In order to avoid certain death at the jaws of underwater predators, small schooling fish such as sardines form what is called a bait ball. When they sense a threat in their immediate environment, they swim in a tight, spherical formation with a common center. This exposes the least number of fish, and is quite handy in open spaces with no cracks or crevices to hide. While a bait ball typically lasts about 10 minutes or less, it can be quite massive, reaching up to 10 meters deep and 20 meters in diameter.
However, due to the fact that it’s difficult to ignore a massive bait ball swimming by, such a formation may end up attracting other hungry predators. Interestingly enough, crafty aquatic hunters have found ways to turn this defensive measure into a free meal ticket. In fact, some predators work in groups to intentionally scare small fish into forming a bait ball. Dolphins, for instance, are known to do this, forcing the fish into a bait ball while picking them off one at a time.
Another effective strategy is carousel feeding. Notably practiced by Norwegian killer whales, carousel feeding involves predators scaring herrings into a bait ball. Afterwards, they use their tail flukes to slap the ball, stunning or killing up to 15 herrings per strike.
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Cover photo: Edwar Herreno
- Reeves RR, Stewart BS, Clapham PJ and Powell J A (2002) National Audubon Society Guide to Marine Mammals of the World Chanticleer Press. ISBN 9780375411410.
- Similä, T. & Ugarte, F. (1993). “Surface and underwater observations of cooperatively feeding killer whales”. Can. J. Zool. 71 (8): 1494–1499. doi:10.1139/z93-210
- U.S. Department of Commerce, National Oceanic and Atmospheric Administration, National Marine Fisheries Service. “Coastal Stock(s) of Atlantic Bottlenose Dolphin: Status Review and Management Proceedings and Recommendations from a Workshop held in Beaufort, North Carolina, 13 September 1993 – 14 September 1993” (PDF). pp. 56–57.
- Webb, Charles Harper (Summer 2015). “BAIT BALL”. The Georgia Review. 689: 271