Epibenthic Fish and Crabs

One of the primary interests of the Estuarine Ecology Lab are long-term, annual and seasonal changes in community structure, response of the population and individual species to environmental variables (such as temperature or salinity), and migration patterns and habitat utilization of select species. In order to collect the necessary data on these topics, the Estuarine Ecology Lab initiated a sampling method called "Trawl Surveying" on the Rhode River in 1981.


Trawl samples are collected three times a month, from March through December, from one station in the Chesapeake Bay, two stations at the mouth of the Rhode River and one station at the head of the river. Our Trawl survey uses a 16 ft. semi-balloon otter trawl square-mOtter Trawl netesh net, commonly referred to as an "Otter Trawl", which is much like the shrimp nets used by commercial fishermen in the South Atlantic and Gulf of Mexico (see illustration right). At the head of the net are the two trawl doors that serve to weigh the net down, keeping it on the bottom, and to hold the net open, so it "fishes" properly. A tickle chain is next in line, and is attached to the trawl doors, stretching between them. The tickle chain serves to force animals lying on the bottom or buried just beneath the surface up off of the bottom just a little bit so the net will catch them as it is pulled by. The net, which is also attached to the trawl doors, stretches 16 ft. from one side of its mouth to the other. The bottom half of the mouth is lined with lead weights that force the net to skim along the surface of the river sediment, while the top half is lined with floats, so the net mouth remains open during trawling. As animals are captured by the net they are funneled down to the "Cod End", which is square-mesh netting of a smaller diameter than the rest of the net and is securely tied-off at the end.

       pulling in trawl net and sorting catch




The otter trawl is deployed for ten-minute intervals, and pulled for a distance of 900 meters, usually behind the R/V Saxatilis. Upon completion, the net is pulled into the boat, the cod end is untied, and the catch is poured into waiting bins of water. From this point the catch is sorted into fish and crabs. The first twenty fish of each species is measured, the data is recorded, and then the fish is released. Once twenty fish within a species have been measured the remainder of that species is counted, data recorded, and released. Data is collected for all captured blue crabs, including: size (distance from one carapace point to the other), sex (either male, mature female, pre-pubescent female, or immature female), autonomy (presence or absence of appendages), and molt stage. Once this data has been collected the blue crabs are released.

The accumulation of twenty plus years of otter trawl data has resulted in the collection of sixty-one different species of fish and crabs. In particular, bay anchovy, spot, white perch, croaker, hogchocker, and blue crabs account for nearly 87% of the catch (see below). The other fifty-five species are encountered on a less frequent basis, and most of these remaining animals are rarely caught.

trawl data pie chart

These data are providing a unique overview of the population dynamics and community structure of the Rhode River subestuary. Trawl data can be used to examine long-term as well as annual and seasonal variation in community structure of the Rhode River. In addition, trawl data, in conjunction with short-term experimental analysis, provide insight regarding predator/prey interactions within the system, reproductive strategies and molting behavior of blue crabs, and habitat partitioning of fish and crabs.