Each year these attract a run of up to half a million chum salmon. As the salmon run up the river, bald eagles arrive in their thousands to feast at the spawning grounds. This results in some of the world's largest congregations of bald eagles. The number of participating eagles is directly correlated with the number of spawning salmon. Residual nutrients from salmon can also accumulate downstream in estuaries. A 2010 study showed the density and diversity of many estuarine breeding birds in the summer "were strongly predicted by salmon biomass in the autumn." Anadromous salmon provide nutrients to these "diverse assemblages ... ecologically comparable to the migrating herds of wildebeest in the Serengeti". Prospects The future of salmon runs worldwide depends on many factors, most of which are driven by human actions. Among the key driving factors are (1) harvest of salmon by commercial, recreational, and subsistence fishing, (2) alterations in stream and river channels, including construction of dikes and other riparian corridor modifications, (3) electricity generation, flood control, and irrigation supplied by dams, (4) alteration by humans of freshwater, estuarine, and marine environments used by salmon, coupled with aquatic changes due to climate and ocean circulatory regimes, (5) water withdrawals from rivers and reservoirs for agricultural, municipal, or commercial purposes, (6) changes in climate caused at least in part by human activities, (7) competition from non-native fishes, (8) salmon predation by marine mammals, birds, and other fish species, (9) diseases and parasites, including those from outside the native region, and (10) reduced nutrient replenishment from decomposing salmon. In 2009, NOAA advised that continued runoff into North American rivers of three widely used pesticides containing neurotoxins will "jeopardize the continued existence" of endangered and threatened Pacific salmon. Global warming could see the end of some salmon runs by the end of the century, such as the Californian runs of Chinook salmon. A 2010 United Nations report says increases in acidification of oceans means shellfish such as pteropods, an important component of the ocean salmon diet, are finding it difficult to build their aragonite shells. There are concerns that this too may endanger future salmon runs. In popular culture In a 1982 video game called Salmon Run, the player takes the role of Sam the Salmon, swimming upriver to mate. Along the way he encounters waterfalls, a bear, fishermen, and seagulls. In the Disney animated feature film, Brother Bear, Kenai