Spatially referenced trap arrays detect sediment disposal impacts on lobsters and crabs in a New England estuary

Abstract

We employed spatially referenced trap arrays and side-scan sonar mapping of the sea floor to assess the impact of late autumn sediment disposal on local American lobster Homarus americanus and rock crab Cancer irroratus abundance in the neighborhood of a disposal site for harbor dredge spoils in Penobscot Bay, Maine, USA. The assumption that impacts on lobsters would be minimized if disposal were conducted in late autumn, when the commercial lobster fishery is at a seasonal low and lobsters are believed to be moving to deeper water, was tested. Previous mark-recapture experiments conducted nearby established that trap catch rates strongly track trends in abundance. To our knowledge this is the first study in which geo-referenced trap-based sampling and seabed mapping have been used to assess the impact of an environmental perturbation on mobile benthic megafauna. We set 3 arrays of 24 traps: 1 array over the disposal area and 2 on adjacent unimpacted areas, all about 0.6 km2 in size. Two weeks prior to the onset of disposal and for 4 wk thereafter, we counted lobsters and rock crabs caught in the traps weekly and released them at the same trap location. Post-disposal side-scan sonar surveys revealed a mound of soft sediment not evident in pre-disposal surveys covering an estimated 44 170 to 108 881 m2 of seabed. No statistically significant impact of the sediment disposal was detected on lobster abundance. In contrast, within a few weeks of the onset of disposal, the overall catch of rock crabs increased dramatically in the disposal area and specifically on the patch of newly deposited sediment. These results indicate that spatially referenced trap arrays used in conjunction with seabed mapping provide a useful tool for evaluating change in the distribution and abundance of benthic megafauna at spatial scales as small as 10s of meters. The findings also indicate that even for ecologically similar taxa, e.g. lobsters and crabs, the impact of such disposal events can be strongly species specific. While direct impacts to lobsters may be minimized after the autumn offshore migration, crabs, still abundant in the area, may be attracted to the food-rich sediments at the disposal site. © Inter-Research 2007.