Marine birds and mammals of the Pacific Subarctic Gyres

The importance of the subarctic gyres of the North Pacific Ocean to marine birds and mammals is poorly known because of a paucity of data spanning app ropriate scales of time and space. The little information that is available indicates the western subarctic gyre (WSAG) is more productive than the ea stern subarctic gyre (ESAG), In summer the WSAG supports a greater density and higher biomass of seabirds than the ESAG, including at least two specie s that are more abundant at nesting colonies in the eastern subarctic. Perh aps most revealing of the seabird distributions in this regard is that of s outhern hemisphere shearwaters (Puffinus spp.) that overwinter in the North Pacific. Their biomass is an order of magnitude greater than that of any n orthern hemisphere species and is three-fold greater in the WSAG than in th e ESAG, Several species of cetaceans also appear to be, or to have been pri or to commercial depletions, more abundant in the WSA, Among the many prey species consumed by marine birds and mammals, squids and fishes in the fami ly Myctophidae predominate overall. Other forage species, notably euphausii ds, Pacific saury (Cololabis saira) and Atka mackerel (Pleurogmmnus monopte rygius) are important at times to certain species. The principal exceptions to this generalization are baleen whales and small seabirds that consume z ooplankton. Interannual and decadal-scale variability in the physical envir onment and food web production affect seabirds and marine mammals at sea an d at coastal breeding locations around the margins of the gyres. (C) 1999 E lsevier Science Ltd. All rights reserved.