Oncorhynchus

Many fish live in cold water.

These fish are called salmon and trout. They swim in the big ocean. Then they swim back to rivers.

Some males grow a hooked jaw. This happens during mating season. These fish are very strong. They move long ways to find home. They are a great food for people.

It is amazing to see them swim.

Many fish live in cold waters. We call this group Oncorhynchus. It includes six types of Pacific salmon and ten types of trout.

These fish often travel between the ocean and rivers. Some salmon are anadromous. This means they live in the sea but return to rivers to spawn, or lay eggs.

Other fish might stay in fresh water for their whole lives.

During mating season, some males grow a hooked jaw. This part is called a kype.

These fish are very important for food. However, many salmon populations are shrinking. In some places, there are less than 10% of the fish seen in 1850.

Fishing and lost habitats cause this decline. Also, salmon are getting smaller. This happens because fishing often catches the biggest fish. Smaller fish survive to have babies, so the group grows smaller over time. This can make it harder for the fish to thrive.

The Oncorhynchus genus is a group of amazing fish. It includes six species of Pacific salmon and ten species of Pacific trout.

These fish are mostly found in the cold waters of the North Pacific. Their homes stretch from the Bering Sea down to Taiwan and Mexico.

Some fish in this group live in the ocean and move to rivers to lay eggs. This way of moving is called being anadromous. Other fish might stay in fresh water for their whole lives. They are important predators in the water where they live.

Many of these fish follow a special path to find their way home. This is called natal homing.

Some salmon are semelparous, which means they die after they spawn. During mating season, males often grow a hooked jaw called a kype.

This hook is a secondary sexual characteristic. However, some fish like the steelhead are iteroparous. This means they can spawn more than once in their lives.

Parasites can sometimes make the journey harder for these fish. If they get sick, they might not finish their long migration.

Scientists have looked at the history of these fish for a long time. They found fossils of trout-like fish in the Clarkia Lake beds in Idaho.

These fossils date back to the Late Miocene, about 7 million years ago. This shows that these fish were in these areas long ago. One extinct species was called Oncorhynchus rastrosus.

It was a very long fish known as the spike-toothed salmon. The split between these fish and Atlantic salmon happened even earlier. Some experts think this split may have happened 20 million years ago.

Today, many Oncorhynchus populations are facing hard times. In the Columbia River basin, populations dropped below 20% of their original levels by 1933. By 2008, some experts estimated stocks were less than 10% of what they were in 1850.

Many fish are also getting smaller in size. For example, the mean body mass of sockeye salmon fell by 10% between 2000 and 2010.

This happens because fishing often targets the largest fish. Smaller fish survive to breed, which changes the whole group over time.

We can see how these fish connect to the world around us. People all over the world eat salmon as a food source.

Because of this, many people use hatcheries to grow more fish. Hatcheries try to help by raising fish to be released into the wild. However, hatchery fish can be much larger than wild fish.

These large fish might compete with wild fish for food or space. This can upset the natural order of the river. Protecting wild populations is a big job for many countries.

It helps keep the whole ecosystem healthy and strong.

The genus *Oncorhynchus* consists of sixteen living species of ray-finned fish. This group includes six species of Pacific salmon and ten species of Pacific trout.

These fish are mid-level predators that live in the coldwater tributaries of the North Pacific basin. Their native range is vast. It stretches from the Bering Sea coasts south to Taiwan in the west. It also reaches as far south as Mexico in the east. Most species prefer high-latitude cold waters. They are often found between the Russian Far East and the Pacific Northwest.

Many *Oncorhynchus* species follow unique life cycles involving migration. Most are migratory, meaning they move between different environments. Some are anadromous, which means they live in the ocean but migrate to freshwater to spawn. Others are potamodromous, meaning they migrate entirely within freshwater systems.

A defining trait of many species is natal homing. This is the ability of fish to return to the specific stream where they were born. Many Pacific salmon are also semelparous. This term describes organisms that spawn only once and then die. However, some fish like the steelhead are iteroparous. This means they have the ability to spawn multiple times during their lives.

Physical changes often occur during the mating season. Males frequently develop a secondary sexual characteristic called a kype. This is a hooked shape on the tip of the lower jaw. The name *Oncorhynchus* actually reflects this feature. It comes from Ancient Greek words meaning "bend" and "snout."

Migration can be a dangerous process for these fish. Parasites can infect individuals and make them weak. This often leads to a process called culling. In culling, infected fish are less likely to complete their long migration. This can result in a shortened lifespan for the fish.

The evolutionary history of this genus is quite deep. Fossil evidence suggests *Oncorhynchus* was present in inland areas long ago. Trout-like fossils found in the Clarkia Lake beds in Idaho date to the Late Miocene. These fossils are about 7 million years ago. This evidence shows that salmon and trout lineages diverged before the Pliocene epoch. Some scientists suggest the split from the genus *Salmo* happened even earlier. This divergence may have occurred as far back as the Early Miocene, about 20 million years ago.

One notable extinct species was *Oncorhynchus rastrosus*. Known as the spike-toothed salmon, it was a very long species from the Late Miocene to the Pliocene.

Scientists use different methods to study how these species formed. Mitochondrial DNA (mtDNA) research has been used to look at many trout and salmon species. However, these molecular results do not always agree with fossil research. Some researchers believe the divergence of different lineages followed the uprising of the Pacific Rim. This geologic event caused changes in climate and habitat. These changes acted as stressors that led to adaptation and speciation. One interesting example is the kokanee salmon. These are landlocked sockeye salmon that have evolved into a distinct biological species. They stay reproductively isolated by spawning at different times or using different food sources.

Many *Oncorhynchus* populations have faced significant declines since the mid-19th century. In the Columbia River basin, populations dropped to less than 20% of their pre-1850 levels by 1933. By 2008, experts estimated Pacific Northwest stocks were below 10% of those original levels. These declines are caused by overfishing, habitat loss, and competition with non-native species. For example, the yellowfin cutthroat trout became extinct due to the introduction of rainbow trout. Additionally, there is a widespread decline in the physical size of the fish. The mean body mass of sockeye salmon decreased by 10% between 2000 and 2010.

Human efforts to help often involve using fish hatcheries. Hatcheries aim to increase production and restore wild stocks. However, hatchery fish can create new problems for wild populations. Hatchery fish are often much larger than wild fish because they are selected for size. These large fish can out-compete wild salmon for food and space. They may also show more aggressive or predatory behavior.

Furthermore, fishing practices often target the largest fish. This creates a selection pressure where smaller fish are more likely to survive and breed. This change in body size can hurt the success of commercial fisheries and the health of the entire ecosystem.