If you put all your fish in one barrel, they might be inclined to align themselves, if they are not too far from the magnetic north pole or south pole, suggests this study:
“Magnetic Alignment in Carps: Evidence from the Czech Christmas Fish Market,” Vlastimil Hart [pictured here with a non-magnetically aligned dog], Tomas Kusta, Pavel Nemec, Veronika Blahova, Milos Jezek, Petra Novakova, Sabine Begall, Jaroslav Cerveny, Vladimır Hanzal, Erich Pascal Malkemper, Kamil Stıpek, Christiane Vole, Hynek Burda, PLoS ONE 7(12):e51100. The authors, variously at the Czech University of Life Sciences and Charles University in the Czech Republic, and the University of Duisburg-Essen in Germany, report:
“We examined alignment of common carps (Cyprinus carpio) at traditional Christmas sale in the Czech Republic. The sample comprised measurements of the directional bearings in 14,537 individual fish, distributed among 80 large circular plastic tubs, at 25 localities in the Czech Republic, during 817 sampling sessions, on seven subsequent days in December 2011. We found that carps displayed a statistically highly significant spontaneous preference to align their bodies along the North-South axis. In the absence of any other common orientation cues which could explain this directional preference, we attribute the alignment of the fish to the geomagnetic field lines.”
(Thanks to investigator Tom Sheldon for bringing this to our attention.)
BONUS (possibly unrelated): Video showing a technique for growing fish in a barrel:
BONUS: A Mythbusterian analysis of the efficiency of shooting fish in a barrel (thanks to investigator Tom Ulrich for bringing this to our attention.) The analysis used non-aligned, artificial fish.