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The latest in the series of Studying the Delaware Bay fisheries surveys articles is available on the NJDEP Division of Fish and Wildlife website.
By Brian Neilan, NJ Bureau of Marine Fisheries
Tags: inshore

(The following piece by NJDEP Bureau of Marine Fisheries’ Assistant Biologist Brian Neilan was released on February 25, 2016. The complete report can be found here).

The Delaware Bay is New Jersey's largest estuary system. It is a semi-enclosed body of water on the south western coast of New Jersey where freshwater from the Delaware River mixes with saltwater from the Atlantic Ocean. It serves as spawning and feeding grounds, nursery areas, and migratory routes for many important recreational and commercial fish and invertebrates such as striped bass, weakfish, American shad, and blue crabs.

Bureau of Marine Fisheries biologists within the New Jersey Department of Environmental Protection’s (NJDEP) Division of Fish & Wildlife conduct several yearly surveys to study and assess the status of important game and forage fish stocks within the estuary. One of these surveys is the Delaware Bay Juvenile Finfish Trawl Survey.

The Delaware Bay Juvenile Finfish Trawl Survey began in 1991 in order to develop annual abundance indices for important recreational and commercial fish species. This survey was designed to mirror a similar study being conducted by the State of Delaware's Division of Fish and Wildlife along the western shore of Delaware Bay. Sampling stations were selected along the New Jersey coast of the bay in the shallow near shore waters preferred by many developing larval and juvenile fishes.

The data collected from this survey is used by biologists to develop relative abundance indices and length frequency estimates of estuarine dependent finfish. These are important tools used by fisheries biologists to predict future stock trends and harvest potential for a variety of species.

The number of stations and their locations varied during the early years of the survey due to weather conditions and personnel constraints. Since 1997, 11 near-shore stations have been sampled once a month from the Cohansey River in the upper bay extending down to Villas in Cape May in the lower bay.

Sampling is performed over shallow shoals (depths generally ranging from 2-8 ft.) as this typically yields larger catches and greater species diversity than trawl tows in deeper water.

Sampling is performed once a month from April through October at each of the 11 stations within the bay for a yearly total of 77 trawl tows. A 16-foot otter trawl is towed for 10 minutes at each station behind the R/V James W. Joseph, a 46-foot research vessel operated by Division staff.

When underway, the otter trawl's "lead line" remains in contact with the bay-floor while the "float line" buoys the top of the net to keep it open vertically. Two wooden "doors" are attached to either side of the net's opening which act as spreaders to keep the net open horizontally while it is being towed. The rear-most section of the net called the "cod-end" is made of a small mesh where the fish are eventually collected and hauled on deck.

At the end of each 10-minute tow the net is retrieved by hand and the cod-end is emptied onto a sorting table. All of the fish and invertebrates are identified down to the species level, sorted, counted, and measured.

If a large number of any particular species is caught, a randomly selected subsample of 50 individuals is measured while the rest of the catch for that species is only counted. Lengths are measured in millimeters using fork lengths (tip of the nose to the inside fork of the tail) for species with forked tails (e.g. bay anchovy, bluefish) and total lengths for species without forked tails (e.g. summer flounder, Atlantic Croaker).

Surveys like the Delaware Bay Juvenile Finfish Trawl are the groundwork for the overall stock assessment process. The data gained from such fisheries surveys can be used at a state level to assess local fish stocks or combined with data from other states to in an effort to better understand coastwide population trends and harvest potential.

Data collected from this trawl survey as well as similar surveys from other states have reflected an increase in striped bass numbers along the entire east coast. Conversely, a decrease in bay anchovy numbers, an important forage fish for predators, has been demonstrated over the course of the survey.

As with any ecosystem, data collected from the Delaware Bay estuary have shown fluctuations among most of the species that have been caught. Large estuaries such as the Delaware Bay are dynamic systems in which a myriad of physical, chemical, and biological factors can influence fish populations. These natural factors along with recreational and commercial fishing and harvesting influence these fluctuations. It is the job of the fisheries biologist to interpret these trends and fluctuations in order to manage fisheries to prevent overharvesting. This is one way to ensure that subsequent years experience sustainable fisheries, healthy ecosystems, and robust natural populations.

Click on the links below for species related data:
Atlantic Croaker (pdf, 260kb)
Atlantic Menhaden (pdf, 245kb)
Bay Anchovy (pdf, 225kb)
Blue Crab (pdf, 240kb)
Striped Bass (pdf, 400kb)
Weakfish (pdf, 315kb)
Unique Species (pdf, 97kb)
Summary table of all species caught, 1991-2015

Information for this article was provided by Bureau of Marine Fisheries Biologist Brian Neilan, lead investigator, and Bureau of Shellfisheries Biologist Craig Tomlin and Technician Andrew Hassal, co-investigators. Funding for this survey is supported by the Sport Fish Restoration Program administered by the U.S. Fish & Wildlife Service. Click here to read the full report which also includes water quality information and links to other information care of the NJDEP.