Unlocking the story of Chinook salmon using dorsal fin rays

You may be aware that Chinook salmon return from the ocean to their spawning grounds in Idaho, but what you might not know is the Idaho Fish and Game uses Chinook dorsal fin rays to obtain their saltwater age. 

Chinook salmon are born in streams throughout Idaho and emigrate to the ocean to spend most of their adult life. These fish journey back to their natal streams to spawn and die naturally. The streams are walked by biologists and technicians, and dorsal fin rays are collected off the carcasses (Figure 1). Below, we will go through how IDFG uses these fin rays to age adult Chinook and why this aging method is useful in fisheries research.

Why we use fin rays to age Chinook salmon

If you read the blog post Read between the scales: An introduction to aging fish and why it’s important by Emily Cates, you might be wondering why we use fin rays instead of scales. The answer is that aging fish using fin rays has proven to be the more accurate method of aging Chinook salmon. Aging fish using dorsal fin rays helps biologists understand the age, growth, and population dynamics of species, providing insights for effective fisheries management. 

Structure of a Fin Ray

The freshwater mark of the fin ray usually resembles a jellyfish shape and can be found just above the inside arc (Figure 2). This mark allows us to distinguish between their juvenile growth in freshwater and the adult growth in saltwater.
 

Just like you can count rings of a tree to estimate its age, you can count the bright white bands on the fin ray, called annuli, to determine the saltwater age of the fish (Figure 3).

Aging fin rays

Step 1: Preparation

When the fin rays are received, we begin collecting age information. First, the fin rays are dried, peeled, and cast in epoxy. After the epoxy has hardened, the fin rays are cross sectioned using a saw. Six cuts are made near the knuckles (where the fin rays intersect the spine) since this is the region that is best for aging. Then the fin ray slices are mounted to microscope slides (Figure 4). 

Step 2: Imaging

After the fin rays have been prepared in the lab, they are examined under a microscope to get a better view of the annuli. A microscope with a camera is used to obtain digitized, detailed pictures of each fin ray sample.

Step 3: Aging

Individuals in the lab observe each fin ray image and count the annuli. Each annulus corresponds to a single year of growth and appears as bright white bands. Once the annuli are counted, the age is estimated by adding up the total number of annuli (Figure 5). Obtaining accurate ages from fish is important because it helps provide valuable information on their life history, age composition, and population structure.