The art of ageing a rockfish

How old is my fish? How long do fish live? When do they mature and start reproducing?

These are all questions that can be answered by ageing a fish’s ear bone, called an otolith. Otoliths are hard, calcium carbonate structures that are found in fluid-filled semicircular canals near the fish’s brain and help with gravity, balance and orientation. These hard structures are like small data loggers. As a fish grows, banding patterns are formed in the otolith¬† similar to how rings are formed in the trunks of trees. When the fish is growing fast, these bands are spaced far apart and when the growth of the fish slows as it gets older, the bands become narrower and closer together. For most temperate fish species, like those found in California, these bands are deposited annually so that we can tell the age of the fish at capture by counting the number of annual growth bands.

This summer I assisted researchers at the NOAA National Marine Fisheries Service in Santa Cruz, California to production age Cowcod Rockfish (Sebastes levis). Cowcod were historically important in California fisheries, but their population numbers dropped so low in the 1990s that the species was declared overfished in 1999 and no-take fishing regulations were enacted to recover the species. The Cowcod population will likely be re-assessed in the coming year by NOAA scientists and part of this process is collecting age data on this species from fish that were caught in scientific surveys.

A Cowcod and Flag rockfish caught on video during a submersible survey.

We use what is called the “break-and-burn” method to age otoliths. We start with the dry otolith, break it in half through it’s center and then carefully burn it over an alcohol flame (kind of like toasting a marshmallow!). This helps us see the banding patterns. Then we line up the otoliths in a petri dish, standing them up in clay, and look at the banding patterns under a microscope to tell the age.


Cowcod are NOT an easy species to age, so no worries if you have a hard time seeing the growth patterns! For production ageing, we always have two readers who independently age each otolith and then come back together and discuss any differences in age estimates. Sometimes it feels like fish ageing can be more of an art than a science when the growth patterns are hard to identify!  Cowcod are reported to be one of the long-lived rockfish species. Out of the 350 or so otoliths we aged, we saw one over 50 years old, but most were in the 15 to 30 year old range. Hopefully good news for a recovering species!

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