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Studying the Delaware Bay - 2011 Report

By Jennifer Pyle
Assistant Biologist
June 1, 2012


The Delaware Estuary is New Jersey's largest estuary system. The Delaware estuary is a semi-enclosed body of water where freshwater from the Delaware River mixes with salt water from the Delaware Bay. It serves as nursery areas, spawning and feeding grounds, and migratory routes for many recreational and commercial fish.

Bureau of Marine Fisheries biologists within the New Jersey DEP's Division of Fish & Wildlife conducts several surveys each year to study the status of species populations within the estuary. One of these surveys is the Delaware Bay Finfish Trawl Survey.

In 1991, the Division began a Delaware Bay finfish trawl survey of juvenile finfish species to develop indices for comparing the relative annual abundance of selected stocks. The survey was designed to complement a similar effort being conducted on the western side of Delaware Bay by the State of Delaware's Division of Fish and Wildlife and sampling stations were set up within the shallow, near shore waters on the New Jersey side of the bay. Data collected allows biologists to develop relative abundance estimates and length frequencies of estuarine dependent finfish necessary for predicting future fishery trends and harvest potential. (See New Jersey's Priceless Resource - Studying the Delaware River for the report on a related survey.)

Click on the links below for more Delaware Bay Finfish Trawl Survey information:


    The number of stations and their locations have varied over the course of the survey. Currently, there are eleven sampling stations located on shoals near the shoreline, extending from Villas in Cape May to the Cohansey River of Cumberland County. These near-shore stations have typically provided greater yields and more species diversity than sampling attempts in deeper waters.

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    Net being retrieved
    Trawl net diagram
    Click images to enlarge
    Sampling is performed with a 42-foot research vessel, the R/V Zephyrus. As was the case with the number of stations, the months sampled have also varied. During the survey, samples are collected at each location once a month from April to October. Single ten-minute tows are conducted against the tide at each station.

    All species collected are identified, counted and measured. If counts are high, 50 individual lengths are randomly selected and recorded. For finfish, from 1991 to 1999, total lengths (tip of nose to end of tail) were measured. Since 2000, fork lengths (tip of nose to inside fork of tail) have been recorded for all species with a forked tail. For species with no forked tail, such as Atlantic croaker, a total length is measured.

    The net used is a 16-foot otter trawl. The top of the net opening is buoyed with fish net floats. The bottom of the net's mouth is weighted with a chain so that the net can be pulled along the floor of the bay. There is a wooden "door" that acts as a spreader off each end of the net. They ensure that the net stays open while fishing. The doors are attached to the chain from the bottom of the net and nylon towline from the top of the net. The cod end is made of small mesh so that the fish will be retained in this section.

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    The Division has hauled 1,553 tows and caught 388,875 fish for an average of 250 fish per tow since the survey's inception in 1991. A total of 89 different species have been identified with the five most abundant being bay anchovy, Atlantic croaker, weakfish, blue crab and Atlantic herring.

    *NOTE: The number of stations, station locations and months sampled from 1991 to 1996 were inconsistent due to personnel constraints and weather conditions. This inconsistency will have an affect on the number of individuals collected and on the annual relative abundance indices for those years, especially for finfish that utilize the Delaware Bay on a seasonal basis.

    Summary table of all species caught (pdf, 11kb)

    Click on the links below for species related data:

    Bay Anchovy (pdf, 24kb)
    Atlantic Croaker (pdf, 32kb)
    Weakfish (pdf, 31kb)
    Blue Crab (pdf, 49kb)
    Atlantic Menhaden (pdf, 34kb)
    Striped Bass (pdf, 30kb)
    Unique Species (pdf, 97kb)

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

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    Water quality levels are recorded at each station. These parameters include salinity, water temperature and dissolved oxygen (DO). Measurements were taken at the bottom of the water column at each station. They were recorded using a YSI brand dissolved oxygen meter. The YSI meter was not functional during the 2007 survey period, therefore there is no water quality data for 2007.

    In this survey, there are 4 stations that are considered the "upper" bay (area north of Egg Island Point), and 7 stations considered "lower" bay (area south of Egg Island Point). Since the upper bay is farther from the mouth of the ocean, the salinity in the upper bay is lower than the salinity in the lower bay. When the water quality measurements taken during this survey are examined by area, there is only a slight difference in their averages. Therefore it makes sense to look at this region as a whole.

    Salinity is a measurement of the amount of salt in a body of water. In the Delaware Bay trawl survey, it is measured in parts per thousand (ppt) which is the measure of grams of salt per liter of water. There are many factors which influence salinity. Freshwater run-off from rain storms lowers levels, while droughts lead to an influx of saltwater from the ocean. Survey averages have ranged from 14.98 ppt in 2000 to 20.24 ppt in 2002. The lowest salinity recorded was at station 12 in 1994, and had a reading of 1.0 ppt. This station is located farthest from the ocean's saltwater influx. The highest salinity recorded was at station 60 in 2006, and had a reading of 30.7 ppt. Opposite of station 12, station 60 is located closest to the ocean. Overall, trends show that the salinity of the Delaware Bay has remained fairly constant through the years.
    Salinity Graph (pdf, 12kb)

    Water temperature is also affected by many factors, including water depth, tides, and the weather. In this survey, temperatures are measured as degrees Celsius (C). On average, the water temperature during this survey, has remained consistent over the years. Average temperatures range from 19.6 degrees C in 1997, to 23.32 degrees C in 2005. The lowest recorded water temperature was 8.2 degrees C at station 30 in 2009, and the highest water temperature was 31 degrees C at stations 39, 54 and 60 in 1993.
    Water Temperature Graph (pdf, 13kb)

    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 from the surrounding air. This can occur by various ways, including: tides, run-off, boat traffic, wind or storms. Trends show hardly any change in the overall average of DO in the Delaware Bay. The survey averages ranged from 5.45 mg/L in 1999 to 8.19 mg/L in 2000. The lowest recorded DO was 2.2 mg/L at station 47.1 in 2004. The highest recording was 12.8 mg/L at station 47.1 in 1994..
    Dissolved Oxygen Graph (pdf, 13kb)

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    Surveys like this are just the beginning of the stock assessment process for many species. For example, data from this and similar surveys in other states have reflected an increase in the striped bass population along the entire East Coast. They have also shown a decrease within the bay anchovy population.

    As with any ecosystem, data collected from the Delaware Estuary surveys show fluctuations among all species. There are constant changes in the size of fish populations due to many environmental factors. While it is not always completely certain why any species population decreases or increases, the Division is planning future research to examine these variations.

    Fishery surveys, such as the Delaware Bay Finfish Trawl, are important for ecosystem management. They provide biologists with information relative to the annual abundance and population trends of many species. Without these surveys, biologists would not be aware of the population increases or decreases that predict future fishery management needs. Being conscious of these fluctuations is important when creating recreational and commercial fishing regulations.

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    Information for this article was provided by Principal Biologists Jeffrey Normant, lead investigator and Jason Hearon, co-investigator of the program. Funding for this survey is supported by the Federal Aid to the Sport Fish Restoration Program administered by the U.S. Fish & Wildlife Service.
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    Last Updated: June 1, 2012