Field Notes: Salmon surgeries

Where I’m from, the term “salmon” conjures images of vacuum-sealed pink flesh sitting in the local grocery store. It wasn’t until my first year as a graduate student at UC Davis that I saw my first live salmon – a wriggling juvenile about the size of my palm. It was hard for me to believe that such a small silver fish could have a profound impact on the lives of people worldwide. But especially out here, in Northern California, salmon are both an economic and symbolic source of sustenance.

Adult spring-run Chinook salmon can be found hundreds of kilometers upstream during spawning season. [Source]
As a member of the Biotelemetry Laboratory at UC Davis, I have had my fair share of opportunities to interact with salmon of all life stages. For the past several years, the lab has received funding from several governmental agencies to monitor the movements of this fish. This is important because in salmon, movement behavior is directly linked to survival and reproduction, and therefore the success of the population. Salmon are anadromous, which means that they spawn (give birth) in freshwater, but return to the ocean as juveniles to feed and grow. The amount of time the fish spend in each habitat varies by species and, in some cases, by individual. In our lab, we often study Chinook salmon (Oncorhynchus tshawyscha), and this species will spend anywhere from three months to two years in freshwater and about two to four years at sea1. Juveniles will feed on insects and crustaceans, shifting their diet to primarily fish as adults. As an adult, the average salmon is at least 3 feet long and weighs around 40 lbs, although weights of 120 lbs have been reported1. Salmon exhibit natal philopatry, meaning they return to their own birthing location to spawn. After laying their eggs, females will guard them for a month in a nest known as a “redd”, dying shortly thereafter.

It is this unique life cycle that makes salmon so fascinating, and this species uses an amazing combination of senses to find their way hundreds of miles upstream to their birthplace to spawn. In the open ocean, they are thought to navigate using the earth’s magnetic fields.2 As they move from ocean to river, olfactory navigation kicks in and chemical cues lead them back to their natal streams. Furthermore, this movement is consistently seasonal across generations. There are spring, fall, late-fall, and winter “runs” of salmon, based on when each population of salmon makes its migration upriver. However, because they depend so heavily on returning to their birthplace, their survival is currently compromised in places like the Central Valley of California. In this heavily agricultural area, a rather impressive water-diversion system has been created to serve the needs of local farming communities. This infrastructure uses dams and canals to redirect and control the amount of water flow in California’s prominent waterways. For the salmon, however, this almost certainly marks disaster.

It is now widely known that salmon cannot adapt to drastic changes in their natal habitat. After following the initial instinct to move upstream, many adults will simply get lost (known as “straying”) and die. In a human-modified system, like that in the Central Valley, salmon may stray into areas that don’t have suitable spawning habitat. Even if the female is able to reproduce, alterations of the waterways in upstream habitat affect the ability of juveniles to find their way to the sea. Hatchery practices can also contribute to high rates of straying. Juveniles are often translocated (via truck) and released many miles downstream to increase juvenile survival to the ocean. Consequently, these fish can’t properly imprint on the chemical cues along the route from their birthplace. While some hatchery-produced fish do return upstream as adults each year, smolt-to-adult return rates are often very low in hatchery populations. Because of this, we still don’t fully understand if we can supplement wild populations of salmon with hatchery fish – we simply don’t know if they will survive. That’s what the Biotelemetry Lab at UCD is trying to find out, focusing specifically on the survival of hatchery-raised spring-run salmon in the San Joaquin River.

Hatchery-raised fish are kept in large tanks, such as these at the base of Friant Dam, until they have reached sufficient size to be released.

Last week, my fellow lab members and I traveled to the Sierra Nevada foothills, just north of Fresno. Our destination was Friant Dam, part of the federal Central Valley project that serves to divert water flowing from the upper San Joaquin River into canals downstream3. Standing at 219 feet high with a crest length of nearly 3,500 feet, the dam was completed by the Bureau of Reclamation in 19423. Its purpose: to transform the Central Valley into an “agricultural powerhouse” by nourishing the valley’s major crops4. However, the dam also caused up to 60 miles of the San Joaquin river to go dry, leading to the loss of salmon runs in California’s second-largest river.3 This sparked a controversy that spanned for several decades, pitting the environmental and fishing communities against local farmers4. After 18 years of litigation, the debate was finally settled in 2006 with the San Joaquin River Restoration Settlement Act. The goal of the act was two-fold: to establish robust and self-sustaining fish populations below the dam, and to minimize water supply impacts to the farmers4,5. At least on behalf of the salmon populations, the success of this legislative action remains to be seen. That was where my lab came in.


In an odd parallel to the water-diversion system, the Biotelemetry Laboratory has created its own sort of infrastructure in the Central Valley waterways. As our name implies, we use biotelemetry methods (attaching transmitters to individual animals) to monitor the movements of a variety of aquatic and marine species. These include salmon, sturgeon, and even sharks. Once tagged, the animals can be detected on “receivers” anchored in different locations underwater. This way, we can see which animals are moving through which areas. This has proven to be particularly important for anadromous salmon. Consequently, during this trip, we found ourselves at the Satellite Incubation and Rearing Facility, located just downstream of Friant Dam6.

The purpose of our visit was to tag and release spring-run Chinook salmon smolts (juveniles). In its second year, our study allows us to monitor the ability of these hatchery-raised fish to survive the long trek out to sea. With the receivers already in place along the river, we simply had to get our transmitters attached to approximately 1,000 individuals. Salmon smolts are quite small, anywhere between 60-85 millimeters length from the nose to the fork of the tail. Thus, our tags have to be significantly smaller – about the size of a grain of rice. In order to ensure that these tiny transmitters do not come off en route, they must be implanted surgically. Thus, for the next week, a trailer at the base of Friant Dam became a miniaturized salmon hospital; practiced researchers conducted surgery after surgery, while research assistants and students monitored recovery times and recorded data.


Tiny transmitters are surgically inserted, and we take great care in keeping this environment sterile and the salmon safe.

After tagging comes a waiting game. These tagged salmon will be transported to different locations along the river, along with other hatchery-raised fish. A group of 100,000 smolts (1,000 of which are tagged) will be released upstream, with a supplementary release near the Sacramento-San Joaquin River Delta. We will be able to detect their movement as they move downstream. Ideally, our results will have critical implications for the future of this species. They will inform our understanding not only of salmon navigation and life history, but also provide insight into the ability of hatchery-raised fish to join wild populations in surviving a constantly-changing world. Stay tuned!


Special thanks to Eric Chapman and Gabe Singer for their expertise in leading the study and their contributions to this piece.

  1. Chinook Salmon (Oncorhynchus tshawytscha). (n.d.) Retrieved from
  2. Putman, N. F., Scanlan, M. M., Billman, E. J., O’Neil, J. P., Couture, R. B., Quinn, T. P., … & Noakes, D. L. (2014). An inherited magnetic map guides ocean navigation in juvenile Pacific salmon. Current Biology24(4), 446-450.
  3. Friant Dam. (n.d.) Retrieved from
  4. Friant Dam. (n.d.) Retrieved from
  5. San Joaquin River Restoration Settlement Act. (n.d.) Retrieved from
  6. San Joaquin Hatchery. (n.d.) Retrieved from

For main photo: [Source]

Alexandra McInturf is a second-year PhD student in the Animal Behavior Graduate Group, and a member of the Biotelemetry Laboratory. Though her primary research focuses on using biotelemetry to monitor the movement of sharks, skates, and rays, she has also volunteered for different projects involving a variety of fish species including, of course, salmon.

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