Oncorhynchus spp. and Salmo salar
The global demand for salmon – that most excellent of table fish – has led to a considerably- sized salmon farming industry in both the northern and southern hemispheres, with large numbers of fish being reared in net-pens. Although the Pacific waters have five endemic salmon species, many salmon farmers prefer the Atlantic salmon, as it is easier to culture.
Inevitably, farmed Atlantic salmon escape from the pens, often in large numbers. This has the effect of having an alien species competing with native species for spawning and rearing habitat. Atlantic and Pacific salmon are genetically different, so interbreeding is unlikely, but when farmed Pacific salmon escape, they can interbreed, and may dilute genetically based survival traits of the wild stocks.
The graceful Atlantic salmon (Salmo salar) is a native of the North Atlantic Ocean basin and is widely distributed from the Arctic Circle to Portugal in the eastern Atlantic, from Iceland and southern Greenland, and from northern Quebec south to the Connecticut River. The Atlantic salmon is anadromous, that is they spend most of their adult lives in salt water, and migrate to freshwater rivers and lakes to reproduce. Other anadromous fish include the alewife, (Alosa pseudoharengus), American shad (Alosa sapidissima), striped bass, (Morone saxatili) and sturgeon (Acipenser spp.).
Salmo salar lives in fresh water for the first 2 or 3 years of life before migrating to sea. Their movements at sea are not well understood. Tagging has shown that while some salmon wander, the great majority return to the river in which they were spawned. Marine scientists do not fully understand how salmon are able to carry out this remarkable feat. Most believe that, given the salmon’s extraordinarily acute sense of smell (1000 times greater than a dog’s) they somehow retain the characteristic odour and taste of their parent stream via imprinting during migration.
Sea-run salmon grow up to 9kg, with the average commercially caught fish averaging around 5 kg. In salt water the fish are blue-green overlaid with a silvery coating. In freshwater the silvery coat is lost and becomes greenish or reddish brown mottled with red or orange. Atlantic salmon also have numerous cross-shaped black spots, scattered around the body. Colour shape and body markings on salmon can vary with age, sexual maturity and habitat.
Pacific salmon differ from Atlantic salmon in that they spawn only once and die soon after, whereas the Atlantic salmon may spawn more than once. The Pacific salmon belongs to the genus “Oncorhynchus”, in which there are five species. Although there are slight variations within the species, the each follow the same basic life cycle. Up to 4,000 eggs are buried in gravel nests, the size of the nest depending on the size of the female parent. The embryos incubate and hatch within the nest, emerging as salmon ‘fry’ in the spring. Each of the species follows a slightly different life plan, with, for example, some migrating to the sea almost immediately upon leaving the gravel nest, whilst others remain in freshwater for a number of years before migrating. Of the 4,000 eggs, only 800 will hatch, only 200 will reach the sea and ten will reach maturity, with barely two adults returning to spawn.
Chinook Salmon (Oncorhynchus tshawytscha) are also known as spings or kings. They live up to eight years, weigh up 36 kg, and are the most sought after game salmon. They begin migration to the sea within a few months of emerging from the gravel bed and can be identified by their small eyes, black gums at the base of their teeth and long black spots along their back and tail. While in salt water, the chinook has a dark back with a greenish-blue sheen. As it returns to its parent stream, the body colour darkens and it develops a reddish hue around the fins and belly. The teeth of adult spawning males become enlarged and the snout develops into a hook. In saltwater, chinook feed on large zooplankton, herring, sand lance and many other fish.
Chum Salmon (Oncorhynchus keta) are also known as keta or dog salmon. They live up to five years and weigh up to 11 kg. Chum migrate seaward almost immediately upon leaving the gravel nest and can be identified by their large pupils and mouth, a lack of spots on their back and tail and silver streaks on their tail. The base of the tail is also very narrow. Spawning chum show reddish bars across their sides, with some specimens also sporting blotches of grey or black.
Coho Salmon (Oncorhynchus kisutch) are also known as silvers or bluebacks and are highly prized both commercially and as a sports fish. It is the most widely dispersed of the Pacific salmon species. They live three years and weigh up to 10.5 kg. Coho can be identified by the whiteness at the base of their teeth with black at the edge of their gums. Adult cohos have silvery sides and a metallic-blue back with irregular black spots. Spawning males undergo colour changes and may exhibit bright red on their sides, bright green on their back and head, with darker colouration on their belly. They also develop a hooked jaw with sharp teeth. Most coho tend to remain close to the coast and prefer warm water – often moving south in the fall and winter months
Pink salmon (Oncorhynchus gorbuscha) are the smallest of the Pacific salmon and have the shortest lifespan – two years. Weighing up to 4.5 kg, pinks are also known as humpies as a result of the exaggerated humped back developed by males as they return to spawn. They are also the most abundant of the Pacific salmon species and can be identified by their small scales over silver bodies and large oval spots on their back and tail. As they mature, humpies develop blue backs with heavy oval blotches on the tail and upper body.
Sockeye Salmon (Oncorhynchus nerka) are the slimmest and most streamlined of the Pacific salmon; they are the most highly prized for their commercial value as a food product. Sockeye live four to five years and weigh up to 5.5 kg. Their slender bodies have small black speckles on their back with distinct large scales and a dark, spot free tail. Sockeye can begin life as far as 1600 km from the sea, often in rivers that feed into lakes, or in the outlets and spring-fed beaches of lakes.
After emerging from the gravel nest, the young sockeye spends up to three years in lakes usually downstream from their spawning area. Migrating juveniles, known as smolts, make the long run to the sea during May and June. They then range far out into the Pacific and the Gulf of Alaska, thousands of miles from their parent streams. As they return to spawn, they turn varying shades of red with pale green heads. The males develop large teeth and hooked jaws.
The best way to identify an Atlantic salmon is to look for the large black spots on the gill covers and back and absence of spots on the tail. Atlantic salmon have 8-11 rays in their anal fins while Pacific salmon have 11-13 rays.