Wednesday, December 29, 2010

Using Otoliths As A Fish Identification Tool

Otoliths are a calcareous structure within the inner ear of fish. The word calcareous simply means a structure made primarily of calcium; thus otoliths are a hard, bony structure. These structures are sensitive to changes in acceleration and gravity. They play an important role in allowing the fish to detect changes in horizontal movement and acceleration.

The shapes of fish otoliths vary highly among different species but not within species. Layers that are mineral-rich and mineral-deficient are deposited in alternating succession around the original center of the otoliths, creating bands that resemble rings on a tree. The bands show up as dark and light areas when viewed under a light. The rates of deposition vary during different times of the year. During the summer months, warmer water and rapid growth cause the otoliths depositional rates to be more rapid than during winter months. By examining the otolith, we can tell how many seasonal changes a fish has experienced during its life, allowing us to determine the fish's age. This is an important tool that is used not only to determine age, but to also identify different stocks.

Water chillers at Quilcene NFH
Manipulating the rearing temperature at hatcheries allows otolith bands to be formed. One technique is to expose the fish to cold temperatures; this causes dark bands to form on the otoliths. This process is repeated for a specific number of times and timed intervals. Since the formation of these bands is controlled by hatchery personnel, a unique pattern of rings is formed that is not seen in wild stocks. This technique is a cost-effective way to accurately differentiate between hatchery and wild fish, as well as allowing the identification of hatchery release groups.

Kokanee egg trays at Quilcene NFH
Hatchery personnel from Issaquah State Fish Hatchery and Quilcene National Fish Hatchery will use this technique to mark Lake Sammamish kokanee salmon. When these fish return as adults, these otolith marks will allow the biologists to identify where the fish originated.