Marine Distribution and Behavior of Juvenile and Adult Marbled Murrelets off Southwest Vancouver Island, British Columbia: Applications for Monitoring

We examined the marine distribution and behavior of newly fledged juvenile (hatching year; HY) and adult (after hatching year; AHY) Marbled Murrelets (Brachyramphus marmoratus) off southwest Vancouver Island, British Columbia, to test whether assumptions associated with estimating productivity are met in this area. Productivity estimates for murrelets use HY:AHY ratios from marine surveys, which assume limited emigration and that juvenile and adult birds are similarly distributed. We examined observations from June–August, 1994–2005. Behavioral data were collected from land-based surveys via instantaneous scan sampling. Locations of murrelets at sea were mapped from cliff-top vantage points using a theodolite; Geographic Information Systems (GIS) was then used to compare the distances from shore of juveniles and adults. Data from boat surveys were analyzed using GIS kernel density analysis to compare adult and juvenile murrelet distributions at sea. At fine scales (1–100 m), juveniles were associated with adults; however, they were found significantly closer to shore than adults. At coarse scales (1–10 km), juvenile and adult distribution overlapped on a daily basis but showed less overlap with annually averaged distributions. Juveniles typically were solitary foragers, whereas adults often foraged in pairs or larger groups. Our results indicate that monitoring HY:AHY ratios using boat transects off southwest Vancouver Island as an indicator of breeding success need not take into account possible "nursery areas," although the proximity of juveniles to the shoreline means that monitoring must consistently include waters closest to the shore. Sequential ratios should be used to account for emigration of adults due to postbreeding dispersal.