State Fish Hatcheries and Invasive Species Don’t Mix


The Vermont Fish and Wildlife Department is responsible for the protection and management of the state’s fish populations. Cultured fish are stocked in natural and manmade areas that are excellent for growth and survival but have little or no spawning habitat. Other stocking areas include natural water bodies where spawning and nursery habitats have been destroyed or isolated from growth habitats by structures such as dams. But perhaps the most visible role of cultured fish is to provide recreational angling for the public. Each year specific water bodies succumb to heavy fishing pressures. Cultured fish can reverse this adverse condition. The Ed Week Fish Culture Station is the newest and largest of the five Vermont state fish hatcheries. It began raising fish in 1991 and releases nearly three-quarters of a million fish per year. Species include brook trout, brown trout, lake trout and rainbow trout as well as steelheads and landlocked Atlantic salmon. Most fish are released as 15-30 cm (6–12 inches) yearlings for statewide stocking. However, Ed Weed is also the home of about 5000 Vermont Trophy Trout. These two-year-old brown and rainbow trout are released every spring for immediate recreational angling. Excess fish are sold to surrounding states for similar purposes.


The Ed Weed Fish Culture Station is located on Grand Isle in the middle of Lake Champlain in Northwest Vermont. This large glacial lake has depths reaching 120 meters (394 feet) and experiences an annual autumnal overturn between the top warm epilimnion layer of the lake and the deeper cold hypolimnion layer. The unique design of this culture station takes advantage of the different water temperatures at different depth in Lake Champlain. A deep water inlet located 55 meters (180 feet) below the surface supplies cool water all year round and a shallow inlet just 9 meters (30 feet) below the surface supplies warm water in the summer. The water temperature differs by nearly 17º C (30º F) between the two inlets during the summer months. Blending water from these two inlets in varying proportions provides the right temperature for rearing the trout and salmon without energy intensive temperature control measures. The initial design called for water to enter the hatchery’s pump station by static lake level pressure through a 90 cm (36-inch) pipe from each of the two inlets. The water was then pumped up gradient to three large rotary micro screens with 21-micron (0.0008 inch) polyester woven elements. Each rotary screen is 1.5 meters (5 feet) in diameter and 3.7 meters (12 feet) long. From the rotary screen filters, the water was fed by gravity to a chamber where liquid oxygen was introduced to super saturate the water with O2. Next the water entered ten raceways (long shallow open-topped concrete tanks) each 30 meters (100 feet) long where the fish were reared. After flowing the length of these tanks, the water cascaded to ten more raceways downstream where more fish were reared. Each of the twenty raceways contains approximately 20,000 fish. Water leaving the final ten raceways flows to an on-site wastewater treatment plant that consists of chemical dosing, clarification and polishing pond before discharge back to the lake. The hatchery system is permitted for a maximum flow of 42,000 m3/day (11 MGD) but typical rates are 32,000 m3/day (8.5 MGD).


Sometime between 1993 and 1994 zebra mussels, Dreissena polymorpha, were introduced to the south end of Lake Champlain. Because the rotary micro screens allow a certain amount of water to by-pass the screening media, they were found to be ineffective at preventing all life forms of zebra mussels from entering the hatchery. For two seasons the Ed Weed Fish Culture Station did not use the shallow water inlet for fear of contaminating the hatchery with zebra mussels. Because of this, the propane-fired boiler used in the wintertime had to be used all year long to raise the temperature of the deep water to the desired level. This greatly raised the cost of operating the station. Something had to be done to reduce the hatchery’s operating costs while assuring no zebra mussels on the premises.


The first alternative for controlling zebra mussels at the culture station was to use various chemicals such as biocides or strong oxidants. This method was quickly thrown out do to the potentially disastrous effects it could have on the fish being reared.

The second alternative investigated for the protection of the hatchery from zebra mussels was ozone treatment of the incoming lake water. This method of treatment is an effective control for zebra mussels but was found to be too expensive and the high dosage and long contact times1 required made it impractical at this facility.

The third method for zebra mussel control looked at was ultraviolet light (UV). While effective at killing the mussels during most of their life stages, required exposure time was a factor in eliminating this approach. Finally, mechanically removing all viable life forms of zebra mussels with pressure filtration was examined. This method resulted in no chemical residue, was not energy intensive, provided positive physical removal (or destruction) of all zebra mussel life stages and was proven reliable in similar applications. This method using Amiad Automatic Self-Cleaning EBS Filters was chosen for the control zebra mussels at the Ed Weed Fish Culture Station and began operation in November 1996.

Customer comments

No comments were found for State Fish Hatcheries and Invasive Species Don’t Mix. Be the first to comment!