Read between the scales: An introduction to aging fish and why it's important

Most people know that you can count the rings of a tree to estimate its age, but did you know you can do the same with fish? As fish grow, their scales grow bigger in ever-increasing layers. Scales carry a record of the fish's life in concentric rings, which are counted to estimate age. Below, we will dive into the detailed steps of how the Idaho Department of Fish and Game ages fish using scales, including the preparation, interpretation process, and the purpose of aging fish.

Why Age Fish Scales?

Accurately aging fish is important because it provides valuable insights into fish populations, such as growth rates, migration patterns, and life history. This information is a key tool for fish biologists to better understand and study these aspects.

The Structure of Fish Scales

Fish scale growth is visible in a series of rings called circuli (Figure 1). 

When anadromous fish, like steelhead and Chinook, head out to the ocean their growth increases and we can also see this change. The ocean entrance is visible on the scale, where the circular spacing shifts as fish grow faster (Figure 2). 

When fish growth slows, the circuli become closer together. This region of slow growth usually coincides with the wintertime due to food scarcity and lower temperatures. This region is called an annulus and because this happens every year it can be used to track time (Figure 3). Counting annuli can reveal how long they have spent in freshwater and saltwater, providing insights into their migration and life history.

The Ageing Process

Step 1: Scale Preparation

Once the scales have been collected, they must be cleaned and prepared for examination. Cleaning involves removing any tissue or debris from the scale's surface. This can be done by soaking the scales in water and gently rubbing them to remove any excess biological material. After cleaning, the scales are dried and mounted on microscope slides for better visibility. 

Step 2: Examining the Scales

After the scales are prepared, they are examined under a microscope. A microscope is essential for analyzing the fine details of the scale. Microscopes with image-capturing capabilities help enhance the analysis by allowing individuals to zoom in on specific scale areas and document the annuli more accurately.

Step 3: Counting Annuli

To determine the age of the fish, individuals in the laboratory will carefully count the annuli on the scale. Each pair of annuli generally corresponds to one year of growth, which appear as dark and light bands on the scale. The first dark band typically marks the end of the first year of growth (the winter spent in freshwater). Successive bands mark each additional year as the fish grows and migrates. After counting the annuli, individuals can estimate the age of the fish by adding up the total number of annuli.