Lobster Trap Fouling Study
Over the course of a four month field study starting in July 2010, SSCW researchers went on a local commercial lobster boat to monitor the growth of fouling organisms on the surfaces of lobster traps. SSCW staff went out from July to October to haul 70 total trawls. These trawls, containing ten traps each, were placed in Salem Sound, and in some areas outside of the Sound. Each trap was first photographed and then manually examined to record the abundance and distribution of any fouling organisms growing on the sides, top and netting of each trap. The difference in geographic location of traps seemed to be the most important explanation for the differences in observed fouling. Bottom type and depth on their own were not as clearly related to differences in fouling.
Each month saw a steady increase in fouling, with September and October seeing a comparably heavy amount of fouling. All four months saw large amounts of incidental fouling from algae washed into many traps as well as a lot of mobile animals captured in the traps, including lobsters (Homarus americanus), rock and jonah crabs (Cancer species), hermit crabs, Buccinum whelk snails, and several species of fish, mostly skates and sea ravens. By September, all 53 trawls in Salem Sound were generally well-fouled and showed a large variety of species such as encrusting sea squirts (also called tunicates), hydroids, bryozoans, snails, flatworms, and amphipods. The 17 trawls that were outside of the Sound, on the southern side of Marblehead Neck, showed much less fouling by the end of the study; some appeared to be nearly as clear of fouling in October as they were at the start. Most fouling seemed to occur in the middle months of the study, probably due to warmer temperatures in the shallower waters of the first months of the study.
Invasive species played the key role in fouling of lobster traps. Traps that were in shallower parts of Salem Sound, particularly those from Salem Harbor, showed a great deal of fouling, particularly by the invasive solitary sea squirt Ascidiella aspersa, with all areas fished having at least some amounts of three invasive colonial sea squirt species: Botrylloides violaceus, Diplosoma listerianum, and Botryllus schlosseri. Additionally, the invasive lacy bryozoan (a type of thin encrusting colonial animal) Membranipora membranacea was commonly found on many of the traps we checked. Oddly, Only one small patch of Didemnum vexillum, a very common and problematic invasive colonial sea squirt, was on a single trap, despite our expectations to the contrary.
Many native fouling species were observed, particularly very common hydroids (simple colonial animals that are related to sea anemones), as well as small jingle shell clams (Anomia species), different snails, various seaweeds, and a few barnacles. One of the more interesting observations was an unexpected number of very small black flatworms that were sometimes found in large numbers on otherwise relatively unfouled traps.
In the future,SSCW hopes to expand this project to tie these results to normally occurring benthic communities on natural substrates such as rocks. A hint of the results to be expected comes from a couple of cement bricks pulled up from the bottom of the Sound in an area that was part of the trawling. The bricks were covered with a fouling community very similar to that which was seen on lobster traps. Though a preliminary work, this study was very successful at increasing understanding of the state of the marine realm below the surface of Salem Sound, particularly regarding invasive species. SSCW's close collaboration with local commercial fishermen proved a great success as well which we hope to continue.
Occurrences of fouling species
over the course of the study
click to see larger version of table.
Abundances of most common fouling organisms in three study areas
click to see larger version of graph.
This study was made possible by a grant awarded to Salem Sound Coastwatch by the Massachusetts Office of Coastal Zone Management in order to broaden our study of marine invasive species in Salem Sound.