Gulf Of Maine Data Suggest Trawling Impacts

In their paper, Cynthia H. Pilskaln and Lawrence M. Mayer, both of the UMaine School of Marine Sciences, and James Churchill of WHOI, used information from sediment traps deployed well above the sea floor for nine months in 1995 to suggest that further research should be done to pin down the sedimentological and chemical effects of trawling.

The authors discuss the scale of trawling in the Gulf and use their data to discuss possible impacts to the Gulf’s chemical and biological cycles and the food chain. Those possibilities include changes to nitrogen and silica cycles and the types of plankton which are the foundation of the marine ecosystem. The amount of primary production, a rough measure of plankton growth, could also be affected.

The sediment traps were located 80-100 feet off the seafloor in two Gulf basins. They collected sinking particulate material which scientists retrieved at two-week intervals. The samples were analyzed at UMaine.

In an area of the western Gulf known as Wilkinson Basin, the trap samples contained large benthic worms that are normally found only at the surface of or within the bottom sediments. The spring peak in benthic worm collection in the 1995 trap samples coincided with seasonal period of peak trawling activity in Wilkinson Basin according to bottom trawling records obtained from the National Marine Fisheries Service for 1990-1993.

In contrast, trap samples collected in Jordan basin, which is located in the eastern region of the Gulf of Maine, contained very few benthic worms. Trawling activity documented by NMFS records is substantially less frequent in this region of as compared with the Wilkinson Basin area.

According to Pilskaln, the paper examines the likely effects that bottom trawling has on sediment resuspension and sedimentary nutrient fluxes in the Gulf. It discusses potential restructuring of benthic communities by high levels of trawling activity which in turn may affect sediment geochemistry.

“Sediments are important in nutrient cycling and if they are being stirred up this process could have an appreciable impact on Gulf-wide nutrient budget,” says Pilskaln.

“At the moment, we have only preliminary calculations based on previously published nutrient data sets and estimates of sediment resuspension via bottom trawling. These calculations suggest that we need to do some more detailed work to quantify exactly how much sediment and pore water is resuspended per year in the Gulf as a whole, and specifically within heavily trawled regions.”

“The worms provide an indirect demonstration of the resuspension and excavation strength of bottom trawling. We can’t account for those worms in the traps in any other way. We know that trawling affects the community of organisms that live on the sea floor by decreasing overall benthic community diversity, but no one has really looked at the chemical aspects of bottom trawling.

“Trawling may very well augment or help maintain the permanent particle resuspension layer in the Gulf of Maine, a natural phenomena in which sediment particles are kept in suspension by strong tidal mixing within a layer just above the bottom. Particle resuspension is not necessarily a negative as there are organisms that have adapted to feed in particle resuspension layers found throughout many of the world’s oceans. The creation of a thicker or more dense resuspension layer may increase the abundance of such organisms, but no one has studied that,” says Pilskaln.

“Ideally, the next step that we should take would be to conduct a laboratory experiment within a large flume tank in which we simulate excavation by trawling and examine the sediment particle dynamics and the changes in sedimentary pore water chemistry. The problem with working in the field is that you obviously cannot control all the interconnected environmental parameters. This is something that we can largely overcome in a controlled laboratory-based experiment,” she adds.

The majority of work presented in the Journal of Conservation Biology paper was financially supported by the NOAA Regional Marine Research Program which was initiated through the “Mitchell Bill”, sponsored by former Maine Senator George Mitchell.