Distribution of murres (Uria spp.) and their prey south of St. George Island in the southeastern Bering Sea during the summers of 2003–2005

The distributions of murres (Uria spp.) and their prey were examined on board T/S Oshoro maru in the southern part of St. George Island, southeastern Bering Sea during the summers of 2003–2005. Murre distribution was determined by sight-counting survey, and the biomass of fishes and plankton was estimated simultaneously using acoustic measuring. Fish were abundant in intermediate (30–90 m) layers around the inner front or the northern edge of Pribilof Canyon, while plankton were abundant in deep (>90 m) layers around the northern edge of Pribilof Canyon or the shelf-break front. In each year, murres foraged around the inner front or the northern edge of Pribilof Canyon, which had high productivity and abundant fish, as has been revealed previously. Non-linear regression analysis on the vertical distribution of prey and distribution of murres suggested that murres especially targeted on fishes concentrated in intermediate layers below the thermocline. Furthermore, comparison of abundance and distribution of prey revealed that fish abundance was lower and distributed in deeper layers in 2005 than in 2003 and 2004. Low abundance and deep distribution of fish prey may be related to poor breeding success of murres on St. George Island in 2005.