A public aquarium in Japan is having a problem with its sardines, which have become so used to their safe environment that they no longer behave as they're supposed to.
The 35,000 sardines at the Port of Nagoya Aquarium have stopped forming the famous 'fish tornado' as they come together to feed - an amazing sight and a big attraction for visitors (scroll down for video showing tornado).
Such shoaling behaviour is a natural form of defence against predators for sardines. The ones in this display come from the Kuroshio Current off the coast of Japan, where rocks and other forms of shelter are scarce.
The 14 x 5m aquarium also contains predators from the same habitat, but apparently these rarely bother with the sardines because they're so well fed by staff.
Rogue sardines have recently started to break away from the rest of the group, swimming in a different part of the tank. Staff think the fish have realised they're unlikely to be eaten and this easy life has led to them changing their behaviour.
According to reports, some of the sardines have even started competing with much larger Bonito for food - fish that would actually eat them in the wild!
So aquarium staff have introduced 11 Bluefin tuna to the aquarium in an attempt to shock the sardines back into behaving more normally.
Akira Ogushi, a worker at the Port of Nagoya Aquarium, said: "It will take the Bluefin tuna about a week to get acclimated. When they start to swim in a school, that will put pressure on the sardines to all swim together as a large school."
The video below shows a sardine tornado at the aquarium.
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