Science Figures Interpreted and Analyzed by RealClearScience
"It's as easy as shooting fish in a barrel" is a common American saying. However that saying originated, surely it did not envision how much easier it could be to shoot those fish if they all lined up in a nice, orderly way. Yet, scientists have now found a fish that is willing to do just that. (That is, line up in a nice, orderly way, but presumably not for the purpose of getting shot.)
Researchers from the Czech Republic and Germany took advantage of the fact that central and eastern Europeans love to eat fish, particularly carp, at Christmastime. (The Polish, incidentally, also like to eat herring, which is absolutely disgusting.) The scientists wandered the streets of Prague and Bohemia looking for fish vendors, and when they happened upon one, they photographed the carp they were selling. Overall, they gathered 817 photos from 80 different barrels.
Then, the researchers analyzed how the fish were aligned. The fish preferred to align according to earth's magnetic field, specifically in a north-south direction. (See photograph.)
The explanation for why fish that aren't going anywhere prefer to line up in a north-south direction is unknown. The authors suggest that it may be important for navigation or coordinated movement in the school of fish.
And just in case you are wondering, it is indeed quite easy to shoot fish in a barrel. You don't even have to be a good shot to kill them, as the shock wave generated by a single bullet is probably enough to kill most of them.
Source: Hart V, Kušta T, Němec P, Bláhová V, Ježek M, et al. (2012) Magnetic Alignment in Carps: Evidence from the Czech Christmas Fish Market. PLoS ONE 7(12): e51100. doi:10.1371/journal.pone.0051100