Project Goal:

The goal of this project is to understand how fish and crustaceans use the deeper waters of the Rhode River and adjacent upper Chesapeake Bay.  

Epibenthic Fish and Crustaceans

One of the primary interests of the Fish and Invertebrate Ecology Lab are long-term, annual, and seasonal changes in population and community dynamics of epibenthic fish and crustaceans. "Epibenthic" refers to organisms that live on or just above the bottom sediments in a body of water. These organisms, many of which support commercial and recreational fisheries, tend to forage on the creatures that live in or on the sediments.  We sample epibenthic species using a trawl survey that began in 1981.

The Rhode River

The Rhode River is an embayment of the Chesapeake Bay located on the western shore of Maryland. The primary source of freshwater is Muddy Creek (headwaters of the Rhode River) ; the Rhode shares a common mouth with the West River where both connect to the mainstem of Chesapeake Bay. The Rhode River has an area of 550 hectares (1 hectare=2.5 acres) and an average depth of 2 meters, with salinity ranging from nearly freshwater (0-1) in Muddy Creek during the spring to more than 20 (half that of full strength seawater) at the mouth of the Rhode and West rivers during the fall.  (Gallegos, CL, TE Jordan, and DL correll.   1992. Limnology and Oceanography, Vol. 37:813-828).

Trawl Surveys

Our trawl surveys use a 16 ft. semi-balloon otter trawl square-mesh 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. At the head of the net are 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. 

 The otter trawl is deployed for ten-minute intervals, and pulled for a distance of 900 meters, usually behind the R/V SERC 41. When the net is pulled into the boat, the cod end is untied, and the catch is poured into waiting bins of water.  The catch is then separated into fish and crabs.  The first twenty fish of each species is measured.  After being measured, each fish is released alive back into the river.  Once twenty fish within a species have been measured the remainder of that species is counted and released. Data are collected for all captured blue crabs, including: carapace width (distance from one carapace point to the other), sex (either male, mature female, pre-pubescent female, or immature female), autonomy (leg loss), and molt stage. All blue crabs are released once these data have been collected.  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.

What have we found?

The accumulation of thirty 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, Atlantic Croaker, Hogchoker, and blue crabs account for nearly 87% of the catch. The other fifty-five species are encountered on a less frequent basis.

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 and short-term experiments provide insight regarding predator/prey interactions within the system, reproductive strategies and molting behavior of blue crabs, and habitat partitioning of fish and crabs.