The tranquility of a Muskoka chair, fishing rod, and glassy lake. What could disrupt this middle-aged man’s dream? The line is tugged, reeled back, and instead of yielding dinner – a corpse. The body of a salmon, missing skin, muscle tissue, an eye, and covered in fungal infection, yet it’s still fighting the line. Little does the fisherman know, he has caught something much more important than dinner. He is witnessing the final stage of a ruthless, upstream battle for reproduction against the odds of nature. Even in decay, the fight goes on.
The reproductive journey of an Atlantic Salmon’s life, colloquially called a “salmon run”, demonstrates many facets of nature’s ingenuity. After hatching in the fresh water in which they were laid, salmon migrate to sea where they spend most of their lives, anywhere from one to seven years depending on species (NPS, 2019). Here they store up the resources they will need for the culmination of their lifecycle, the salmon run. To spawn, salmon return to their birthplace upstream with remarkable precision. It is suspected that this is accomplished through chemical or solar cues, but one interesting theory involves magnetoreception (NPS, 2019). Magnetoreception allows animals to detect Earth’s magnetic field, which in this case grants salmon an internal compass (Lohmann, 2023). Suspected magnetite has been detected in nerve tissue of Atlantic Salmon, in quantities appropriate for magnetoreception, in regions that suggest biogenic origin and not of some contaminant origin (Moore, et al., 1997).
In addition to their incredible navigational ability, salmon display ridiculous amounts of rapid, morphological change in their journey upstream, particularly in males. In only 14-24 days, male salmon of various species experience a complete change in pigmentation, the growth of a hump, increase in snout length, increase in body depth, increase in adipose fin size, and even a complete transition in skull shape, with the development of a hooked snout in order to fight with other males (Hendry and Berg, 1999; Witten and Hall, 2003). Some of these morphological changes are depicted in Figure 1.
Figure 1: Morphological differences between pre-spawning and spawning adult Coho Salmon, one of the many species that display this transition (Ragan, 2023). Here, hooked snout development and pigment change are especially notable.
The freshwater environments to which salmon return for spawning do not contain the resources to feed or sustain large populations. They instead rely on energy reserves and catabolism to fuel their journey, often using 40-80% of energy reserves in their period of reproduction (Morbey, Brassil and Hendry, 2005). Additionally, high levels of plasma cortisol have been detected in spawning salmon, which can lead to tissue degeneration and immune system suppression (Hruska, et al., 2010). After suffering wounds from competing males, immunosuppressed salmon are incapable of properly fighting infection, and are often found infested with fungus. The result of this rapid catabolism, tissue degeneration, and immunosuppression is known as senescence, and creates the infamous “zombie salmon”, as shown in Figure 2 (Thao, 2018).
Figure 2: Late-stage senescent salmon (Luke Gordon [@manta_luke], 2022)
The reproductive process of Atlantic salmon is extreme at best, presenting feats of navigation, competition, and sacrifice. They are a wonderful demonstration of the unyielding will to reproduce present throughout nature, teasing death for the life of the next generation.
References
Hendry, A.P. and Berg, O.K., 1999. Secondary sexual characters, energy use, senescence, and the cost of reproduction in sockeye salmon. Canadian Journal of Zoology, 77(11), pp.1663–1675. https://doi.org/10.1139/z99-158.
Hruska, K.A., Hinch, S.G., Healey, M.C., Patterson, D.A., Larsson, S. and Farrell, A.P., 2010. Influences of Sex and Activity Level on Physiological Changes in Individual Adult Sockeye Salmon during Rapid Senescence. Physiological and Biochemical Zoology, 83(4), pp.663–676. https://doi.org/10.1086/652411.
Lohmann, 2023. Magnetoreception | The Lohmann Lab – University of North Carolina at Chapel Hill. Available at: <https://lohmannlab.web.unc.edu/magnetoreception/> [Accessed 20 November 2023].
Moore, A., Freake, S.M., Thomas, I.M. and Bone, Q., 1997. Magnetic particles in the lateral line of the Atlantic salmon (Salmo salar L.). Philosophical Transactions of the Royal Society of London. Series B: Biological Sciences, 329(1252), pp.11–15. https://doi.org/10.1098/rstb.1990.0145.
Morbey, Y.E., Brassil, C.E. and Hendry, A.P., 2005. Rapid Senescence in Pacific Salmon. The American Naturalist, 166(5), pp.556–568. https://doi.org/10.1086/491720.
Thao, D., 2018. Spawn of the Living Dead. [online] FISHBIO | Fisheries Consultants. Available at: <https://fishbio.com/spawn-living-dead/> [Accessed 20 November 2023].
United States National Park Service (NPS), 2019. The Salmon Life Cycle – Olympic National Park. [online] Available at: <https://www.nps.gov/olym/learn/nature/the-salmon-life-cycle.htm> [Accessed 20 November 2023].
Witten, P.E. and Hall, B.K., 2003. Seasonal changes in the lower jaw skeleton in male Atlantic salmon (Salmo salar L.): remodelling and regression of the kype after spawning. Journal of Anatomy, 203(5), pp.435–450. https://doi.org/10.1046/j.1469-7580.2003.00239.x.
Images Cited
Luke Gordon [@manta_luke], 2022. They become zombie fish, swimming with no eyes, or skin, driven by the innate urge to swim further upstream and spawn. The hatchery needs the strongest and fittest fish, healthy fish which are ready spawn in the next few weeks are caught. #zombie #salmon #nanaimo https://t.co/ErpStHa0g8. [Tweet] Twitter. Available at: <https://twitter.com/manta_luke/status/1483612471080325123> [Accessed 20 November 2023].
Ragan, 2023. Alaska’s Five Species of Pacific Salmon, Alaska Department of Fish and Game. [online] Available at: <https://www.adfg.alaska.gov/index.cfm?adfg=wildlifenews.view_article&articles_id=714> [Accessed 20 November 2023].