Anadromous Fish and Dams in NJ’s Raritan River

By Bianca T. Esposito, NWNL Research Intern
(Edited by Alison M.  Jones, NWNL Director)
All photos © Alison M. Jones

Bianca is a Syracuse University senior studying Biology and Economics. Her NWNL summer research on watershed biodiversity also yielded these NWNL blogs:  Wild v. Hatchery Salmon; Buffalo & Bison; Papyrus & Phragmites; Deer & Elephants and Water Hyacinth & Zebra Mussels.

Jones_150726_NJ_3308.jpgView of Raritan River below Nevius Street Bridge


Native fish species have been returning to New Jersey’s Lower Raritan River Basin and Raritan Bay after dam removals that began in 2008.  As the Raritan River flows to the Raritan and Hudson River Bays, it runs through central New Jersey, the heart of the most densely populated state in the US.  Current decommissioning of dams in this NWNL case-study watershed reflect concerns over declines of New Jersey’s aquatic species since the 1740’s when small mill dams were built to grind grain and other commodities, to control flooding and to store water.


Since the 18th century, New Jersey dams have played a role in water control, commercial power and irrigation. However, abandoned and non-functioning dams contributed to water-quality degradation. Even relatively-low dams from the early Colonial Days have degraded water quality by creating abnormal temperature fluctuations in slow-moving, or still reservoirs. Thus, Raritan dams have altered river functions and habitats for its river species by trapping sediments and burying rocky riverbeds.

Dams on any river obstruct species’ upstream-downstream passage through rivers. Dam impacts on riverine fauna include restricted migration and limited access to habitat. As a result, dams restrict the ability of aquatic species to seek food and escape predators. These  impacts can lead to local extirpations of a species and can isolate populations within a species, resulting in a “bottleneck effect” or “genetic drift.”

One benefit of dams, or similar water impediments, is that they can help prevent introduced, non-native and often invasive species from colonizing in new areas and potentially outcompeting the native species. Thus, one initial investigation for those who plan dam removals is to check whether secluded streams harbor any non-native species that could spread and jeopardize a larger ecosystem.

The best option for eliminating the separation of upstream and downstream species is dam removal. The next-best alternative is construction of fish ladders or fish passages for each dam. In sum, by restoring natural streamflows, dam removals support the health of rivers and streams by reconnecting artificially-isolated native populations and by improving water quality for all native species.

Jones_150722_NJ_2954.jpgRaritan/Hudson Bay’s striped bass, oyster, herring & fluke. Courtesy Raritan Baykeeper


In the Lower Raritan River and Raritan Bay, there have traditionally been many anadromous fish species (those that live at sea and then return to native freshwater tributaries upstream to spawn). Once anadromous fish spawn, they die and their life cycle is carried out by the next generation of juveniles who migrate from their upstream tributary down to the sea – only to return upstream later.

Anadromous species native to the Raritan River include sea lamprey, American shad, hickory shad, alewife, blueback herring and striped bass. While striped bass and shad are most favoured by recreational anglers, herring and American eel provide important food sources for other species. American eel are also present in the Raritan Basin; however, they are catadromous. (Catadromous, the opposite of anadromous, means the American eel spawns in the Sargasso Sea in the Atlantic Ocean, and then returns to its freshwater river basin.

Jones_070702_OR_6069.jpgThe mouths of anadromous lamprey eels

Bunker, (aka menhaden) are a critical basis of the Lower Raritan’s aquatic food webs and a critical food supply for predator fish like striped bass. Known for schooling together and being indicators of predatory larger fish nearby, bunker spawn in the ocean; but their ‘fry’ grow up in the bays and estuaries, such as the Raritan Bay. Frighteningly, bunker declined 86% within 30 years (c. 1980-2010), reaching dangerously low levels. With no limit on how many bunker could be removed from the water via nets, there has been heavy overfishing of this vital species. There are several reasons for this decline.

Bunker have long been netted as predominant baitfish for recreational fishermen, commercial fishing fleets and lobstermen in the Raritan Bay. Those ‘takes’ account for 20% of recent losses of bunker. But 80% of that loss is for commercial reduction into food for aquaculture, pets and livestock – as well as for dietary supplements. Some describe this commercial harvesting process as “strip-mining” of our waters.

Jones_150728_NJ_3822.jpgRecreational fisherman in the Raritan Bay

Local fishermen and bay stewards are divided on the best methods of protection. As of 2011, there was no limit or cap on taking bunker; but as of 2018, there is no reduction fishing allowed in NJ bays. Yet, a few 90’ grandfathered “carry boats” from Massachusetts do still haul full loads of bunker from the Raritan Bay to Maine to feed lobsters there. Captain Paul Eidman of the Anglers Conservation Network and the Menhaden Defenders proclaims, “No Bunker – No Bass,” citing the need for moderation and conservation. Others call for a total ban on all bunker netting in NJ bays.

The overfishing and decline of bunker has been related to the cause of decline in the Raritan’s anadromous fish populations, adding to the other negative impacts on them of dam construction and years of pollution. It is necessary for bunker to be able to naturally reproduce and keep their populations healthy in order to maintain aquatic food webs involving predator fish, dolphin, whales, waterfowl and marine birds. Bunker are vital to maintaining a healthy Raritan River ecosystem.

