The delicate web of the Arctic biosphere is complex and one small change can have big affect.
One of the main effects of the warmer sea surface temperatures was a suppression of phytoplankton productivity, which led to reductions in shrimp, crab, and populations of smelt fish, such as capelin.
Other fish populations, particularly cod and pollock, increased at the same time.) The reduced numbers of smelt may have led to declines in the numbers of Steller's sea lions and harbor seals, which were one of the main food items for killer whales.
The declining numbers of sea lions and seals apparently led to increased predation on sea otters by killer whales, resulting in a drastic reduction in the population of Alaskan sea otters of more than 70%. These low sea otter numbers allowed sea urchins (a major part of the sea otter diet) to proliferate.
Sea urchins feed on kelp, so the kelp forests in the Gulf of Alaska and the Bering Sea were overgrazed, which affected all of the organisms associated with the kelp, including seabirds like puffins and kittiwakes.
Note: The Gulf of Alaska and Bering Sea fisheries are the largest in the United States, and some researchers also blame overfishing for the decline in marine mammal populations. Because of the climatic regime shift, it is difficult to conclusively determine the primary cause.
One small change can have effects to the largest of creatures and those again can have affect on the smallest of creatures. This causes failure in harvesting of one species, but can have positive affect to another. Valuable commercial fish like Cod and Pollock lose their juvenile habitats due to increase in Sea urchins, but again, harvesting of Sea urchin roe might become a substitute economical activity.