Harris Lake, a reservoir in New Hill, North Carolina, covers 4,100 acres in southwestern Wake County and southeastern Chatham County. Hydrilla was first observed in the lake in 1988, and since that time a substantial infestation has developed. Hydrilla is an aquatic weed included on the Federal Noxious Weed List and is recognized by the N.C. Department of Environmental Quality as an Aquatic Weed. The possession or sale of hydrilla is prohibited in North Carolina and in the United States. Harris Lake is a source population for the spread of hydrilla to other waterbodies in our state, where the long-term environmental and economic impacts can be substantial. With the goal of mitigating hydrilla’s impacts, the N.C. Department of Environmental Quality’s Division of Water Resources (DWR) – Aquatic Weed Control Program will be implementing hydrilla control in Harris Lake. This effort will be conducted in collaboration with the Wildlife Commission. The Commission is also initiating a substantial effort to enhance aquatic habitat for fish and wildlife, especially largemouth bass, black crappie, and waterfowl to utilize in the absence of hydrilla.
What is hydrilla?
Why is hydrilla classified as a noxious weed?
History of hydrilla in Harris Lake
Why treat Harris Lake for hydrilla now?
Fishing at Harris Lake
Harris Lake aquatic habitat
Stakeholder and constituent collaboration to enhance aquatic habitat
An aquatic plant native to Asia, hydrilla can grow in most soil types, thrive in low light conditions, colonize in shallow or deep water (down to approximately 30 feet in clear water, 15-20 feet in Harris Lake) and can utilize multiple reproductive mechanisms.
A submersed plant that grows completely under the surface of the water, hydrilla roots into the bottom-growing stems that run laterally along the bottom and vertically toward the surface. Plant stems that reach the surface tend to have increased leaf density and develop a canopy just beneath the surface of the water. A single plant can grow one inch a day. Mature plants growing in shallow water will often fill the entire water column. Leaves are less than an inch in length and grow in whorls along the stem.
In North Carolina, hydrilla spreads rapidly by fragmentation, by which a length of stem broken from the plant can drift to a new spot, grow roots and begin a new colony. In addition, hydrilla’s creeping rootstalk produces pea-sized shoots, also referred to as tubers, that can be readily found in the lake bottom soil wherever hydrilla plants have matured. As a perennial plant, hydrilla’s vegetative growth dies back every year, but survives through the winter season as tubers that can remain viable in lake bottom soils for six to eight years. The flowers are small, white to semi-transparent and can be found on the surface of the water in late summer and fall.
In North Carolina, hydrilla was first identified in a Wake County pond in 1980. Since that time, hydrilla has be found across North Carolina in waters ranging from small ponds to major reservoirs and even several rivers. And hydrilla continues to spread in North Carolina and in other states, successfully invading many states in the Mid-Atlantic and even into the Northeast. Hydrilla is spread to other water bodies by fragments floating downstream, being moved by waterfowl, inadvertently transported to other waterbodies via boats, boat trailers and other equipment or intentionally by individuals wanting to improve habitat.
Hydrilla can form extremely dense stands, filling the water column from the bottom to the surface, crowding and outcompeting native vegetation, as well as reducing habitat quantity and quality for native freshwater aquatic animals. Hydrilla infestations may lead to several undesirable events, including the loss of municipal and recreational use of waters and habitat alterations. Advanced infestations decrease the diversity of native submerged vegetation, cause severe changes in water quality, decrease the available volume of water, inhibit recreational activities and have the potential to foul water withdrawal intakes. Boat ramps and public use areas can become unusable during the summer and fall months when surrounded with dense hydrilla growth.
History of hydrilla in Harris Lake
First observed in Harris Lake Reservoir in 1988, hydrilla was thought to have been introduced to the reservoir on a boat or boat trailer and then slowly spread throughout the reservoir. By the mid 1990’s, hydrilla became the dominant submerged aquatic plant species in the reservoir. In 2015, a submerged aquatic plant survey identified approximately 942 acres of hydrilla. In September 2018, DWR conducted a submerged aquatic plant survey at Harris Lake that identified 232 acres of hydrilla. Even though a reduction in vegetative growth has been observed recently, a large tuber bank is present, which can result in expanded growth in the future.
