Salmon, Cod and Trout. Salmon have a most interesting life. One that takes them from the rivers and streams to the high seas of the Ocean, and back.

Salmon, Cod and Trout

Salmon have a most interesting life. One that takes them from the rivers and streams to the high seas of the Ocean, and back again. In fact, right back to the very place they were born. How they find their way back from the immensity of the Ocean is a small feat in itself. 

Salmon are anadromous fish - they live in the sea but reproduce in fresh water (in a stream or lake). They are amazing fish that live in fresh water during their early life, mature in salt water, and then return to fresh water to breed (and then die). Some salmon travel up to 1,600 km upstream in order to spawn.

Starting out as small eggs in a stream bed, they hatch and begin their journey downstream towards the ocean. They spend a couple of years in the streams and rivers growing from small alevin to juvenile smolts. At the mouth of the streams and rivers, the smolts school together and ready themselves for the trip out into the ocean. During this time, their bodies change to adapt to the seawater. The young adult salmon then head out to sea and spend several years swimming in the ocean. Once they have fully matured, they will swim back to their original stream or river where they re-adapt to the fresh water and swim back up the stream to reach their spawning grounds. Sometimes this means swimming up rugged rivers with miles of rapids and even waterfalls to leap. Once they get back to their natal stream, they breed and lay their eggs. After spawning they generally die within a week, fertilizing the stream and creating a nutrient-rich environment for the new infant salmon that are about to hatch.

Eggs (Ova) Salmon are born in gravel nests at the

bottom of stream and river beds in the form of a slightly translucent eggs about the size of a pencil eraser. The eggs are usually red to pink in color and spherical in shape. During the 2-3 month period it take the eggs to hatch, their eyes and other organs can be seen developing through the translucent shell of the egg.

When the salmon egg is ready to hatch, the baby salmon will break free of the egg's soft shell retaining the yolk as a nutrient-rich sac that hangs below it's body. At this stage, they are called Alevin and are about one inch in length. During the next month, the alevin will remain hidden in the gravel nest and feed from the nutrient-rich yolk sac until it is completely absorbed.

The tiny salmon leave their gravel nest and begin to swim and feed for themselves. At this stage they are called Fry and take the form of tiny fish. It's also at this time that they start their journey downstream. The first part of their journey is a difficult one as the small vulnerable fry must hide under rocks and among vegetation to avoid predators such as birds, insects, and other fish. At the same time, they must find feed to survive.

After several months, as the Fry feed and grow, they develop vertical markings on the flanks of their bodies. At this stage they are called Parr and are about six inches in length. Though a bit bigger they still must hide from predators and continue their journey towards the ocean. Parr will continue to feed for 1 to 3 years before they are ready venture out into the ocean.

At this point, the juvenile salmon loses its vertical markings on its body and turns silvery in color. Now considered Smolt, they will school together in large groups. It's at this time that the young salmon will adjust their bodies to saltwater, allowing them to swim out into the Ocean to feed and grow into adult salmon.

Adult salmon spend 1 to 4 years in the ocean swimming and feeding. They grow to their adult size and develop unique adult markings that are different for all species of salmon. There ocean journey is long and hazardous, as they are constantly hunted for by seals, orca whales, and fishermen. After swimming throughout the Ocean they return to their original spawning grounds to spawn. In some cases, young adult salmon return early before they have fully grown.

Upon reaching their birth rivers and streams, the adult salmon re-adapt to the fresh water and begin their upstream journey to their natal stream where they were born. At this time, they cease to feed and live on the stores of fat within their bodies. Their upstream journey is a challenging one, swimming upstream against rugged rapids, leaping over rocky waterfalls, traversing fish ladders, avoiding fishermen nets and hooks, and staying clear of hungry bears. When they finally reach their natal stream they have reached sexual maturation and are ready to spawn. The female adult clears a spot in the streambed by sweeping her tail back and forth creating a gravel nest that is referred to as a redd. She will then lay her eggs in this redd and the male adult salmon will fertilize and protect them until both salmon die within a couple of weeks and leave the embryos to fend for themselves.

Reproduction: Salmon live most of their life in the sea, but when they are mature and ready to breed, they enter fresh water to spawn (reproduce), traveling to a stream or pond high in oxygen. The female digs a nest in the gravel (called a redd) with her tail. She then pushes her thousands of eggs into the nest and the male milks the eggs, fertilizing them. Most salmon die after spawning.

