Salmon are amazing fish that undertake incredible migrations across thousands of miles, all in an effort to reproduce. After swimming upstream to their spawning grounds, salmon stop eating altogether and eventually die shortly after spawning. This seeming act of self-sacrifice is actually an ingenious evolutionary adaptation that maximizes the survival chances of their offspring.
The Salmon Lifecycle
To understand why salmon fasting is so crucial, it helps to first understand the salmon lifecycle. Salmon are anadromous fish, meaning they hatch in freshwater streams and rivers, migrate to the ocean to feed and grow for 1-5 years, and then return to their natal freshwater streams to spawn.
The journey upstream against the current to the spawning grounds where the salmon hatched can be arduous. Salmon cease feeding during this migration and rely entirely on their energy reserves to battle upstream. Their bodies undergo an incredible transformation as well – they develop breeding coloration and protuberances and their flesh starts deteriorating.
Once at the spawning site, female salmon dig nests called redds in the gravel riverbed. They lay their eggs in these nests while male salmon fertilize them. Some species like Pacific salmon die immediately after spawning while others like Atlantic salmon may survive to spawn again.
The fertilized eggs are covered up with gravel by the female and left to incubate over the winter. Come spring, the eggs hatch into tiny salmon fry called alevins that stay buried in the gravel, living off their egg yolk sacs. Once the yolk is absorbed, the baby salmon called parr emerge from the gravel and start feeding. They will spend 1-3 years in their natal streams before migrating out to the ocean as smolts.
Why Salmon Stop Eating
With their bodies already deteriorating, why don’t salmon just continue eating to gather energy for the exhausting spawning process? There are several key reasons why salmon fasting is crucial:
Metabolic Changes
On their journey to the spawning grounds, salmon undergo metabolic changes that cause them to stop eating. Their stomachs shrink and their intestines shorten. Digestive enzymes stop being produced. Essentially, the salmon’s digestive system shuts down.
This metabolic change serves an important purpose – it allows the salmon to divert all their energy towards migration and reproduction rather than digestion. Without food intake, they rely on stored energy reserves and start deteriorating metabolically.
Difficulty Feeding
Even if their digestive systems were functional, it would be challenging for salmon to feed once they start the journey home. As they swim upriver, salmon battle strong currents and must leap up waterfalls and dams. Expending all their effort on migrating makes it hard to hunt for food.
Spawning salmon also undergo physical changes like developing a hooked jaw or humped back that make it tough to feed. With their bodies already falling apart after the long journey, they lack the ability to chase down prey.
Deteriorating Flesh
A salmon’s flesh starts deteriorating during the arduous journey to the spawning grounds. This is because they stop eating and their bodies start breaking down their own muscle proteins for energy. Males in particular undergo a more extensive deterioration.
By the time they reach the spawning grounds, their flesh quality is extremely diminished. Even if salmon did try to eat at this advanced stage of deterioration, their flesh would not be nutritious or palatable.
Lack of Appetite
With all their focus and instincts directed towards reproducing, spawning salmon simply lack any appetite to feed. Their sole biological drive is to mate and reproduce, even if it comes at the cost of their own survival.
Interestingly, their brains start producing large amounts of endorphins – natural “feel good” chemicals associated with pleasure. So while not eating or taking care of themselves, spawning salmon may be in an elevated euphoric state.
Short Lifespan After Spawning
Most salmon species are semelparous, meaning they die after one spawning. Some species like chum and pink salmon usually only survive a few weeks after reproducing. Even species that can spawn more than once like sockeye and Atlantic salmon may only live a few months more.
With such an imminent death approaching, there is no biological reason for salmon to eat and survive longer. Their sole evolutionary purpose is to pass on their genes by spawning. Once reproduction is complete, they deteriorate and die soon after.
Evolutionary Advantages
While it may seem paradoxical that salmon don’t eat after embarking on an energetically taxing migration, their fasting behavior is actually an ingenious evolutionary adaptation with several advantages:
Maximizes Reproductive Success
If salmon had to spend time and energy hunting for food during their final life stage, it would detract from their goal of migrating and spawning successfully. Fasting enables them to divert all their physical resources towards reproduction.