Jones_090621_NJ_0979.jpgHead of dead fish washed up onto banks of Raritan River Estuary


Dam removal improves water quality, reduces pollution, and benefits fisheries by once again allowing a free flow of water throughout the watershed. In the last decade, three Raritan River dams that were key in blocking anadromous fish migrations were deconstructed and removed. This reparation of Raritan waters stemmed from coordinated efforts to increase species’ populations in the Raritan watershed.

Calco Dam, in Bridgewater, was built for industrial dispersal of chemicals in the 1930’s. It was the first dam to be removed in a series of dam deconstructions intended to promote anadromous fish passage throughout the Raritan. Calco Dam was successfully removed in 2011.

Roberts Street Dam, spanning Bridgewater and Hillsborough, was removed next. This dam was built in the 1920’s and deconstructed in 2012, the year following the Calco Dam removal.

Nevius Street Dam, in Hillsborough, was owned by Duke Farms. It wasbuilt at the turn of the 20th Century to support 9 man-made lakes on the 2,740-acre Doris Duke Estate. It was the third dam removed. It came down in 2013. Constructed in 1900, this was the oldest of these three dams.

Jones_080511_NJ_0758.jpgRockaway Stream falling over a former canal diversion in the Upper Raritan River Basin

The elimination of these three barriers has allowed nine miles of river to again flow freely. This restored access for American shad, hickory shad, blueback herring and alewife – both predator and bait fish – to 28 miles of combined mainstream and tributary waters. Once again, these anadromous fish could reach their historical spawning grounds.

These removals triggered feasibility studies and plans for dam removals even further upriver: for the Headgates and Weston Mill Dams and Island Farm Weir. When the Weston Mill Dam (aka Weston Causeway Dam) was deconstructed in August 2017, about ten miles of free-flowing mainstream river were again open to anadromous fish. These dam removals were funded, not by taxpayer dollars, but by financial settlements paid for by former polluter companies, due to their responsibility for natural-resource damages.


Just as these dam removals benefit anadromous fish in the Raritan, they also provide habitat diversity for other species, such as the American eel, bald eagle and blue heron.  Basically, the entire food chain within these new-flowing waterways benefits. The elimination of dams in the Raritan Basin also improves the river’s overall health by reducing water temperature and siltation, thus increasing oxygen levels and improving vegetation growth.

These recent dam removals also helped the Raritan watershed meet critical goals of ensuring clean drinking water. By establishing safer and cleaner waterways, dam removals have also increased local communities’ recreational use of the river, including kayaking, canoeing and fishing. These are great achievements for the nation’s most densely-populated state.

Partly due to these dam removals, 2017 was a banner year for New Jersey shad catches ─ thus underlining increases in anadromous fish runs. Dam removal also has allowed for improved sediment transportation which creates healthier aquatic habitats.  The Raritan today is in far better shape than during its period of “open sewer” status during colonial days. Referring to this newly-improved Raritan watershed, NJ’s Musconetcong Watershed Executive Director exclaimed, “If we can do it in New Jersey, we can do it anywhere!”


• Alderson, Carl. The New Jersey Section of the American Water Resources Association (NJ-AWRA), accessed on August 8, 2018, via link.
• Bowers-Altman, Jeanette. NJ Division of Fish and Wildlife, accessed on August 8, 2018, via  link.
• Byers, Michele. NJ Advance Media, accessed on August 8, 2018, via link.
• Crouse, Shawn. NJ Division of Fish & Wildlife, accessed on August 8, 2018, via link.
• Dauwalter, Daniel. Trout Unlimited final report to National Fish and Wildlife Foundation, accessed on August 8, 2018, via link.
• Digilo, Luke. Montclair State University, accessed on August 8, 2018, via link.
• Eidman, Paul.
Hatch Magazine, accessed on August 9, 2018, via link.
• Environment News Service, accessed on August 8, 2018, via link.
• Jengo, John. Association of State Dam Safety Officials, Inc., accessed on August 8, 2018, via link.
• Litton, Bruce Edward. “New Jersey dam removals improve fisheries.“ My Central Jersey, accessed on August 10, 2018 by AMJ, via link.
•, Message Board, May 5-6, 2018.  Accessed by AMJ 10-3-18 via link.
NY/NJ Baykeeper, accessed on August 8, 2018, via link.
• Turpin, Craig. NJ Advance Media, accessed on August 8, 2018, via link.
• Waldman, John. American Fisheries Society Symposium 51, accessed on August 8, 2018, via link.
• Wichert, Bill. NJ Advance Media, accessed on August 8, 2018, via link.

2 thoughts on “Anadromous Fish and Dams in NJ’s Raritan River

  1. Your statement above about anadromous fish dying after spawning is not totally correct. Unlike the Pacific salmon species, which do die after spawning, the majority of Atlantic coast diadromous fish species are repeat spawners. Sea lampreys and American eels are the exceptions.
    Glad to see more river restoration work being done!

  2. Hello. Today 3/26/23 we saw multiple large fish (24+ inches) repeatedy and unsuccessfully attempting to swim up across the 3 tier weir/spillway on the Raritan River just South of the Raritan Millstone water treatment plant (at the confluence of the Raritan River and Millstone River) They were clearly attempting to swim upstream, and were being completely obstructed from doing so by this spillway. It was sad to witness the heated repeated failed attempts. They appeared to be salmon or very large Trout

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