The largest cod attack on capelin was recorded by biologists from the Massachusetts Institute of Technology and the Institute of Marine Research in Norway. 

Scientists observed the collision during the seasonal spawning of capelin in the Atlantic Ocean off the coast of Norway. The spawning of this fish usually takes place between February and March. Before the capelin migrate to these waters, their main predator, the cod, moves in.
The capelin, as biologists noted, formed a shoal of about 23 million individuals. The cod also gathered in a group and, by attacking, devoured about 10.6 million capelin individuals in about four hours, the study said.
Abrupt changes in fish behaviour, biologists warned, could have a significant impact on the abundance of prey populations. It could also affect the stability of the ecosystem.