Given their imminent death after spawning, it makes the most evolutionary sense for salmon to ensure their genes get passed on rather than extending their own survival a bit longer after reproducing.
Provides Nutrients to Eggs
As salmon deteriorate during migration, their bodies break down fat, protein and other nutrients. Some evidence suggests that this liberated nutrition may be incorporated into the eggs, improving embryo survival.
So while not eating food directly, fasting may help salmon indirectly nourish their offspring.
Clears Spawning Grounds
If salmon survived for long after spawning, they would occupy space and compete for resources with their offspring. Their dead carcasses also provide an influx of nutrients to the river ecosystem.
By dying soon after reproduction, salmon make space and food available for their progeny to thrive.
Stronger Swimming Ability
Without having to expend energy digesting, salmon can devote their metabolism fully towards swimming upstream. Fasting enables them to effectively utilize their energy stores for the strenuous journey.
Some research indicates that fasting also provides cardiovascular benefits that may improve their swimming performance and survival during migration.
Impacts of Fasting
The fasting adaptation comes at an immense physical cost to individual salmon but provides evolutionary advantages to salmon as a species by maximizing reproductive success. Some key impacts include:
Extreme Weight Loss
Studies across salmon species have found they can lose up to 20-30% of their body weight during migration and fasting. Some sockeye salmon may shrink from 25 pounds down to just 7 pounds!
This starvation and deterioration provides nutrition to their eggs but comes at the cost of their own health.
Diminished Immune Function
Loss of fat and muscle during fasting likely impairs salmon’s immune system. This makes them more vulnerable to viruses, bacteria, parasites, and fungal infections during migration.
Diseases like salmon lice, fungi, bacterial kidney disease, and viruses may rapidly spread between immunocompromised salmon crowded together on spawning runs.
Oxidative Stress
As with starvation in other animals, salmon fasting creates cellular oxidative stress as their bodies start consuming their own proteins and lipids for energy. Oxidative damage accumulates in their tissues.
While not eating provides evolutionary advantages, it comes at a high cost of oxidative stress to individual salmon at the cellular level.
Increased Predation
Bears, eagles, sea lions, and other predators feast on salmon runs. In their weakened state, migrating salmon are easy prey.
While some predators consume post-spawn salmon carcasses, others eat the energy-rich salmon preparing to spawn. Fasting makes them an attractive target.
Population Declines
Many salmon populations are threatened or endangered. Their complex life cycle, reliance on high quality habitat, and fasting behavior makes them vulnerable to environmental pressures like dams, pollution, overfishing, and climate change.
Supporting healthy salmon fisheries while safeguarding fragile spawning populations remains an ongoing challenge.
Unanswered Questions
While we know that fasting provides evolutionary advantages, many unanswered questions remain about how individual salmon are impacted:
- What genes and signaling pathways control the metabolic shifts that cause fasting?
- Why do some species like steelhead continue feeding longer into their spawning migration?
- How does fasting impact salmon susceptibility to disease and pollution exposure?
- What determines whether a salmon can spawn again or undergoes rapid post-spawn death?
- Could shorter migrations and reduced fasting improve survival rates?
Ongoing research into the genetic, hormonal, and environmental mechanisms that control salmon fasting and deterioriation may reveal ways to support healthy populations.
Conclusion
In conclusion, salmon fasting provides an excellent example of the incredible power of evolution. While seemingly maladaptive for individual survival, their fasting enables maximal reproductive success and transfer of genes to the next generation.
Yet this evolutionary strategy comes at great physical cost to the salmon, requiring immense energy reserves to migrate and spawn without feeding. As human impacts like dams and overfishing impinge on salmon habitats and health, it highlights the fragility of their fasting adaptation.
Conserving salmon remains critical, as they provide invaluable marine nutrients to the forests and animals they once called home. Their fasting journey connects marine and freshwater ecosystems, reminding us of salmon’s important place in aquatic food webs and the ongoing need to manage our impacts on their survival.
