Atlantic Salmon and Trout: Life Cycle and Threats
Atlantic Salmon
Introduction: The Atlantic Salmon (Salmo salar) inhabits temperate and Arctic regions of the Northern Hemisphere, from Russia to Portugal. Populations have declined in some areas, and they are now extinct in some locations. They are found in Scandinavia, Ireland, and Northwest Britain. Atlantic Salmon are anadromous, migrating from the sea to rivers to spawn.
Life Cycle
Salmon return from the sea to their home river after one to four years. ‘Grilse’ are salmon that return to the river after only one year. Spawning occurs from October to February (mainly November to December). Females lay 450-750 eggs per pound of body weight. Eggs are laid in redds (depressions in the riverbed). The eggs are fertilized and covered with gravel. 90-95% of adults die after spawning. Depending on water temperature, hatching occurs in the spring. Alevins (newly hatched salmon) stay in the redd for about one week. Parr (young salmon) stay in the river for one to five years, depending on temperature and food availability. Smolts (juvenile salmon ready for migration) leave the river, typically in June, and migrate to the Norwegian Sea. The survival rate from egg to smolt is approximately 1%.
Feeding
Young salmon feed on aquatic insects, invertebrates, and terrestrial insects that fall into the river. Adults in the sea feed on small fish (capelin, eels, sprats, herring) and crustaceans. Adult salmon rarely feed in rivers.
Predators
- Birds: Goosanders, gulls.
- Fish: Cod, pike.
- Other: Sharks, seals.
Other Threats
- Habitat loss.
- Obstacles to migration (dams, weirs).
- Over-fishing of salmon or their food sources.
- Climate change.
Population Dynamics
Salmon numbers have been reduced in many countries. In Scotland, for example, while 10 million smolts go to sea, the return rate has decreased by 65%, representing a loss of 9 million fish at sea. Positive aspects include a 70% catch-and-release rate, and strong river catches on the east coast, particularly on the Tweed, exceeding those of England and Wales combined.
The west coast has seen declines, primarily due to aquaculture. Escaped farmed fish swim in both directions, interbreeding with wild salmon. Farmed fish are often more aggressive. The main issue is disease; both wild salmon and farmed fish are vulnerable to diseases such as Furunculosis, Infectious Salmon Anemia (ISA), Infectious Pancreatic Necrosis (IPN), and Ulcerative Dermal Necrosis (UDN). Sea lice and Gyrodactylus salaris (which can cause 95% mortality in smolts at sea) are also significant threats. Massive losses occur, but the causes are often poorly understood.
Trout
Sea Trout
Sea Trout (Salmo trutta) are the same species as Brown Trout, but are anadromous, migrating to sea after one to five years (usually two to three). Most return after one year. Most die after spawning. Their life cycle is similar to that of Atlantic Salmon, and their numbers are declining.
Brown Trout
Brown Trout (Salmo trutta) inhabit rivers, lakes, ponds, and chalk streams. Chalk streams are fed by chalk aquifers. They require fast-flowing water for spawning, shallow areas with cover for juveniles, and pools with cover for adults. Ranunculus (water crowfoot) provides excellent cover, but its growth is often controlled.
Life Cycle
Similar to salmon, eggs are laid in redds from October to January. Eggs hatch in 100-150 days. Alevins stay in the redd for 30-50 days. Brown trout can grow up to 10lb.
Feeding
Brown trout feed on aquatic insects, invertebrates, and terrestrial insects that fall into the water. River keepers sometimes provide supplementary compound feed.
Release of Reared Fish
The release of reared fish has contributed to the decline in wild trout numbers. Approximately one million Brown Trout are released annually. Rainbow Trout (Oncorhynchus mykiss) have been introduced from the US. Fish released from fisheries can be infertile. Wild trout are sometimes caught without eggs. The Environment Agency is considering banning the release of diploid fish, except for indigenous fish.
Predators
- Birds: Cormorants, herons.
- Fish: Pike.
- Mammals: Mink.
River Management
Siltation and Pollution
Climate change is leading to heavier rainfall. Run-off carries silt, nitrates, and pesticides into rivers. Silt covers the gravel, damaging spawning grounds, and can contribute to algal blooms. Fencing livestock helps prevent bank erosion. Fast-flowing water scours the gravel bed, keeping it clean. Excess water abstraction reduces water flow, leading to less dilution of pollutants. Water companies have duties to manage abstraction. Original abstraction licenses often had no volume limits.
Bank Vegetation
Thinning out trees near the river reduces shading. Tree roots help prevent bank erosion. Dense ground cover should extend slightly over the water. Fisheries sometimes over-manicure banks.
Stream Vegetation
Managing Ranunculus provides cover for fish, helps clean the gravel bed, and allows for good water flow.
Engineering
If a stream is too broad and shallow, it can be narrowed. Barriers can create pools. Banks can be reinforced to prevent erosion. Salmon leaps can be built to aid fish migration. Fishing provides economic benefits (selling fishing rights provides income for river keepers and habitat management), and release programs and fly life are important considerations.