Frank co-authored a paper on the discovery, based on more than 40 years of data, in the June 10 issue of the journal
Science.
And while it's still true that oceans need plankton to form the base of the food chain, Frank said, removing a dominant top predator like cod can cause the critters they used to eat to have a population explosion. In the case of the northwest Atlantic Ocean, the next in line are invertebrates like crabs, which are more numerous than ever.
The concern, Frank said, is that the boom in invertebrates could be a temporary effect that has unpredictable impacts on organisms all the way down to plankton, potentially even altering such basic ocean health matters as algal blooms.
Similar tropic cascades have been observed in lakes, where adding a top predator in greening waters can cause a cascading effect down the food chain that leads to more zooplankton, which eat up the green phytoplankton and cause the waters to clear, explained Michael Pace, an aquatic ecologist for the Institute of Ecosystem Studies in Millbrook, New York.
"I think people are a little surprised," said Pace of the open-ocean discovery.
One reason for the surprise is that there are often more predators there to fill in for those that might be over-fished. Also, not every ecosystem is so influenced from the top-down, he said.
"It's not a universal condition that top predators control everything that happens below," Pace said.
Most surprising is the fact that anyone succeeded in capturing a glimpse of a trophic cascade in the open ocean at all, said Pace.
"They were extremely lucky," Pace said of the Canadian teams' access to decades of scientifically collected data at many levels of the food chain.
Fisheries-independent scientific data — not fish catch data from commercial fisheries — is harder to come by, but provides a much broader and objective picture of what's happening among all organisms, said Pace.
Name: Atlantic Cod (
Gadus morhua)
Primary Classification: class (common)
Location: Northern Atlantic Ocean
Habitat: Cold temperate waters.
Diet: Mainly fish, including haddock, capelin, herring and even other cod.
Size: Approximately 4 ft in length and 26 lbs in weight.
Description: Pale gray, reddish or greenish in color with dark spots; large head; protruding upper lip; barbels on chin; long, slim body; three dorsal fins; two anal fins; squared fluke.
Cool Facts: This species varies in color depending on its surrounding habitat; those that live near the coasts are reddish or green to blend in with algae, while those that live in open ocean are typically pale gray in color.
Conservation Status: Vulnerable
Major Threat(s): Overfishing
What Can I Do?: Visit
The Starving Ocean and
The Ocean Conservancy for information on how you can help.
