Nd:YAG Improves the Fine Print on Fish Tags

Daniel C. McCarthy

Roughly 50 percent of the salmon issuing from US Pacific Northwest rivers and streams are raised in fish hatcheries; the number is higher in Canada. Once released, these fish often disappear into the ocean depths for years, carrying a tiny coded wire tag from Northwest Marine Technology Inc. These tags can detail the fish's origins, release date and other information in the form of marks made by a frequency-doubled Nd:YAG laser from Cutting Edge Optronics.

A solid-state Nd:YAG laser improved the readability and flexibility of codes printed on Northwest Marine Technology's wire tags. The tags help track hatchery-raised fish after they are released into the ocean. Courtesy of Northwest Marine Technology Inc.

The magnetized, stainless steel tags measure 1.1 × 0.25 mm and are inserted into the nose cartilage of small salmon before they are released from a hatchery. Biologists and fishery officials examine catches using a magnetometer to locate tagged fish and read the extricated tag under a microscope.

Since 1971, Northwest Marine has etched coded marks onto the tags using spark-erosion. This technique enabled marking of binary codes on the tiny surface, but not characters, according to Terje Vold, an engineer at Northwest Marine. "With a laser, however, it is practical to micromachine dots or dimples on the wire that are small enough to write clearly legible alphanumeric characters," he said.

These tags are dramatically easier to read and decipher than the original binary-based version because alphanumeric characters are much easier for humans to read than a binary pattern of dots. Vold pointed out that Northwest Marine's customers are interested only in easier-to-read codes. However, he added, "There is a potential with the laser to provide a higher number of codes and more information."

The size of these marks necessitated a laser emitting at short wavelengths, which precluded a CO₂ source. Cutting Edge's Stiletto laser emits in the green at 532 nm.

Vold said he had considered copper vapor lasers but was attracted to Cutting Edge's solid-state device for its ease of use as well as its cost. "There are only a few manufacturers of such lasers, and Cutting Edge Optronics' unit had the specs we needed at the best price," he said.