The population of non-native zebra mussels has recently expanded in the Pickwick Lake and other Tennessee River impoundments. Although we will have to contend with this unwanted invader for a long time to come, some new information suggests zebra mussels may benefit fisheries as well as harm them.

The biology

Zebra mussels, native to the Black and Caspian seas in eastern Europe, arrived in the U.S. in ballast water of transoceanic shipping. First documented in the Great Lakes as early as 1988, they rapidly spread throughout the Great Lakes region and the Mississippi River Valley.

Zebra mussels are prolific; a single female can release 40,000 eggs during an annual spawn. They tightly attach to any hard surface and filter-feed microscopic plankton. Densities of adult zebra mussels commonly exceed hundreds per square foot. 

Zebra mussels tolerate a wide range of temperatures, thriving up to 77°F and persisting up to 86°F. They have been in the Tennessee River for at least 20 years, and the population has remained relatively small. 

Why the sudden outbreak? Biologists speculate that summer temperatures reaching 90°F have suppressed the population. The summer of 2017 was relatively cool, maybe cool enough to allow high survival of young mussels that then grew during the cooler fall through this past spring. We’ll see what happens after this summer.

Problems aplenty

Their ability to tightly attach to hard substrates allow zebra mussels colonize and clog industrial and municipal water intake pipes and related equipment. At TVA dams, they interfere with the operation of gates that control water flow to the turbines that drive electric generators. Keeping water flowing costs money — millions of dollars annually in the Great Lakes — and that cost is borne by people who use water and electricity.

Colonization of hard substrates also results in their selective attachment to native mussels, ultimately preventing the native mussels from opening their shells to feed and breathe. Native freshwater mussels are the most imperiled group of animals in the United States.

As filter feeders, they remove the microscopic plankton that is the foundation of the aquatic food web. A single, dime-sized zebra mussel can remove plankton from one quart of water per day. Removing this much primary production can affect the entire food web.

Angling effects

Filtering microscopic algae clears the water. The good news: with greater light penetration, this often increases the growth of rooted aquatic plants. Although generally a good thing for anglers, this can also result in weed problems for lakeside property owners and other people who use lakes for recreational activities. This, in turn, invokes all the expense and controversy of aquatic plant control. The bad news: in lakes lacking aquatic vegetation, anglers generally will have to fish deeper, 10 or 20 feet deeper in some lakes, as fish move deeper to find preferred light levels. 

Because of the problems caused by zebra mussels, nobody wants them. To prevent them from spreading, boaters in many states are required to clean, drain and dry their boats. In other areas, boaters are required to take their boat to an inspection and cleaning station; these stations may be a long way off the route to your intended fishing destination, and this regulation is obviously a major inconvenience. A far greater effect on anglers are bans on boat launching in some waters. Conceivably, this “solution” could be used to ban anglers who do not live on the lake. 

Benefits to bass 

No doubt, our aquatic ecosystems would be better off without zebra mussels, and any thoughts of spreading them around are unconscionable. But given that we are stuck with them in lakes and rivers where already established, it is fair to consider another benefit in addition to enhancing growth of aquatic plants.

In a conversation about smallmouth bass, accomplished angler Kevin Van Dam — whose career has spanned the colonization of zebra mussels in the Great Lakes — said he thought smallmouth bass growth had improved since zebra mussels became established. Available biological information supports his hypothesis. First, round gobies, another Great Lakes invader, eat zebra mussels. Although round gobies compete with several native fishes, smallmouth eat them like candy. Second, crayfish eat zebra mussels, further enhancing a food source that benefits smallmouth bass growth and abundance. 

Clean, drain, and dry

Please be a conscientious angler and don’t contribute to the spread of zebra mussels or any other aquatic nuisance species. Clean, drain and dry your boat after every fishing trip. Clean: wipe the boat and remove any debris and aquatic plants. Drain: drain the bilge and livewells and lower the outboard motor to drain any water from the lower unit and prop hub. Dry: store the boat uncovered to dry the floors and carpet.