April Mystery of the Month

Photo courtesy of Przemek Bajer

This month’s mystery is another visual puzzler. One of our staff thought it looked kind of like a wing. Another said it reminded them of tree fungus. Does it help at all if we tell you that this is a 40x magnification? Probably not too much…


Once again our mystery involves the fish we love to hate, the common carp. This interesting ringed structure is a carp otolith, which anyone but a fish biologist would call the earbone. And just like the rings in a tree trunk, the rings, or “annuli” in the otoliths of many bony fishes are a cue to the age of the fish.


In the otolith shown above (courtesy of Przemek Bajer of the Sorenson Lab) the light areas correspond to periods of rapid growth (summer) and the dark areas reflect periods of relative inactivity (winter). This particular carp appears to have been 11 years old when it met its fate.

A freshly-extracted otolith from a young carp.

Preparing an otolith for reading is a bit more complicated than cutting into a tree to count the rings. Carp otoliths are quite small and located deep within the fish's cranium. Removing the otolith can be a messy process, particularly for the inexperienced (yes, you are hearing first-hand knowledge). Once the earstones are removed they are thoroughly cleaned and mounted in resin for handling. After the resin has cured, very thin sections are cut with a diamond blade. These sections are examined under high magnification, the rings are counted, and the apparent age is
recorded for the fish.

An otolith from a very old carp.

The otolith pictured at the top of the page is pretty easy to read; the light areas are wide and the dark bands spaced well apart. In older fish, otoliths can be much more difficult to read, however – in this fish, for instance (image to left). The Sorenson lab accounts for this by having three experienced researchers interpret the rings independently and then compare results.


One of the plain coolest pieces of information to come out of this work is just how long-lived carp can be. The oldest fish pulled out of Lake Gervais appears to be 66 years old; and is accompanied by several others aged 60 and older. Understanding the age structure of a population of fish can be incredibly powerful information. One of the most helpful discoveries made by the Sorenson lab is the association between the age groups of carp and the years that egg predators – bluegills – endured a winter-kill. Current research is connecting carp ages and genetics to historical changes in the watershed; more on that to come!