Adult Atlantic salmon return to their river of origin and can enter freshwater throughout the year. The salmon find their home river using the sense of smell (olfaction). The larger fish that have spent more than one year feeding at sea (multi-sea winter: MSW) tend to return earlier in the year. The smaller fish that have spent only one year at sea (grilse) return later. However, the timing and duration of the spawning migration depends on the population, the size and location of the river, and when the conditions in the river are favourable for upstream migration.
Water flow is the main environmental factor stimulating salmon to enter the river, although other factors such as water temperature, tidal cycle and water quality may all be important. Adequate flows are essential for the entry into the river and adults will often wait just off the coast or in the estuary until river flows are suitable. Once the salmon have entered the river, the spawning migration normally includes a quiescent period where fish reside for long periods within pools or below obstructions, where the water is well oxygenated. During this period the salmon do not feed and yet may be caught by anglers using a variety of baits, lures and artificial flies.
The final spawning migration, when the adults move from the pools to the spawning area, is controlled by river flow and temperature but also by the sexual maturity of the fish. During this period the sense of smell becomes more sensitive and the fish begin to detect the pheromones released by other mature salmon in the river. Small increases in river flow due to rainfall (freshets) are also important in stimulating the upstream movement of the fish to the spawning areas.
Various factors can impact on salmon during their upstream migration prior to spawning, for example, obstacles such as estuarine barrages, tidal lagoons, weirs and hydropower structures, as well as changes to the physical characteristics of the river. Climate change can also pose a threat to upstream migration by altering flows and other river characteristics.
Once spawning has been completed, any surviving fish return to the sea. These fish are known as kelts and they can contribute further to the population by returning a second or, exceptionally, third time to spawn. Seaward migration of the kelts occurs after a period recovering from spawning and is also controlled by river flow and temperature.
The adult spawning migration can be affected by a wide range of environmental and man-made conditions in our rivers. For example, barriers to migration such as weirs and renewable energy schemes may delay or inhibit the movement of the adult salmon. However, suitable river flows and temperature are the most critical factors in allowing the salmon to migrate successfully to the spawning grounds. Any reduction in the river flow due to abstraction and climate change (droughts) may also interfere with the timing and duration of the spawning migration.
Understanding the requirements of the returning adult salmon and the challenges they face during the freshwater spawning migration is vital for managing stocks effectively. The use of tags and tracking to follow the movements of adults from the estuary freshwater to the spawning areas has been an important tool in the research on salmon migration. Research on the sense of smell and its role in controlling upstream migration and spawning has also improved our ability to conserve stocks of salmon.
Recent investigations have highlighted the following important issues for returning salmon:
- Suitable river flow and temperature are critical to the successful spawning migration of adult salmon.
- Some sheep dip insecticides and herbicides may affect the sense of smell of returning adults reducing their ability to remember their river of origin and detect the pheromones released by other spawning salmon.
- Barriers to migration, such as barrages, weirs and renewable energy schemes may delay or inhibit the upstream migration of spawning adult salmon.
- Climate change will have a significant effect on the migration of spawning salmon, influencing the amount of rainfall and variations in the water temperature.
Adult Spawning Migration Reference List (PDF, 154 KB)