The Delaware River spans over 330 miles, flowing through New York, Pennsylvania, New Jersey, and Delaware. This extensive waterway transitions from cool mountain streams in its headwaters to a broad, brackish estuary where it meets the Atlantic Ocean. This diverse range of environments supports over 100 species of fish. The transition from non-tidal freshwater to the tidal zone dictates which species thrive in different segments of the river system.
Permanent Freshwater Residents
The non-tidal, upper, and middle sections of the Delaware River are characterized by cooler water temperatures, supporting cold- and cool-water fish species. These permanent residents spend their entire lives in the freshwater environment, often preferring specific stream conditions.
Various species of Trout are prominent in the colder, clear tributaries and the upper main stem. Brown Trout and Rainbow Trout are common, thriving where water quality is high and temperatures remain consistently low. These salmonids often inhabit areas with rocky substrates and fast-moving currents.
Moving into the slightly warmer, large-river sections, Smallmouth Bass and Largemouth Bass become more prevalent. Smallmouth Bass prefer rocky cover and swift water, making them a signature species in the middle Delaware River. Largemouth Bass tend to occupy quieter, vegetated backwaters and tributary mouths.
Other species that complete their lifecycle in these freshwater zones include Walleye, which favor deeper, turbid waters. Common panfish like Yellow Perch and various Sunfish species are also abundant, providing a forage base for larger predatory fish.
Anadromous and Catadromous Migrants
A defining ecological feature of the Delaware River is its support for diadromous species, which migrate between freshwater and saltwater during their life cycle. These migrations are essential for reproduction and involve two distinct types of movement.
Anadromous fish spend their adult lives in the ocean before migrating upriver into freshwater to spawn. The American Shad run is a celebrated annual event, seeing these members of the herring family move far upstream in the spring to reproduce in the river’s non-tidal reaches. The closely related Hickory Shad also makes this upstream journey, typically spawning closer to the tidal-freshwater zone.
Striped Bass, another anadromous species, use the tidal freshwater section of the river for spawning, generally peaking in late May and early June. The Atlantic Sturgeon, a federally endangered species, also enters the river to reproduce, with juveniles spending their first few years in the river before moving out to sea.
Conversely, catadromous fish migrate from freshwater to the ocean to spawn, exemplified by the American Eel. These eels spend years growing in the Delaware River and its tributaries before undertaking a massive migration downriver to the Sargasso Sea in the Atlantic Ocean to reproduce. The adult eels that migrate downriver are known as “silver-phase” eels.
Estuarine and Tidal Zone Species
The lower Delaware River and Bay form a vast estuary, a dynamic zone of mixing freshwater and saltwater. Fish here must tolerate fluctuating salinity levels, providing habitat for resident and temporary marine visitors.
Many species of the drum family, known for their ability to produce a drumming sound by vibrating their swim bladder, are common in the estuary. Weakfish are a significant estuarine resident that uses the bay as a nursery ground. Atlantic Croaker are also abundant, moving into the Delaware Bay and lower river in early summer to feed on bottom-dwelling invertebrates.
Flatfish species like Summer Flounder are found in the bay’s shallow waters and tidal creeks, where they camouflage themselves on the bottom. Various Catfish species, including the native Channel Catfish and White Catfish, are highly tolerant of the estuarine environment and are commonly found in the lower river’s tidal zones. These estuarine species represent a transitional zone, bridging the gap between the river’s freshwater fish and purely marine species.
Notable Non-Native Species
The Delaware River ecosystem has seen the establishment of fish species not originally native to the watershed. These non-native species can impact the river’s ecology through competition and predation on native populations.
The Northern Snakehead, a predatory fish native to Asia, has been documented in the river system, including the Upper Delaware. This species is a predator and can survive out of water for short periods, raising concerns about its potential to disrupt the river’s food web.
Blue Catfish, introduced to the Chesapeake Bay area, have been increasingly detected in the Lower Delaware River. These large fish consume large quantities of other fish, potentially displacing native species. The Freshwater Drum is another non-native fish established in the lower, tidal portions of the river, and its feeding habits may also affect native invertebrates.