Certain scenes at sea remain invisible even when they involve millions of animals. This, however, was measured almost minute by minute. Researchers reconstructed what happened on February 27, 2014 off the northern coast of Norway, in Finnmark, during the reproductive migration of the capelin, a small Arctic fish about the size of an anchovy. Right there, in a few hours, what the authors define as the largest marine predation ever documented in terms of number of individuals involved and extension of the area observed took place. The study is due out in 2024 on Communications Biology.
With the first light, the capelin stopped moving in scattered nuclei and reached a critical density: from that moment they aligned themselves, took a common direction and speed and formed a compact bank over ten kilometers long. Estimates speak of around 23 million individuals and a biomass of around 414 tonnes. The compactness offered an energetic advantage and a form of collective defense, but at the same time it transformed that school into a gigantic signal for their main predator, the Atlantic cod.
Cod responded with the same logic. Even the predators, initially dispersed, have organized themselves into a large coordinated school which has incorporated that of the prey. Within about four hours the number of cod above the critical threshold reached 2.5 million and the estimated consumption reached 10.5-10.6 million capelin, i.e. more than half of the school intercepted. The researchers speak of a very rapid shift in the balance between predator and prey, a sort of collective outburst which on an oceanic scale resembles an avalanche.
What made this massacre visible was the OAWRS, which stands for Ocean Acoustic Waveguide Remote Sensing. In practice, a ship sends sound waves into the water via an acoustic array and collects the echoes with towed receivers, obtaining almost instantaneous maps over very large areas. The decisive novelty lies in the multispectral version used to reread that data: each species with a swim bladder resonates differently, and this acoustic signature allows us to separate the capelin from the cod even when the two fields overlap.
The authors explain the difference with a very simple image. Cod, which has a larger swim bladder, resonates at low frequencies; the capelin, which has a lower case, on much higher frequencies. Translated: the researchers were able to “listen” to two different populations within the same stretch of sea and follow their movements in real time, from the initial dispersion to the moment in which both became an orderly structure, first for defense and then for attack. It is this simultaneous transition, observed on an enormous scale, that makes the work so important also on an ecological level.
How quickly can an ecosystem change?
The observed episode accounts for a small share of the total capelin stock in the Barents Sea, around 0.1-0.2% according to the study’s estimates. On a population scale, therefore, that single assault remains contained. The scientific value, however, lies elsewhere: capelin is a key species for the Arctic and supports the food web of numerous fish, including cod. A local, concentrated and very rapid change can therefore alter the balance of power in a few hours within one of the ecosystem’s hot spots.
This is where the climate comes in. The authors and the team that released the findings report that retreating Arctic ice is forcing capelin to make longer migrations to spawning areas. More distance means more energy consumed, more stress and more exposure to episodes of concentrated predation like the one recorded in front of Norway. If ecological hotspots become fewer or more fragile as a result of climate and human pressures, natural events of this type can leave much more serious scars.
This is why the researchers want to use OAWRS again on other species and in other scenarios. The idea is simple and severe at the same time: when a population approaches collapse, there is often one last large bank left to keep the system afloat. Understanding where it is found, how it is formed and when it breaks can become decisive. In the Barents Sea, on that 27 February, the sea showed a brutal rule: it only takes a few hours for a shoal to become a refuge, a target and the final score.
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