Survey Methods

Area Sampled

In the spring of 1955, the survey sampled a series of stations in the mid-river channel at approximately 5-mile intervals from the mouth of the York River to West Point, Virginia. Stations were sampled irregularly for the rest of that year. Since April 1956, these stations have been sampled almost continuously (April through November). A few stations in lower Chesapeake Bay and in the Pamunkey River were regularly sampled for several years.

Historical photos of VIMS trawling.

In 1962, sampling on the Rappahannock River commenced; stations on the James River were added in 1964. Occasional samples were taken from the Potomac River, Mobjack Bay, and several smaller tributaries. Semi-annual sampling of Chesapeake Bay occurred sporadically until 1988 when regular sampling of the entire Virginia portion of the Chesapeake Bay began. Mobjack Bay was sampled again from March 2010-March 2012.

Currently, 22 stations in the James, York, and Rappahannock River are sampled each month, year-round. Sampling in the Bay varies between 39 and 45 stations per month (depending on season) and does not occur in January or March. 

Maps of trawl stations in a typical month of sampling with the current sampling scheme: Chesapeake Bay, York River, Rappahannock River, James River.


The trawl gear has been modified several times, affecting the kind and size of fish captured. Extensive sampling using these various gear configurations was completed in order to standardize the catch rate associated with each gear combination.

  • A tickler chain, which stirs up the bottom and increases the catch of bottom-dwelling species such as Blue Crabs and flatfish, was added in 1973.
  • A small mesh liner was added in 1979. This enabled the net to retain smaller species such as Bay Anchovy, as well as smaller individuals of other species.
  • The doors, which provide horizontal spread of the net, were changed in 1991. This change did not significantly alter the catch.
The most recent gear change occurred in 2015 when the net was changed from a 9.1-m head line, 4-seam, semi-balloon otter trawl with 38.1 mm stretch-mesh body and a 6.4-mm mesh cod liner to a 5.8-m head line with 40 mm stretch-mesh body and a 6.4-mm liner.

R/V Fish HawkVessel R/V Tidewater

There have been a number of vessel changes throughout the history of the survey. Recently, the R/V Fish Hawk (left) was retired. The R/V Fish Hawk was used from 1990-2015 and sampled 28,569 stations during that time. The R/V Tidewater (right) is now being used. 

Historical vessel and gear changes (1955-2009)        


View a slide show of the trawling process

After each 5-minute tow, the catch is sorted and fishes are identified and measured, then quickly returned to the water. (Subsets of large catches are measured and the rest are counted.) Invertebrates are identified and some are measured*. Habitat type is also noted*. Approximately 70 species are commonly caught, although more than 200 species have been identified during the last 50 years.

Complete list of fish species caught through 2017 (pdf)

*As of March 2012, Horseshoe Crabs, Blue Crabs, and penaeid shrimp are the only invertebrates identified and measured by the VIMS Juvenile Fish and Blue Crab Trawl Survey.  Habitat data are no longer recorded.

From Field to Database

Measuring a Blue Crab on an electronic measuring board.

Until 1987, most fish were preserved in the field and brought back to the lab to be measured and recorded. In 1988, the survey began using electronic measuring boards connected to a computer running a database program; this allowed the survey to work more efficiently. The operator places a fish or crab on the board and touches a magnetic wand to the board at the end of the fish's tail/edge of shell; the length is electronically recorded. This process speeds measuring and eliminates the need for manual recording and entry of fish/crab lengths. Animals are returned to the water quickly and data are available for analysis within a couple of days of the field work.

Deploying YSI to collect hydrographic data.

In addition to animal lengths, hydrographic and station data are also collected. These data include latitude and longitude, depth, tidal current stage, secchi depth, air temperature, wind direction*, wind speed*, weather conditions, sea state, water temperature, salinity and dissolved oxygen.

*As of March 2012, wind data are no longer collected.