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Treebeard's Stumper
20 September 2002

Stay in School

I was entranced by the huge schools of baitfish we saw while snorkeling at Catalina Island. Fish in a school act like a single organism with a purpose. They divide and regroup around us when we dive into them. They all change direction so abruptly that I'm sure they are not following a single leader like a line of pelicans. Flocks of starlings over local vineyards are similar. How do fish stay in school without a single leader to follow? Why do fish form schools at all? I was attracted to these fish, so aren't other predators also attracted to such an obvious target?

Jack Mackerel (Trachurus symmetricus) at Catalina.
Photo by Erik Erikson.
Baitfish school in Key Biscayne, Florida.
Note the threatening shark on the bottom.
Photo from National Parks photo gallery
.

Schooling fish are less interesting up close (left). It's the dynamics that are interesting (right). Note how the school stays together with a sharp boundry despite the threatening shark on the bottom. This is intelligent social behavior, but where is the intelligence? Wouldn't individual fish fare better on their own, so they wouldn't attract such large preditors?

I'm not sure what fish species we saw "in school" while diving at Catalina. There are many small schooling fish including sardines, anchovies, herring, mullet, salema, jack mackeral, blacksmith, topsmelt, grunion, flying fish (really!) and lots more. The nomenclature of these fish is confusing as some names are proper species and others are generic. To a fisherman, these are all baitfish, and that's good enough for me.

I can't take my digital camera in the water, and I had a surprisingly hard time finding photos of local fish schools that I could "borrow" with a clear conscience. I can share some links. Natural history photographer Phillip Colla has a remarkable collection of photos of schooling fish, including many taken at Catalina. There'a another beautiful gallery by Marc Shargel. These photos don't show the beautiful dynamics that make this a real stumper. There is a quicktime movie of schooling anchovies at the Monterey Bay Aquarium, but these are aquarium fish going in a circle. In the ocean, it's more complicated.

Answer


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Copyright © 2002 by Marc Kummel / mkummel@rain.org