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Mianus Pond Fishway


The Mianus Pond Dam located on the Mianus River just north of Post Road was constructed in 1926 and is the first impediment to migratory diadromous fishes as they enter the river. Construction of the dam caused an immediate and drastic reduction in migratory fishes that utilized the river for reproduction, feeding, and residency, and as early as 1970, the Connecticut Department of Energy & Environmental Protection (DEEP) stated that the future of these migrations was in jeopardy. Furthermore, a fishway at the dam would allow the runs to be restored closer to their original sizes.

In 1991, settlement of the Grass Island Wastewater Treatment Plant suit enabled funds to be allocated for construction of a fishway. On May 27, 1993, an Alaskan steeppass, a specific style of Denil fishway, was officially opened and allowed migratory fishes to gain access to ~ 2.1 miles of freshwater stream habitat (Figure 1). The fishway project was sponsored by the Greenwich Conservation Commission and supported by the CT DEEP as well as many local groups.

Figure 1. Mianus Pond Fishway located adjacent to the Greenwich Adult Day Care Center (GADC) and the base of the Mianus Pond Dam.

To further improve fish passage, a Delaware-style eelpass (Figures 1 & 2) was installed adjacent to the existing fishway during the fall of 2003. The eelpass, constructed from a trolling net draped over the face of the dam, enabled juvenile American eels to ascend the spillway by climbing through the net. Support for the eel pass was provided by the CT DEEP.

Figure 2. Close up of eelpass. fishway2.JPG

Additionally, during the fall of 2006, an aluminum walkway (Figure 1) was installed next to the fishway. The walkway allowed town employees and volunteers to gain access to the entire fishway, thereby, providing safer conditions and increased opportunities for monitoring, maintenance, and supervised public education. The Long Island Sound Futures Fund and The Greenwich Adult Day Care provided support for the walkway.

Fishway objectives

The specific management objectives of the Mianus fishway are to restore native fish runs and allow diadromous fishes to spawn and feed upstream of the dam. By doing so, the overall goal of the fishway is to increase biodiversity and health within the Long Island Sound and Mianus River. The original targeted fishes for passage include the alewife (Alosa psuedoharengus), blueback herring (Alosa aestivalis), brown trout (sea run) (Salmo trutta), and American eel (Anguilla rostrata). Other diadromous species that use the fishway may include the gizzard shad (Dorosoma cepedianum), white perch (Morone americana), and striped bass (Morone saxatilis). In addition to improving ecosystem health, allowing passage of sea run brown trout supports and expands the existing trout fishery in the Mianus River. Migration windows for each species are listed below in Table 1.


Each spring, the Mianus Pond Fishway passes one of the largest runs of alewife and blueback herring in Connecticut.  The populations of river herring that spawn in the Mianus River serve as sentinels for the other populations of these species along the Atlantic Coast of North America.  Each day during the spawning period, runs are tallied on an electric fish counter monitored by a team of dedicated volunteers.  Information about fish runs in the Mianus River and other CT streams can be seen in CT DEEP's weekly diadromous fish reports.  In addition to the electronic fish counter, an underwater camera is also used at the Mianus Pond Fishway to monitor fish runs.  Click here for more information on the underwater camera and to view footage of fish migrating through the fishway.

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