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New Jersey's Priceless Resource - Studying the Delaware River

By Jennifer Pyle
Assistant Biologist
May 27, 2008
Updated June 3, 2009


An estuary is defined as a semi-enclosed body of water in which freshwater mixes with salt water. Estuaries are one of the most biologically productive ecosystems on Earth. They serve as nursery areas, spawning and feeding grounds, and migratory routes for many recreational and commercial fish. As the Delaware River flows past Trenton, freshwater begins to mix with saltwater from the Delaware Bay, creating the Delaware Estuary, New Jersey's largest estuary system.

The success of any fish or wildlife species is contingent upon the survival of its young. The Delaware estuary provides a suitable nursery environment for young fish to grow. Monitoring populations of these juvenile fish is essential for fishery managers to estimate abundance and evaluate the viability of the population. These assessments provide a means to predict population trends and future harvest potential of monitored species. New Jersey's Department of Environmental Protection's Division of Fish & Wildlife's Bureau of Marine Fisheries conducts several surveys each year which allow biologists to study the status of a given species population or water body. One of these key surveys is the Delaware River Seine Survey.

The seine survey is a required Fishery Independent Monitoring Project in accordance with the Atlantic State Marine Fisheries Commission's Interstate Fisheries Management Plan for Striped Bass. It is currently the Bureau of Marine Fisheries' longest running fishery-independent survey.

The survey began in 1980 when striped bass stocks were severely depleted, and is primarily a juvenile abundance survey for striped bass. Data collected provides an annual abundance index for this species, reported as the number of young-of-year per seine haul. Results have been corroborated by other independent surveys, such as the Delaware Division of Fish & Wildlife's striped bass spawning stock survey (pdf, 205kb).

A unique aspect of this survey is its longevity - it has been conducted for 29 consecutive years. Data from such a long period of time is highly beneficial to species population studies. Not only does this survey tell us how many fish there are from year to year, but the data also contributes to the development of fisheries management plans. Information collected helps to ensure that fish will be able to provide abundant harvests in the future.

Click on the links below for more survey information:


    The survey area is divided into three regions which have not changed since the beginning of the survey. Changes have occurred to the number of stations, station locations and dates sampled over the years.

    All regions are tidal. Region 1 is the southernmost area of the sampling area. It is a brackish region extending from the spring saltwater/freshwater interface to the Delaware Memorial Bridges. Region 2 is the central sampling area. It is a brackish-to-freshwater area extending from the Delaware Memorial Bridges to the Schuylkill River. Region 3 is the northernmost sampling area. It is a freshwater area extending from Philadelphia to the fall line at Trenton.

    Currently, there are 32 fixed stations (beaches): 8 in Region 1, 16 in Region 2 and 8 in Region 3. Each station is sampled twice a month from mid-June to mid-November, resulting in the current effort of 320 seine hauls each year.

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    Boat deploying net
    Measuring fish
    Click images to enlarge


    Survey methodology has remained fairly consistent through the years. The boat used for this survey is a 20-foot Privateer, Roamer skiff. This fiberglass boat has a side console and a 140-horsepower Suzuki 4-stroke outboard engine. The net used is a 100-foot long, 6-foot deep seine net, with -inch mesh that has a bag in the middle where the fish are collected.

    One end of the net is held close to shore by a crewmember on the beach while the rest of the net is set off the bow of the boat as it backs away from the beach. The boat sets the net with the current, before turning back towards the beach to form a "U" shape. To complete the haul, the net is pulled onto the beach from both ends and the catch is funneled into the bag in the center of the net.

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    After a seine haul is complete, and the net has been pulled onto the beach, all fish captured are sorted by species, counted and subsamples of target species are measured. In addition to striped bass, target species include: white perch, herring, American shad, bay anchovy, Atlantic croaker and weakfish.

    Since the survey's inception, sampling crews have set a beach seine 6,066 times and caught 1,200,471 fish. Biologists have averaged 197 fish per haul since 1980, with 88 different species identified. The five most abundant species caught include: blueback herring, Atlantic menhaden, bay anchovy, white perch and American shad. The primary target species (striped bass) is the eleventh most commonly captured.

