BIOLOGY
FIGURE 2
Atlantic salmon migrate from their birthplaces in freshwater bodies across the North Atlantic to the Atlantic. The North American Stock spends the first phase of ocean life in the Gulf of Maine and off the coast of Quebec. They then travel to the rich feeding grounds of western Greenland, joined by the European stock. The North American sotck then returns to the coast of North America before returning to their spawning grounds. (6) |
Salmon are anadromous fish, meaning that they migrate upstream to spawn (2). As a result, their life cycle is different from most other commercial fish (2). Salmon require cold, sediment free waters, so they start their life cycle as eggs in freshwater rivers and streams (7). The pea-sized eggs take 6-12 weeks to hatch depending on the water temperature, and the eyes can be seen through the semi-transparent membrane (7, 8). When they hatch in early spring, salmon are called alevin, measuring just 2 centimeters in length (7, 8). Alevin feed from the yolk sac still attached to their bodies, and hide in the sediment to escape predation (8). After several weeks the yolk sac dissolves, and the alevin becomes fry (7, 8). Fry develop all 8 fins used to navigate the currents of the river, feeding off microscopic life (8). No longer hidden by sediment, fry are exposed to all types of predators including large fish and birds (7). Fry grow quickly, and reach about 5-8 centimeters before transforming into parr after one year of life (8). Parr develop camouflage in the form of vertical markings separated by red dots, known as parr marks (8). The time spent as a parr depends on the temperature and the availability of nutrients, but mostly takes 1-2 years (8, 9). In cold glacier rivers, salmon can stay parr for 5 years before becoming smolt (9). Once reaching around 18 centimeters in length, parr are ready to undergo smoltification (9).
Smoltification occurs late in the year, and allows the smolt to handle and regulate salt water in its bloodstream (7, 8, 9). Without this transformation, atlantic salmon would never be able to handle the high salt levels in the open ocean (9). Salmon are osmoregulators, meaning that they are able to compensate for water loss and too much water, and also maintain healthy a salinity level in the blood (9). Without being able to osmoregulate, the salinity of salmon’s blood would reflect that of the water, which changes drastically throughout the salmon’s life (9). Smolt feed on larval fish and crustaceans, and are preyed upon by seals and larger fish (9). It is at this stage that atlantic salmon start to become social, and clump together in small schools (9). However, salmon never gather in large enough groups to be called schooling fish (9). Smolt also change in appearance, from a brown-green color to a sleek silver resembling a Herring (9). Smolt head downstream, hanging out in estuaries and brackish waters to cope with the salinity before finally heading into the open ocean around spring (9).
Once reaching the ocean, salmon reach adulthood (10). Only 10% of alevin reach this stage (10). At sea, salmon feast on amphipods, krill, and small fish like herring while at sea (8, 11). Atlantic salmon migrate around the North Atlantic (8, 10). The North American stock move from their coastal feeding grounds like the Bay of Fundy before feeding off the coast of Quebec, and then migrating to the west coast of Greenland along with the European stock (6). Of all of the atlantic salmon’s stages, the adult life is the least explored (7). However, we do know that the time spent in the ocean is devoted to fattening up for the long journey back to their feeding grounds (7). Once back in freshwater, salmon do not feed (7). Once at sea, atlantic salmon are a type of pelagic fish called oceanic fish, inhabiting waters beyond the continental shelf 2-10 meters from the surface (11). After spending 1-2 years at sea, atlantic salmon reach sexual maturity, weighing around 18 pounds and 75 centimeters in length (2). Salmon who return after one year are called small salmon or grilse, and salmon who return after two are called large salmon, and usually carry far more eggs (12). The term SW refers to the the number of winters spent at sea (SW1 means grilse and SW2 means large salmon) (12).
From April to September, salmon travel upstream to the place where they were born, sometimes swimming more than 1,000 kilometers and jumping higher than 12 feet over dams and waterfalls (10). This period of time is called the salmon run, and scientists still do not understand what natal homing mechanism salmon use to find their way back to their spawning grounds, but believe it has something to do with smell (2, 9). During this breeding period, atlantic salmon change from their sleek silver to darker and bolder colors (7, 13). The head elongates, and the lower jaw grows a hook, called a kype (13). Salmon breed in a mass spawning event from July to September (11). This spawning is often referred to as big bang reproduction (11). Once a female accepts the male courtship involving kype display, the females release her roe, consisting of 1,000-3,000 eggs, in gravel depression called redds (10, 11). The roe is then fertilized by the male’s milt (sperm), who then pushes gravel over the redd for protection (10, 11). Of these eggs, only 0.1% at best will survive to reach sexual maturity (10, 11). The temperature the eggs incubate in determine the gender (14). Salmon can also change gender when introduced to chemicals such as pesticides that mimic estrogen and testosterone (14). After spawning, salmon are known as kelt (7, 8). Salmon are r-selective, and give no parental care after spawning (7, 8). Most kelts die within weeks of breeding (7, 8). Some atlantic salmon are iteroparous, meaning that they have multiple reproductive cycles in their lifetime (8). However, only 5% of kelts are able to swim downstream to the ocean, and return the next year to breed again (8). Salmon live to 4-6 years of age (8).
Because atlantic salmon are anadromous, they are a keystone species to the terrestrial ecosystems surrounding rivers and streams (1). During the salmon run, salmon transport millions of tons of nutrients from the rich atlantic waters to the forests of Maine, Europe, and Eastern Canada (1). Numerous organisms like bears, minks, and eagles feast on the salmon, and spread the nutrients throughout the ecosystem (1).
