Experimental support for a new drift block design to assess seabird mortality from oil pollution

Seabird mortality from large oil spills and chronic oil pollution is often significant. Total mortality estimates are derived from counts of dead bird s that wash ashore and are corrected for numbers lost at sea. Past attempts to estimate proportion of birds that die at sea and wash ashore have inclu ded several experiments using carcasses and different types of wooden drift blocks. Results varied greatly depending on environmental conditions and d istance from shore where blocks or carcasses were released. Wind seemed to be the predominant factor determining movement over large distances, wherea s tidal currents influenced deposition on specific beaches. Determining tim ing and location of arrival of dead birds on beaches are crucial for accura te mortality estimates. Drift experiments using beached birds that have alr eady drifted at sea for an undetermined length of time are inaccurate due t o natural buoyancy loss and decomposition. To determine accuracy of drift b lock designs used in the past, we compared drift characteristics and patter ns between four drift block designs and fresh murre (Uria spp.) carcasses. Our experiments showed that drift blocks used in the pact have none of the drift characteristics of dead seabirds, because they have much larger areas exposed to wind and hence drift much faster and farther than murre carcass es. Past mortality estimates using those blocks are therefore doubtful. The drift block design that most accurately mimicked murre carcass drift durin g our experiments was a 9 X 9 X 14.5 cm wooden block with a 450 gram steel weight that adjusts buoyancy and area exposed to the wind. We propose that in areas where murres are predominant victims of oil spills, that block des ign be used for all future estimates of oiled seabird mortality.