    Summary Table of All Species Caught

    Click on the links below for species related data:
    Menhaden and Striped Bass (pdf, 34kb)
    Blueback Herring and Shad (pdf, 39kb)
    White Perch (pdf, 33kb)
    Bay Anchovy (pdf, 31kb)
    Sturgeon (pdf, 24kb)

    For more information about individual species (biology, range, etc), please visit one of the following websites:

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    Water quality parameters such as salinity, water temperature and dissolved oxygen are recorded at every station. A YSI brand dissolved oxygen (DO) meter is used to collect this data. This is a handheld meter that simultaneously measures several different water quality parameters. The DO meter has a probe attached to it with a long cord.

    The probe is lowered over the side of the boat into the water, just below the surface. The DO meter gives readings within a matter of seconds, providing fast, accurate measurements. Because all water quality parameters are influenced by many outside factors, it is necessary to record this data at the site of each seine haul.

    Salinity is the saltiness, or dissolved salt content, of a body of water. In the Delaware River seine survey, it is measured in parts per thousand (ppt) which is the measure of grams of salt per liter of water. In a tidal estuary, there are many factors which influence salinity. Freshwater run-off from rain and snow storms lowers salinity levels, while droughts lead to an influx of saltwater from the ocean.

    In this survey, Region 1 is the southernmost area, closest to the Delaware Bay. Therefore it makes sense that this region has the highest salinity averages, ranging from 0.901 ppt in 2004 to 6.48 ppt in 1995. Region 2 is a more brackish water area, with averages ranging from 0.0 ppt in 1996 to 1.612 ppt in 1997. While still tidal, Region 3 is almost completely freshwater. Survey averages have ranged from 0.0 ppt in 1990 to 0.102 ppt in 2000. Overall, trends show that the salinity of the Delaware River has remained fairly constant through the years.
    Salinity Graph (pdf, 13kb)

    Water temperature is also affected by many factors, including water depth, tides and weather. On average, the water temperature during this survey, measured in degrees Celsius, has increased over the years. The lowest average temperatures in all three regions occurred in the early 1990s. In Region 1 high averages were collected in 2007 measuring 23.9 degrees. In Region 2, it was 2001 at 23.8 degrees. Finally, Region 3 peaked out in 2005 at 24.0 degrees.
    Water Temperature Graph (pdf, 11kb)

    Dissolved oxygen (DO) is a measure of the amount of gaseous oxygen that is dissolved in a body of water. It is measured in milligrams per liter (mg/L). Oxygen gets into the water by diffusion from the surrounding air. This happens through a number of mechanisms, including rapid water movement (tides, run-off or boat traffic) and air movement above the water (wind or storms). Trends show a gradual decrease in the overall average of DO in the Delaware River. Region 1 peaked out at an average of 7.867 mg/L in 1996 and bottomed out in 2007 at 6.478 mg/L. Region 2, had its highest average in 1990 at 7.378 mg/L and lowest in 2008 at 6.286 mg/L. Like Region 1, Region 3 had its highest average in 1996 at 8.01 mg/L and its lowest in 2001 with a value of 5.94 mg/L.
    Dissolved Oxygen Graph (pdf, 11kb)

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    Regions 1 and 2 are historical striped bass spawning grounds. Results of this seine survey in recent years have confirmed that these regions are just as important now as they were in decades past. In fact, data from this and similar surveys in other states have reflected an increase in the striped bass population along the entire East Coast. Surveys like this are just the beginning of the stock assessment process for many species.

    Since the inception of the survey, the abundance of several species has declined including spot, American eel and channel catfish. While it is not completely certain why these species populations are decreasing, the Division of Fish & Wildlife is planning future research to determine the underlying causes.

    Seine surveys, as with all fishery surveys, are important for ecosystem management. Not only do they provide information on species abundance, but they also provide a broader source of data on interactions with other species and associations with environmental factors. Without these surveys, biologists could not identify species interactions that might predict future fishery management needs. With consistent monitoring along the entire New Jersey coast, we can alleviate any future declines in abundance before it's too late.

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    Last Updated: December 7, 2009