Atlantic salmon can survive and reproduce multiple times, unlike their Pacific salmon relatives. While many do die after spawning, a significant portion recovers and returns to freshwater to spawn again. This ability is central to their life history.
Understanding Iteroparity
Atlantic salmon exhibit iteroparity, a reproductive strategy where an organism undergoes multiple reproductive cycles over its lifespan. This stands in direct contrast to semelparity, a strategy seen in most Pacific salmon species, where reproduction occurs only once, followed by death. Iteroparity is favored when there is high variability or low survival rates among juvenile offspring, allowing individuals to spread their reproductive risk across several years. Conversely, semelparity may be favored when adult survival is low, making a single, massive reproductive effort more advantageous.
The evolution of iteroparity in Atlantic salmon reflects a trade-off in energy allocation between current and future reproduction. Unlike semelparous species that invest all their energy into one reproductive event, Atlantic salmon conserve enough resources to facilitate recovery and subsequent spawning attempts. This biological difference allows them to contribute to multiple generations, enhancing their long-term reproductive success in fluctuating environments.
Physiological Keys to Survival
Atlantic salmon possess physiological adaptations for post-spawning survival. Their energy management during spawning migration is crucial; they rely on stored energy reserves, primarily lipids. Both male and female Atlantic salmon experience substantial energy loss during spawning, yet retain sufficient reserves to begin recovery.
Their ability to manage and recover from the physiological stress of spawning involves less severe hormonal changes compared to semelparous species and a robust immune system. Atlantic salmon also demonstrate tissue repair and regeneration, vital for healing physical damage sustained during migration and spawning. Mesenchymal stromal cells (MSCs) in salmon skin play a role in wound healing and tissue remodeling, indicating a broader regenerative capacity in their tissues, including gills.
Atlantic salmon can re-adapt between freshwater and saltwater environments through osmoregulation. After spawning, their gills and kidneys undergo physiological changes, allowing them to regulate salt and water balance upon returning to the ocean. This reversible adaptation, managed by ion transport in their gills, is crucial for survival in varying salinities.
The Spawning and Recovery Process
Atlantic salmon migrate from the ocean to their natal freshwater rivers to spawn. During this upstream journey, they navigate challenging currents and obstacles, relying on stored energy as they cease feeding. Females dig “redds” in gravel to lay eggs, fertilized by males. This exertion and lack of food cause significant physical deterioration and injuries.
After spawning, Atlantic salmon, known as “kelts,” begin recovery. They regain strength and heal from injuries sustained during spawning. This recovery is crucial before their downstream migration back to the ocean. In the marine environment, they resume feeding, replenishing energy and rebuilding body condition. Successful recovery allows them to return for future spawning.
Environmental Influences on Survival
While physiological adaptations aid Atlantic salmon survival, external environmental factors impact their post-spawning success. Water quality and temperature in rivers and estuaries are important for recovery; unsuitable conditions increase stress and reduce survival. Elevated temperatures can negatively affect osmoregulation and stress responses, leading to mortality.
Predation threatens Atlantic salmon during and after spawning. Migrating salmon face predators like seals, otters, fish, and birds. Food availability in the ocean post-spawning is important for kelts to regain energy for subsequent migrations. Their ocean growth rate also impacts marine survival.
Human activities influence Atlantic salmon survival. Fishing pressure, habitat degradation, and dams can reduce their ability to return to spawning grounds or recover. Migration barriers like dams can delay or prevent salmon passage, affecting spawning success and survival.