Resource: Warmer Oceans Affect Food Web

Print Background Essay

Before March 1989, harbor seals were a far more common sight in Prince William Sound than they are today. Like other parts of the local ecosystem, harbor seals have yet to recover to their pre–Exxon Valdez oil spill population. Since the spill, many other maritime animals have also experienced significant declines, including sea lions and seabirds. What might come as a surprise for many to learn is that the oil spill alone—albeit still a significant disturbance—might not entirely explain the ecosystem's condition in the following years. It turns out that each of these species was already in decline before the spill occurred.

The causes of population declines in southern Alaskan waters are under investigation. One hypothesis concerns changes in sea surface temperature that took place beginning in the late 1970s. Warmer sea surface temperatures in the North Pacific Ocean and the Bering Sea are thought to have prompted a dramatic change in the Prince William Sound ecosystem—the effects of which continued at least through to the 1989 spill. Chief among the affected species were populations of sand lance and other small schooling fish often referred to as forage fish, or "food fish." These food fish swim in huge schools and provide marine mammals and seabirds with a good source of fatty food.

All marine animals have a temperature range within which they grow most efficiently, reproduce, and live. Food fish that prefer cold water migrate to deeper, cooler waters when surface waters warm. This makes them less available to surface-level feeders. Because the sand lance and other types of food fish appear to be sensitive to temperature changes, and because many other species rely on them for food, their own behavior and population changes might explain changes exhibited throughout the ecosystem.

According to this hypothesis, then, climate change may be an influential factor in the health of Alaska's maritime ecosystem. Because population studies of sand lance were not taken immediately before the 1989 oil spill, it is impossible to know for sure the extent of the impact they were having on the ecosystem when the Exxon Valdez ran aground in Prince William Sound. However, because of its possible effects on this ecosystem, climate change might be considered a threat to the cultural fabric of Alaska Native peoples, many of whom adhere to a subsistence hunting and fishing lifestyle.

To learn more about the long-lasting impact of oil around Prince William Sound, check out Oil Contaminants Hidden from View.

To learn more about the scope and effects of the Exxon Valdez tanker spill, check out Oil Spill: Exxon Valdez, 1989.

To learn more about strategies designed to protect the maritime ecosystem, check out Safeguarding Alaska's Waters.

Print Questions for Discussion

• What has happened since the 1989 oil spill to the seabird and harbor seal populations in and around Prince William Sound, and what are some of the possible reasons for what has happened?
• Describe how kittiwake populations can be viewed as "indicators" of predator/prey availability.
• What are forage fish, and what role do they play in the food chain?
• What environmental factor(s) might explain the decrease of the sand lance in this ecosystem?
• When fish, seabirds, and marine mammal populations are stressed to the point that they are declining, what do you think should be done about human activities that might add to the stress, and why?