The atlantic salmon has several symbiotic relationships (1, 2). When lamprey eels lay prepare their nest sites in rivers, they clean the surrounding areas, improving the conditions for both adult salmon and alevin (2). In Maine, the freshwater mussels such as the yellow lampmussel improve the water quality in the salmon’s freshwater habitats (2). Beavers alter rivers and streams in a way that is favorable for spawning salmon, earning them the title of an ecosystem engineer (1, 2). It is not known if the lampreys or the mussel benefit from salmon activity, so these are all examples of commensalism (2).
Smoltification occurs late in the year, and allows the smolt to handle and regulate salt water in its bloodstream (7, 8, 9). Without this transformation, atlantic salmon would never be able to handle the high salt levels in the open ocean (9). Salmon are osmoregulators, meaning that they are able to compensate for water loss and too much water, and also maintain healthy a salinity level in the blood (9). Without being able to osmoregulate, the salinity of salmon’s blood would reflect that of the water, which changes drastically throughout the salmon’s life (9). Smolt feed on larval fish and crustaceans, and are preyed upon by seals and larger fish (9). It is at this stage that atlantic salmon start to become social, and clump together in small schools (9). However, salmon never gather in large enough groups to be called schooling fish (9). Smolt also change in appearance, from a brown-green color to a sleek silver resembling a Herring (9). Smolt head downstream, hanging out in estuaries and brackish waters to cope with the salinity before finally heading into the open ocean around spring (9).
Once reaching the ocean, salmon reach adulthood (10). Only 10% of alevin reach this stage (10). At sea, salmon feast on amphipods, krill, and small fish like herring while at sea (8, 11). Atlantic salmon migrate around the North Atlantic (8, 10). The North American stock move from their coastal feeding grounds like the Bay of Fundy before feeding off the coast of Quebec, and then migrating to the west coast of Greenland along with the European stock (6). Of all of the atlantic salmon’s stages, the adult life is the least explored (7). However, we do know that the time spent in the ocean is devoted to fattening up for the long journey back to their feeding grounds (7). Once back in freshwater, salmon do not feed (7). Once at sea, atlantic salmon are a type of pelagic fish called oceanic fish, inhabiting waters beyond the continental shelf 2-10 meters from the surface (11). After spending 1-2 years at sea, atlantic salmon reach sexual maturity, weighing around 18 pounds and 75 centimeters in length (2). Salmon who return after one year are called small salmon or grilse, and salmon who return after two are called large salmon, and usually carry far more eggs (12). The term SW refers to the the number of winters spent at sea (SW1 means grilse and SW2 means large salmon) (12).
From April to September, salmon travel upstream to the place where they were born, sometimes swimming more than 1,000 kilometers and jumping higher than 12 feet over dams and waterfalls (10). This period of time is called the salmon run, and scientists still do not understand what natal homing mechanism salmon use to find their way back to their spawning grounds, but believe it has something to do with smell (2, 9). During this breeding period, atlantic salmon change from their sleek silver to darker and bolder colors (7, 13). The head elongates, and the lower jaw grows a hook, called a kype (13). Salmon breed in a mass spawning event from July to September (11). This spawning is often referred to as big bang reproduction (11). Once a female accepts the male courtship involving kype display, the females release her roe, consisting of 1,000-3,000 eggs, in gravel depression called redds (10, 11). The roe is then fertilized by the male’s milt (sperm), who then pushes gravel over the redd for protection (10, 11). Of these eggs, only 0.1% at best will survive to reach sexual maturity (10, 11). The temperature the eggs incubate in determine the gender (14). Salmon can also change gender when introduced to chemicals such as pesticides that mimic estrogen and testosterone (14). After spawning, salmon are known as kelt (7, 8). Salmon are r-selective, and give no parental care after spawning (7, 8). Most kelts die within weeks of breeding (7, 8). Some atlantic salmon are iteroparous, meaning that they have multiple reproductive cycles in their lifetime (8). However, only 5% of kelts are able to swim downstream to the ocean, and return the next year to breed again (8). Salmon live to 4-6 years of age (8).
Because atlantic salmon are anadromous, they are a keystone species to the terrestrial ecosystems surrounding rivers and streams (1). During the salmon run, salmon transport millions of tons of nutrients from the rich atlantic waters to the forests of Maine, Europe, and Eastern Canada (1). Numerous organisms like bears, minks, and eagles feast on the salmon, and spread the nutrients throughout the ecosystem (1).
The atlantic salmon has several symbiotic relationships (1, 2). When lamprey eels lay prepare their nest sites in rivers, they clean the surrounding areas, improving the conditions for both adult salmon and alevin (2). In Maine, the freshwater mussels such as the yellow lampmussel improve the water quality in the salmon’s freshwater habitats (2). Beavers alter rivers and streams in a way that is favorable for spawning salmon, earning them the title of an ecosystem engineer (1, 2). It is not known if the lampreys or the mussel benefit from salmon activity, so these are all examples of commensalism (2).
FIGURE 3
The external anatomy of an atlantic salmon is similar to most fish, save for its adipose fin. This small fin located between the caudal and dorsal fin does not seem to have any importance, and is a characteristic of the Salmonidae family. Atlantic salmon have streamlined bodies, and also a distinct array of black spots above the lateral line. Like all salmon, the atlantic salmon has a small head that makes up ⅕ of its body. Their pelvic fins are also very prominent. (15) |