Just in time for Halloween, scientists have recorded a spectacular sight of bloody carnage on the high seas. Researchers tracked a huge school of cod off the coast of Norway when the fish intercepted and quickly ate millions of migrating capelin fish. This appears to be the largest predatory kill of its kind ever recorded by humans.
Many species of marine life congregate for protection and migrate between territories, creating opportunities for resourceful predators to find food. But only recently have scientists been able to reliably observe the movement of large populations of fish, also called shoals, which would allow us to see these floating buffets in real time. Using a sonar technique called ocean acoustic waveguide remote sensing, or OAWRS, oceanographers from Norway and the Massachusetts Institute of Technology were able to capture one such school.
The researchers tracked populations of capelin (Mallotus villosus), a small anchovy-like fish, as they came into contact with large Atlantic cod (Gadus morhua), their primary predator, in coastal waters off Norway during the height of their spawning season in February 2014. Scientists first observed how the initially disorganized schools of capelin had coalesced into a shoal of about 23 million fish that stretched for many kilometers. This huge concentration of fish prompted cod to also gather in a group and feast on capelin. It is estimated that the cod ate about 10 million capelin in just four hours after the capelin first formed the shoal.
“Investigating species interactions over different areas and time periods using OAWRS can contribute to a new and better understanding of the functions of large marine ecosystems, as well as support the quantification of key processes in the assessment and management of marine resources over large areas,” the researchers write in their paper published today in the journal Nature Communications Biology.
Luckily for capelin, this particular food frenzy hasn’t had much of an impact on their overall numbers. Billions of capelin are believed to migrate between the waters of the northeast Atlantic Ocean, so researchers estimate that the cod probably only ate about 0.1% of the total capelin population in the area that year. These large-scale events are also an important part of the balancing act between predators and prey inherent in any ecosystem.
Unfortunately, there is evidence that at least some capelin and cod populations have experienced declines in recent years due to factors such as commercial overfishing. And, as in many other cases, it is possible that warming due to climate change will further impact capelin and make these events even more dangerous to the health of the entire population.
“In our work, we see that natural catastrophic predation events can change the balance between local predators and prey in a matter of hours,” senior author Nicholas Makris, a professor of mechanical and ocean engineering at the Massachusetts Institute of Technology (MIT), told MIT News. “This is not a problem for a healthy population with many spatially distributed settlements or environmental hotspots. But as the number of such ‘hotspots’ [declines] due to climate and anthropogenic stresses, the natural ‘catastrophic’ extinction of a key species that we have seen could have dramatic consequences for that species and for many species that depend on it.”
At the very least, the availability of this technology will make it easier for researchers and others to monitor the health of these important fish and other marine ecosystems in the future.
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