The Age of  Fish

Ever wonder how old that fish you caught was? Dr. Skip, a Gulf Coast marine biologist has written a nice article about that subject :

How to Tell a Fishes’ Age Without Asking

Fish are born, live and die like we do only you can’t ask a fish its age. When a fish is spawned in a hatchery the manager knows how old it is. That’s called a known age fish. If you catch a fish in the wild telling it’s age is a different story.

Fish have bones in their heads called otoliths (oto’ meaning ear and lith’ meaning stone). These bones help the fish to keeping its balance in the water. When an otolith is removed from a fish, sectioned into thin slices and viewed through a microscope, it reveals a pattern of light and dark concentric rings.

When a fishes’ grow slows in the winter a darker denser ring forms. In the warmer months when a fish is growing faster and a clearer ring is formed. These yearly growth rings are called annuli, which are similar to the rings found in tree stumps. You can count the rings in the otolith or stump and determine age. Unfortunately, for the fish it had to die to allow you to determine its age. Other hard parts in a fish’s body can also be aged in a similar way such as fin rays or scales. Unlike bones, scales in a fish tend to get knocked off or fall off over time and have been proven not to be as reliable as otoliths for aging. Otoliths can be small, thin and clear like in marlins or as large as half your thumb and opaque like the red drum (redfish).

A scientific study called “Age and Growth” allows a fishes’ age to be determined without killing it – except for those used in the initial study. For each species otoliths are collected as soon as possible after they are spawned to as old a fish as can be obtained. Data including the gender and length of the fish are collected along with the information from the otoliths. This information is analyzed for each species and gender and an age assigned for a given range of lengths. While not perfect it’s a good indicator of a fish’s age. A separate analysis is done for males and females because in fish, generally speaking, females grow slower, are larger and are longer lived than males. When you hear or read about a large red snapper or king mackerel being caught it’s a good guess it was female.

When age and growth data are combined with commercial and recreational harvest information, the egg producing capacity of the females (fecundity) and other information the result is called a stock assessment. Stock assessments provide regulating agencies with the data necessary to set angler regulations like length limits, number per day and total harvest. The intent of the regulations is to allow for perpetual renewal of the stock.

Fishy Facts

A three foot-long dolphin fish can be less than one year old.

It’s better to throw the big ones back (females) as they produce more eggs a year than the smaller ones.

Otoliths contain heavy metals like lead and can be used as indexes of pollution.

Otoliths from Indian villages thousands of years old can provide insights into the base level of heavy metals like lead.

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