by Mollie Ogaz

The undead, honing back to its natal stream. (Photo credit: Ken Davis)

Imagine you are on the bank of a river or stream in California’s Central Valley. It is just past sunset, leaves rustle overhead, and you feel a tingling along your spine. Suddenly a zombie fish leaps past you, patches of decomposed flesh visible as it streaks by. It’s a thing of nightmares; just a figment of the imagination brought on by the spooky atmosphere, right?

No.

It is a Pacific salmon (Oncorhynchus spp.), making its upstream migration to natal spawning grounds. This arduous journey to reach the stream in which they were born is attempted by each Pacific salmon that manages to survive to adulthood in the ocean. Only 1 in 2,000 will complete this journey. Unlike their Atlantic relatives, Pacific salmon are semelparous, meaning they reproduce only once in their lifetime, so the stakes of failure are high.

These spawning dead remind us about the unlikely success of this species, and why dwindling populations recover over decades, not single seasons. By understanding the challenges these fish face throughout their life cycle, we can appreciate how our natural resource management decisions are really about laying the groundwork for future generations, rather than enacting short-term fixes for ourselves.

The life cycle of a Pacific salmon begins and ends in the same freshwater stream – if everything goes well. Fertilized eggs buried in the gravel hatch into alevin, which derive energy from the attached yolk sac as they gain strength. After leaving the gravel, the small salmon, called fry, rise to the water’s surface to fill their swim bladders and begin to feed.

Depending on the species, fry can spend anywhere from a few weeks to a couple of years in freshwater. As they approach the ocean, juvenile salmon undergo smoltification, a physiological transformation that allows them to live in saltwater, where they spend the majority of their lives feeding in the highly productive marine environment. Entering the ocean marks the end of the first major migration Pacific salmon make in their lifetime, one they will have to complete in reverse in order to reproduce.

Life cycle of Pacific salmon.

Lucky adults that have managed to avoid anglers, disease, predators, and many other dangers make their way back toward their natal stream, initiated by cues that are poorly understood, but likely have to do with reproductive maturation and environmental changes. Once a salmon reaches freshwater, it stops feeding entirely and relies on energy stores accumulated during years in the ocean to support its homeward migration. This natal homing, or ability to navigate upstream to the very waters in which they were born, is one of the most amazing feats in the animal kingdom. To do this, young salmon imprint on the unique smells of their natal stream, which they use as a sort of map to successfully navigate their return.

Here is when things get spooky. Since the salmon have stopped feeding and are putting all of their energy into reaching their spawning grounds and subsequent reproduction, their bodies begin to shut down. The normal energy balance that is kept between growth, survival, reproduction, and body maintenance shifts to solely focus on reproduction. It’s spawn or bust for these salmon; there are no second chances.

The decrepit carcass of an adult salmon. (Photo credit: Ken Davis)

Robust, silvery fish turn into living zombies, flesh decomposing and falling off their battered bodies as they struggle against the current and over obstacles toward spawning grounds. The journey is long and tough, but ultimately worth it if they successfully reproduce.

Upon reaching their natal streams, females build a redd, or nest, in the gravel where they deposit their eggs that are then fertilized by competing males. Shortly thereafter the adults die, although females may guard the nest for a week or two first. The carcasses become a source of nutrients for the aquatic and riparian communities of the stream, including the offspring that will soon hatch. It is a rare cycling of nutrients from the marine environment to freshwater streams, and only possible due to the salmon’s anadromous lifestyle.

Video shows Chinook salmon spawning in Lower Putah Creek, Yolo County, in fall 2014. No sooner does a female lay her eggs than she is flanked by two males, mouths agape as they release clouds of sperm. The female attempts to bury her eggs as several rainbow trout attempt to eat them. Poor girl even hits her head on a rock while covering the eggs. Video by Ken Davis/Wildlife Survey & Photo Service

This Halloween you can add a spooky stop to your list and head to Putah Creek in Winters to see some zombie fish for yourself, where record numbers of spawning Chinook salmon have been observed over the last few years. Last fall there were an estimated 1,000 to 1,600 spawners, a truly remarkable number for a creek that had a high of about 70 returned salmon a little over a decade ago! This rebound shows that changes in management with long-term vision can succeed in returning salmon to local streams. Next up: achieving major management changes in larger, dammed rivers with high political controversy.

Mollie Ogaz is an assistant specialist at the Center for Watershed Sciences.

Further reading

Alvarez, F. 2016. “Record numbers of salmon are spawning in Putah Creek”. The Davis Enterprise. December 28, 2016

California Trout. 2017. “SOS II: Fish in Hot Water”

Cooke S.J., Crossin G.T., and Hinch S.G. (2011) Pacific Salmon Migration: Completing the Cycle. In: Farrell A.P., (ed.), Encyclopedia of Fish Physiology: From Genome to Environment, volume 3, pp. 1945–1952. San Diego: Academic Press.

Moyle, P.B. 2015. “Salmon finding a home in my backyard – Could it be?”. California WaterBlog. March 11, 2015

Tilcock, M. 2016. “The Horror of a Salmon’s Wheel of Misfortune”. California WaterBlog. Oct. 30, 2016