In a study designed to assess the impacts of dredged sediment disposal on lobsters at the Rockland Disposal Site (RDS) in late autumn, Dr. Richard Wahle and his research team concluded that while the disposal locally altered bottom topography and may have even attracted crabs, there was no detectable effect on the lobsters in the area.
During the fall of 2002 harvesters expressed concern over the possible impact of disposing dredged silt, clay and gravel from Penobscot Bay harbors may have on lobsters near the RDS (midway between Rockland Harbor and North Haven).
To address these concerns, the Army Corps of Engineers commissioned Wahle to employ a novel, trap-based mark-recapture methodology (WWF Dec. 02) that he had been developing with Sea Grant support in the waters surrounding Vinalhaven during the previous two summers.
The new methodology integrates three tools: acoustic mapping of the seabed to assess substrate features; a spatially referenced grid of traps spread over the area to determine the link between lobster catch and bottom type; and finally, a lobster mark-recapture protocol that permits estimates of population size and movements.
The researchers used sidescan sonar to make a digital map of the disposal site before and after the onset of disposal. The post-disposal maps revealed a new, low profile mound of soft sediment covering between 53,000 and 130,000 square yards, an area roughly equivalent to between 10 and 24 football fields.
Wahle’s team compared catch rates and population estimates of lobsters between impacted and non-impacted parts of the seafloor in relation to the onset of disposal. Over the course of the study (from Nov. 1 to Dec. 19, 2002) they estimate that the lobster population declined from several thousand to several hundred individuals over the entire 1.1 by 0.5 mile area of the trap grid; however declines occurred even in areas not impacted by disposal.
Therefore, they concluded, the change in lobster numbers could not be attributed to disposal activity. A more likely explanation is that lobsters were moving out of the bay as part of their yearly late-fall exodus to deeper water. Recapture of tagged lobsters outside the study area by harvesters corroborates this explanation.
In contrast with the declining lobster catch, the catch of rock crabs (Cancer irroratus) increased during the study. To the researchers’ surprise, crabs increased most dramatically in the immediate area of disposal activity. Apparently the crabs were attracted to the mound, which likely provided a richer source of food than the surrounding area.
The study did not address whether lobsters would be impacted by sediment disposal during the warmer months when they are at peak abundance in the Bay. The Army Corps’ decision to delay disposal activity until the colder months, when most lobsters have exited the disposal site, seems to have minimized possible negative impacts.