Structure — above-bottom features such as aquatic plants and brush — benefit fisheries by providing added substrate for invertebrates that are important foods for fish and by providing refuge from predation to aid survival of young fish. Structure also benefits anglers by concentrating predatory sportfish that are attracted to forage fish.
As reservoirs age, structure declines in those where aquatic vegetation fails to become established. Many fisheries agencies have initiated programs to add structure to their aging reservoirs.
Mississippi’s flood-control reservoirs — Sardis, Enid, Grenada and Arktabutla — are an extreme situation of structure deficiency. A recent study by the U.S. Geological Survey, Mississippi Cooperative Fish and Wildlife Research Unit and Mississippi State University fisheries researchers assessed the effects of the limited existing structure and added structure on the fish assemblage in Enid Lake.
Like Mississippi’s other flood-control reservoirs, Enid has extensive mud flats — areas that are de-watered during the annual winter drawdown to provide floodwater storage capacity — with very limited structure. In the first year of this two-year study, researchers compared the fish assemblages between mud-flat areas with no structure and areas with some, although scarce, natural structure, such as willow and cypress trees and flooded grass.
More species of fish were collected, and the fish were more abundant in sites with structure than in the open sites. Largemouth bass, bluegill, orangespotted sunfish, blacktail shiner and spotted gar were more abundant in sites with existing structure; brook silverside, common carp and gizzard shad were more abundant in open sites.
In the second year, Mississippi State’s research team compared the fish assemblages in areas with added structure — cypress and cedar brush piles — to areas lacking structure. Again, fish were more abundant in the areas of added structure. Further, fish abundance was positively related to the volume of the brush piles: bigger brush piles held more fish. And larger structures had wider size ranges of fish than smaller structure.
Black crappie, white crappie, bluegill, orangespotted sunfish, largemouth bass, blacktail shiner and emerald shiner had an affinity for larger structure volumes.
Structure deficit disorder
Structure, whether natural or added, increased the abundance of fish. No real surprises here: as found in numerous evaluations in other reservoirs, structure attracts fish. The fish attracted to — and that presumably benefit from — structure include the primary sportfish in Enid Lake. Unlike studies in other reservoirs where the added structure is available year-round, fish in Enid can only use the structure when the mud flats are seasonally inundated.
The question asked of all efforts to provide structure for fish is whether the structure merely attracts and concentrates fish or whether the added structure actually increases the production and abundance of fish. Certainly, adding structure can do no harm, but long-term effects on fish abundance and production will require additional study.
Recent work in Texas has demonstrated that adding large amounts of structure does increase fish production. I think it is time to look beyond the question of “attraction or increased production?” and try to determine how much structure needs to be added to realize increases in fish abundance, growth, and production.
A second problem warranting attention for Mississippi’s flood-control reservoirs is providing structure to benefit fish year-round. The Mississippi State study documents the beneficial effects of structure in the barren water-fluctuation zone. But the mud flats are de-watered in the fall, and the fish spend late fall through spring in a structure-deficient lake. Realizing the benefits of structure addition to the mud flats may also require addition of structure to the permanently inundated basin of the lakes.
Adding structure to habitat-deficient reservoirs is laborious and expensive. In Table Rock Lake, the Missouri Department of Conservation spent $4 million adding structure to improve habitat. The effort was considered highly successful by biologists and anglers. Other reservoirs have benefited from habitat enhancement by cooperative efforts of anglers and state or federal management agencies. These voluntary efforts of anglers can make these projects more affordable.
Mississippi’s flood-control reservoirs have long benefited from structure additions during late-winter Habitat Days. Organized and supported by the U.S. Army Corps of Engineers and with technical assistance provided by Mississippi Department of Wildlife, Fisheries, and Parks biologists to determine best locations for structures, volunteers install cedar tops, Christmas trees and stake beds. These efforts have benefited from participation of as many as 250 volunteer workers. More effort leads to better fishing.
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