Harris Lake aquatic habitat
Harris Lake has diverse aquatic habitat, including rock outcroppings, flats, roadbeds, creek channels and a variety of aquatic vegetation. The existing habitat provides forage, refuge, spawning, nesting and nursery areas for species that utilize structure, such as Black Crappie, Largemouth Bass, Bluegill and other sunfish. Harris Lake's habitat is also vital to waterfowl.
Native aquatic plants can play a major role as a food source and home for aquatic invertebrates and other wildlife, along with juvenile and adult fish habitat. This can be dependent on the species and abundance of both the fish and the vegetation. Native aquatic plants can also improve water clarity and quality and can reduce rates of shoreline erosion and sediment re-suspension. In October 2017, a preliminary vegetation survey focused on emergent and rooted floating leaf plants along the shoreline found White Oak Creek, Little White Oak Creek and Tom Jack Creek arms had approximately 60 percent of shoreline with aquatic vegetation other than hydrilla. Common emergent plants included giant cutgrass, squarestem spikerush and cattail. Rooted floating leaf plants included American lotus, white water lily and spadderdock.
Fishing at Harris Lake
Harris Lake has an abundant Largemouth Bass and Black Crappie fishery. In 2017, it was ranked by Bassmaster Magazine as first in the southeast region and fourth in the nation for best bass lakes in America. Studies have shown that hydrilla removal does not have long-term effects on Largemouth Bass populations. A study focusing on shifts in populations of largemouth bass, Response of a Reservoir Fish Community to Aquatic Vegetation Removal, showed little change in Largemouth Bass population after hydrilla was removed. Another study, Changes in Behavior, Movement, and Home Ranges of Largemouth Bass Following Large scale Hydrilla Removal in Lake Seminole, Georgia, showed Largemouth Bass switched habitat preference primarily to large woody debris once large quantities of hydrilla were removed. Other prior studies also indicate that the population will likely not change, rather fish behavior will likely change with the removal of hydrilla.
Removal of hydrilla in other reservoirs has been shown to affect Largemouth Bass behavior, but not abundance or size structure. The reduction of hydrilla may result in changes in fish behavior and could result in decreased catch rates if anglers do not alter their strategies. It is important to note that while anglers may associate decreased catch rates with negative fishery impacts related to hydrilla removal, actual changes in fish behavior are the more likely culprit. Other natural and artificial structures, including the introduction of native aquatic vegetation for fish to utilize, will be provided by the Commission to help maintain angler catch rates and satisfaction.
Presently, there are five fish attractor reef sites on the lake that consist of a variety of artificial fish attractors, including Mossback trophy trees, stake beds and barrel structures. As part of the habitat enhancement plan, approximately 400 to 700 fish attractors will be placed throughout the reservoir at varying depths and habitat features (e.g., flats, creek channels, points, roadbeds) to ensure seasonal use by a variety of fish species. The exact number of structures placed will depend on the amount of current habitat available, the amount lost from hydrilla removal and the amount of area that is available for fish to use throughout the year. Sites will be marked with buoys and GPS coordinates will be available on the Commission’s website.
Aquatic vegetation will be established in two phases. Phase 1 involves mapping existing vegetation, identifying areas for re-vegetation throughout the lake and planting and monitoring a variety of plant species within and outside of small protective fenced enclosures to see which species survive and expand the best. Monitoring during Phase 1 will help ascertain the levels of protection needed from grazers, herbivores such as turtles, muskrats, beavers and of course sterile grass carp, and determine which species will likely result in the successful establishment of founder colonies. This information will dictate the best course of action to take during subsequent growing seasons (Phase 2). The size and number of protective exclosures will be expanded in Phase 2 and should result in the successful establishment of at least 0.4 hectare of all founder colonies. Implementing the science-based methods from the study, Techniques for Establishing Native Aquatic Plants, these colonies will expand outside of cages over time and through the establishment of new colonies from fragmentation and seed propagation.
Stakeholder & constituent collaboration to enhance aquatic habitat
Public involvement will be key to developing and implementing the habitat enhancement plan. The Commission will also work with angler and hunting groups to identify specific habitat enhancement locations and the type of structures to be included in the habitat enhancement plan. The amount of work being proposed is extensive so to implement this plan, volunteers will be needed to help build and place artificial structures and to help establish native aquatic vegetation.