Home

About

Blog

The survival of salmon as a species is hanging in a delicate balance due to their very specific needs and many threats to their habitats. Scientists are investigating another potential threat that has yet to be fully understood. Hatcheries in various forms have been used to supplement wild salmon stocks in Alaska for over a century, but the implications of the interactions between hatchery-raised fish and wild fish has never been fully understood.

 

Now, the Alaska Department of Fish and Game as well as the Prince William Sound Science Center and the Sitka Sound Science Center are working together on a study to determine how hatcheries in south east Alaska are influencing wild salmon stocks. One vital source of information for this study is the small ear bone in a salmon called an otolith.

 

While the salmon is alive, the otolith assists with balance and hearing; once the salmon dies it is a treasure trove of information for scientists. The otolith allows scientists to determine the salmon’s origins, and investigate the issue of hatchery-raised fish straying into wild streams.

 

Some Things You Need to Know

Salmon are anadromous fish, which means they hatch in fresh water, migrate to the ocean for their developmental years, return to the stream in which they hatched to spawn the next generation and, soon after, die. The salmon industry is a mainstay for Alaska’s economy and salmon are an integral part of Alaskan’s culture. Since the late 1800s, commercial fishermen have experimented with hatching salmon in captivity and releasing them into streams to supplement the wild salmon stocks. Despite their long history, it is still unclear how interactions between hatchery and wild fish impact the wild salmon population.

 

Recently, concerns have been raised about hatchery salmon degrading the genetics of the overall salmon stock. In Alaska, hatcheries are intentionally located away from streams where important natural stocks of salmon spawn so that the fish hatched in captivity are less likely to interact with wild salmon. However, studies have shown that hatchery salmon are straying into wild streams and reproducing with wild salmon at unprecedented rates.

 

Scientists from the PWSSC, SSSC and ADF&G created the Alaska Hatchery Research Program in 2013 to conduct a study to determine how much interbreeding has occurred between hatchery and wild salmon, the proportions of wild fish to hatchery fish, and the impacts of interbreeding on the fitness of natural salmon stocks. Due to the salmon lifecycle, tracking salmon is extremely difficult. Marking the otoliths of hatchery-raised fish has made tracking salmon a much more manageable task.

 

Gathering Information from Otoliths

The otoliths are thin and ovular, and vary in size from fish to fish because each year the otolith grows another ring, the width of which depends on how much the fish grew that year. Hatcheries take advantage of this growth pattern and create “thermal marks” on each generation of salmon they raise before releasing them.

 

According to Stormy Haught, supervisor at the Alaska Department of Fish and Game lab in Cordova, “Otoliths are marked at the hatchery with a unique pattern by changing the temperature of water at specific stages in a fish’s life history.” Each hatchery uses a unique sequence of heating and cooling so that scientists can determine which hatchery the fish came from.

 

It takes a water temperature shift of just three degrees Celsius (5.4 degrees Fahrenheit) to create distinctive markings on a salmon’s otolith. Thermal marking is much more efficient than previously popular marking methods because it can be done to large groups of fish at once instead of individually as other methods require. Hatcheries have used thermal marking since the 80s and now almost every hatchery fish is marked before they’re released.

 

For the AHRP’s study, PWSSC crew members retrieved otoliths from fish in five streams around the Prince William Sound and sent them to ADF&G scientists to be analyzed. Most salmon’s otoliths are about the size of a black bean, so rings that hold all of this data can only be seen under a microscope. The scientists grind the thin oval bones down to flatten them on one side and affix them to microscope slides. Through the microscope’s lens, peering at an otolith is very similar to looking at the rings on a tree.

 

Following Genetic Lines

This study was proposed in response to previous research that found hatchery salmon were straying into wild streams more often than expected and concerns that hatchery salmon genetically weaken wild salmon populations. To determine the genetic implications of hatchery and wild salmon interbreeding, scientists use DNA collected from tissue samples to determine the parentage of the salmon who successfully returned to spawning sites. As long as their parents have been sampled, salmon’s lineage can be determined using polymorphic DNA markers. This is the same method used for paternity tests in humans.

 

The scientists collected genetic data from 7,941 fish between 2013 and 2016. The species of salmon in this study -- pink and chum -- have two-year life cycles, so they took samples from parent broods in 2013 and 2014 and the offspring of those broods in 2015 and 2016. In Spring of 2019 they released the first report from the study.

 

They have continued collecting data in the subsequent years and will be sampling from more streams in the future to get a wider swath of data. Scientists are still genotyping fish from 2015 and 2016 to establish more parent-offspring connections, and eventually include the grandparent generation in their analysis.

 

As further research is conducted, scientists will develop a more conclusive understanding of how interactions between hatchery-raised salmon and wild salmon impact wild salmon populations. With that knowledge, they can make any needed adjustments to hatchery management policies to increase the sustainability of the salmon industry.