of increased production - in a reservoir that was overstocked with bass.

Further assessments of the benefits of different amounts of structure, particularly aquatic macrophytes for which extensive efforts are made both to establish and to reduce coverage, are needed. In the meantime, simply monitoring angler catch rate will provide useful information about whether added structure has any adverse effects. Adverse effects are unlikely as long as catch rates remain high.

Research by Dr. Brian Graeb and graduate students Jason Breegeman and Chance Kirkeeng at South Dakota State University offers insight to rejuvenating an aging impoundment. The study was conducted in a 110-acre private impoundment in east Texas managed for trophy bass. The lake was stocked with Florida bass. In addition to a diversity of forage fishes, bluegill, gizzard and threadfin shad, tilapia, and red swamp crayfish were intensively stocked to maintain abundant forage for the bass. Grass carp had been stocked to control aquatic vegetation. The impoundment was not producing bass to the owner’s satisfaction, and the owner stocked additional, 9- to 10-inch Florida bass (about six per acre). The bluegill population crashed, and bass growth slowed.

Mossback Fish Habitat structures were added to limited existing wood and macrophytes to achieve 20% areal coverage of combined structure in the littoral zone, water 2- to 10-feet deep. Within one year after adding the structure, bluegill abundance increased several fold, and bass growth rate improved. Adding structure appeared effective: more bluegill forage, faster bass growth. But something else was in play. Radio-tracking studies found bass strongly selected for areas with the added habitat. Graeb suspects that the fast improvement in bass growth rate was not only the result of the increased food supply but also improved foraging efficiency, such that bass could conserve energy by consuming the abundant food in the vicinity of the cover-providing structure.

Graeb, who now applies his science to his lake and wildlife management service (bluewingoutdoors.com), is further evaluating the amounts of structure needed to achieve different fishery endpoints. Graeb’s next quest is to see whether structures suspended in open water will increase the bass’ use of open-water forage like gizzard shad.

*Dr. Hal Schramm is an avid bass angler and fishery scientist who has done extensive research on black bass. He frequently contributes to In-Fisherman publications on a variety of fishery science and conservation topics.