Tales in the scales

Alive without breath, as cold as death

Never thirsty, ever drinking

All in mail, never clinking

The answer to this riddle from J.R.R. Tolkien’s The Hobbit is: Fish. And the “mail,” of course, refers to fish scales.

Scales are fascinating structures that can tell a great deal about the fish in your lake. By looking at scales from fish taken during test nettings, scientists can tell how old the fish are, how fast they have grown, whether they has been seriously ill or stressed, and more.

The types of fish in our area lakes are hatched with all the scales they will ever have. Scales originate from points in the fishes’ skin and overlap like shingles in a roof. They grow larger as the fish ages by adding to the outside edge. The scales show growth rings somewhat like those seen in the cross-section of a tree trunk. A difference is that while trees add just one ring per year, a fish scale may gain multiple rings in a year.

Still, each year does leave a distinct mark, especially in our climate. Because fish are cold-blooded, their growth slows significantly as they spend winter under the ice. A thicker ring, called the annulus, forms during these times.  

Scientists can learn a lot by studying these rings. For example, the distances between the annular rings reveal the approximate length of a fish at each age up to the current one. That’s because the rings are placed in proportion to the total length of the fish. So, suppose a scale from 12-inch walleye, three years old, has its first ring one-third of the distance from its focal point to the outer edge. That fish would have been four inches long at the end of its first year.

Using scales to tell the age of individual fishes, biologists can learn about the growth rate of a lake’s fish population. Because fish grow more slowly when they reach sexual maturity, scientists can use rings on scales to estimate the age at which fish began to spawn. This helps in setting fishing regulations to make sure most fish can spawn at least once before they are caught and removed.

A few other facts about scales: Fish that swim fast and live in fast-flowing water often have small scales, while fish like carp that live in slow or still water tend to have larger scales. Some fish have smaller scales toward the tail, providing more flexibility there. If a fish loses a scale, such as to injury, it grows back, all at once – so that scale will not show growth rings.