The Eggs Hatch and Grow: The newly-emerged salmon (called alevins) still have a food sac attached to them. When the food sac is used up, the salmon fry emerges from the nest - and must find food (like insects) for the first time. As the fry matures, it becomes camouflaged (with parr marks) and is called parr . When it becomes silver-colored, it will be called a smolt. After growing for a while, the smolts swim downstream to the sea.

Adapting to Salt Water: When smolt reach the estuary (where the river meets the sea), a process begins in which their body changes, allowing them to soon live in salt water (this is called smoltification).

Maturing at Sea, then Returning Home: The salmon lives in the sea until maturity (1 to 7 years, depending on the species); some migrate thousands of miles in the sea. They then return to the place where they hatched and continue the cycle. No one knows how salmon return home -perhaps they remember the distinctive set of smells along the way. On their journey home, they do not eat at all, they often change color, their muscles soften, and they will die soon after spawning.

Anatomy: Salmons have silvery skin with spotted back and fins. The biggest salmon is the chinook, which weighs up to 120 pounds (55 kg).

Diet: Salmon are carnivores (flesh eaters) - they eat fish (like herring and pilchard), squid, and crustaceans (like shrimp).

Predators: Salmon are preyed upon by many animals, including bears, people, many birds (like wading birds and kingfishers), and other fish. For every 8000 eggs produced, 4500 alevin survive, from which 650 fry survive, from which 200 parr survive, from which 50 smolt survive, from which only 2 spawning adults survive (who produce thousands of eggs).

Classification: Kingdom Animalia, Phylum Chordata, Class Osteichthyes (bony fish), Family Salmonidae (salmon, trout, and char), Genus Salmo (Atlantic salmon - salmo means jumper) and species Oncorhynchus (Pacific salmon - 5 species; Oncorhynchus means "hooked snout").

1. Do salmon hatch in salt water or fresh water? __________________

2. After maturing enough to swim well, does the young salmon swim upstream or downstream? _______________________

3. What is the name of the area where a river meets the sea (and where salmon undergo body changes that let them live in salt water)? _______________________________

4. Where do salmon live until they are mature and ready to reproduce? ____________________

5. What sense do scientists think that salmon use to return to their birthplace? _________________

The life-cycle of this fish species is well understood.

Larvae hatch from eggs, then develop into juveniles. The juveniles then grow and become adults, which then mature and spawn to give rise to the next generation.

Spawning Adult cod form spawning aggregations from late winter to

spring. Females release their eggs in batches, and males compete to

fertilise them.

Pelagic Larval Phase Fertilised eggs drift with ocean currents and develop into

larvae.

Adults Age of maturation varies between cod stocks, from ages 2 to 4

in the west Atlantic, but as late as 8 years in the northeast arctic.

Cod can live for 13 years or older, but as late as 8 years in the northeast arctic.

Spawning trout require a stream or river with a gravel bottom that is free from silt. The female digs a redd (egg nest) in the gravel by turning onto her side and beating the gravel away with her tail. In this depression or shallow hole, the female will lay as many as 4 000 eggs. The male will release milt on the eggs to fertilize them. Trout do not stay to guard the eggs, but the female will cover them by moving gravel onto them with her tail.

Depending on the species, trout eggs will incubate from three to five months. As a result of this long period, the eggs are subject to disease and predation. The time of incubation depends on the temperature of the water flowing over the eggs. Generally the warmer the water the faster the eggs develop. However, temperatures above 10-15°C result in water with lowered amounts of dissolved oxygen, which may be fatal to the eggs.

Eggs require oxygen-rich water owing to the relatively thick membrane covering them. Fast flowing water is ideal since the water turbulence mixes oxygen into the water, keeping the dissolved oxygen level high. Warm or slow moving streams or lakes are typically not good habitats for trout spawning.

  Eggs hatch in the gravel as alevins or sac-fry. These are about 2.5 cm long with the yolk sac still attached to them.  

Nutrients continue to be absorbed directly into the blood from the yolk sac for about 10-20 days. During this time, alevins remain hidden in the gravel as a protection from predators.

After several weeks, they emerge as fry (5-10 cm) and begin to feed on plankton and bits of free floating organic matter.

Upon reaching a length of 10-15 cm, trout young are referred to as fingerlings