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.

Measuring Tag Retention and Fin Clip Quality

Each year, our office coded-wire tags (CWT) and mass marks (adipose fin clip) over 2 million juvenile fish from the federal hatcheries on the Olympic Peninsula. The CWT is a tool that is used to assess overall success of hatchery practices. The tags are also used to estimate survival and assess run size and timing. Mass marks allow us to distinguish hatchery fish from wild populations. This is a key component in allowing fishing to occur for hatchery-raised fish in areas where they coexist with wild stocks.

Before these fish are released, a quality check must be performed to ensure successful retention of tags and proper clipping of fins. This is accomplished by examining groups of fish 1 month after being marked and tagged. The fish are individually passed over a detector to determine if a tag is present and then fish that received a clip are visually examined for the quality of the clip. This data is then recorded and used to calculate the success of our tagging and marking. Once examination is complete, the fish are returned to the raceways where they are cared for by the hatchery staff.

Following are three short videos demonstrating this process:

Coho Salmon Are Back in the Upper Big Quilcene River

Quilcene National Fish Hatchery, situated on the picturesque Big Quilcene River, has been raising coho salmon for many decades. During this time, most returning coho adults have been intercepted at the hatchery and used for human consumption and raising more fish. In recent years, a few adult coho have been allowed to pass upstream of the hatchery.

These practices are changing, however.  Managers have recognized that passing fish upstream of the hatchery has many benefits: River habitat can be used for natural production of coho salmon; coho eggs, juveniles, and decomposing carcasses in the river have many food web and ecosystem benefits; and people just plain like to see salmon spawning naturally in our rivers and streams!  In 2008 and 2009, hatchery managers allowed 200 adult coho to move upriver. During this time, USFWS biologists performed spawner and carcass surveys to evaluate how successful these fish were at spawning. We also performed snorkel surveys and installed a screw trap to evaluate rearing success of the juveniles.  


USFWS biologist checks the screw trap for fish

A screw trap (see picture) has a rotating cone that captures fish moving downstream. Fish are held in a live box at the back of the trap where they wait to be removed, identified, and measured by a biologist. Although it might look a little scary, these traps are actually very gentle on the fish.

So far, results have been encouraging. In the spring of 2010, we estimated that 1,000 to 2,500 outmigrating smolts resulted from the 2008 spawners. Not bad for a system recognized as having minimal spawning and rearing habitat! We will be continuing our studies next spring to see what results from the adults passed in 2009. Sadly, for various reasons, no adults were passed upstream this year. Nonetheless, passing coho upstream to spawn naturally is something we plan on doing every year as standard practice.


Andrew measures, weighs, and marks the fish captured in the screw trap