Food webs can be tricky things. If a top-level predator is suddenly killed off, then this could cause an unhealthy change in other trophic levels. For example, if a fatal disease spread through a sea urchin-eating starfish population, then this could lead to a sudden spike in sea urchin abundance, who could then in turn could wreak havoc on their source of energy - kelp beds. On the other hand, what would happen to a food web if a top-level predator's numbers steadily increased?
A recent study looked at the consumption of Chinook salmon in the past 40 years by killer whales, harbor seals, California sea lions, and Stellar sea lions, and found that it has dramatically increased! Previously, the question had been asked: why had Chinook populations been decreasing in the Salish Sea despite decreased numbers being caught and ongoing salmon recovery efforts? This paper gave us a possible unexpected answer: the increase in Chinook salmon consumption by marine mammals had offset salmon recovery efforts.
Increased consumption of Chinook may have dangerous consequences for our Southern Resident killer whale populations since 80% of their diet is Chinook, and because they have a much smaller migration range than Alaskan and Canadian killer whale populations. Obviously, there is no simple solution to the intertwined and sometimes conflicting marine mammal and salmon recovery efforts. Further research on how salmon hatcheries affect consumption rates, or if killer whales have preferences for wild or hatchery salmon may shed more light on this conundrum.