Chesapeake Bay Multispecies
Monitoring and Assessment Program
ChesMMAP

ChesMMAP Trawl Survey

Data and Technology

Note: For several of the types of data that we collect, described here, early examples of actual data are presented on the 'Data Summaries' page (click 'Data Summaries' to the left or 'Next' below).

Abundance Data: At a minimum surveys such as this can produce reasonable estimates of relative abundance for a variety of species.  Since this survey is aimed at capturing adult fish, this can provide verification of juvenile abundance estimates from the VIMS Juvenile Fish and Blue Crab Trawl Survey and other surveys.

Distribution Data: The survey covers the entire stretch of the mainstem Chesapeake Bay and samples every other month from March through November.  This will allow us to document the geographic and temporal patterns of distribution for a number of important species.  

Age Data: As this survey targets adult fish we will be able to develop good age distribution structures over the range of sizes/ages selected for by our gear.  Otoliths or other ageing structures are taken from a subsample of specimens.

Diet Data: From a sizable subsample of specimens we preserve stomach contents for later analysis.  As above, the broad geographic and temporal scale of the survey presents a unique opportunity to document and quantify diet habits for a broad variety of species at several trophic levels.  The same specimens are sampled for diet data and age data so we can quantify age-specific diets.

Other Basic Biological Data: Weight, girth, sex (when determinable by simple dissection), and gonad stage are taken from the same subsample of fish used for ageing and diet.

Extended Data:
Models being developed in the FEMAP program require absolute estimates of abundance or biomass, as opposed to the relative indices of abundance that are typically produced from this type of survey.  The formula to calculate abundance (or biomass if you factor in weight) from a trawl survey is: 

where: N = abundance
           A = total survey area
            e = net efficiency
            c = catch per tow
           a = area swept each tow       

In simple terms this calculation estimates mean density (number caught divided by area swept) within your towed areas and expands to the total survey area.  If the gear captured every individual in the tow swath then the efficiency term becomes 1 and drops out of the equation.

Total survey area (A) and catch-per-tow (c) are easily calculated.  Area swept on each tow (a) can be roughly estimated from known (GPS) beginning and ending coordinates and by assuming a standard net opening.  However to increase the reliability of our estimates are are fishing net mensuration gear on each tow.  This gear provides moment-by-moment measurements of net opening and net height.  A schematic of this equipment is shown here (only the circled sensors are used).  A sample of the computer output from the net sensors is shown here.

Efficiency is harder to quantify.  The approach we are taking is to run a simultaneous hydro-acoustic survey with our trawl gear.  On each tow we deploy a 200KHz 12-degree split-beam transducer in front of the net.  Once we establish species-specific acoustic signatures the 'fish finder' should be able to tell us how many fish were available to be captured, and the ratio of number captured to number available is our estimate of efficiency.  A schematic showing deployment of the sonar and tow body is here.  A sample annotated display of the hydro-acoustic screen is shown here.

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Fisheries Science / VIMS
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