Salmon

Salmon are a group of highly important fish found in cold waters.

They belong to the family Salmonidae. This family includes other fish like trout, char, and whitefish. Scientists divide true salmon into two main groups, or genera. The genus Salmo includes the Atlantic salmon. The genus Oncorhynchus includes the Pacific salmon. These fish are known for their incredible life cycles. They are also vital to humans for food and recreation.

Most salmon follow a process called anadromy. This means they move between fresh water and the salty ocean. The journey begins in shallow gravel beds in freshwater streams. Here, eggs are laid and eventually hatch into tiny fish called alevin. These alevin soon become parr, which have vertical stripes for camouflage.

After staying in the river for months or years, they become smolts. Smolts are silvery and ready to move toward the sea. To survive, their bodies must undergo osmoregulation. This is a process where the fish manages salt levels in its body. Hormones like cortisol, thyroid hormones, and insulin-like growth factor I drive this change. These hormones help the gills increase their ability to secrete salt.

Once in the ocean, salmon spend one to five years maturing. They eventually return to their freshwater birthplace to spawn. This return journey is often guided by olfactory memory. This means they use their sense of smell to find their way home. Most salmon return to the exact stream where they were born. However, some fish may stray and spawn in different river systems. The percentage of fish that stray depends on the specific species.

Salmon species vary greatly in size and appearance.

The Chinook salmon is the largest of the Pacific group. It is sometimes called the king salmon. In contrast, the Pink salmon is the smallest of the Pacific species. It usually has an average weight of only 1.5 kilograms. Sockeye salmon are often called red salmon in the United States. Some salmon undergo physical changes before spawning. Males might develop a kype, which is a curved jaw. Others might grow a large hump or canine-like teeth.

Scientists use fossils to study the history of these fish. The oldest known salmonid fossil is Eosalmo driftwoodensis. This fossil was found in British Columbia. It provides evidence that salmon existed at least 40 million years ago. This was during the Eocene epoch. At that time, the Atlantic and Pacific groups had not yet split. Analysis of mitochondrial DNA suggests the two genera diverged later. This separation likely happened 10 to 20 million years ago during the Miocene. This occurred long before the Quaternary glaciation began.

Salmon are economically significant around the world. Many people catch them as game fish in both salt and fresh water. Because they are a major food source, they are also farmed. Norway is currently the largest producer of farmed salmon globally. Chile is the second largest producer.

Some species have even been introduced to new areas. Chinook salmon now live in New Zealand and Patagonia. Coho and sockeye salmon have also established themselves in Patagonia.

Understanding salmon helps us understand complex ecosystems. They connect the nutrients of the ocean to freshwater rivers. Their migration patterns are a key part of the natural world. Some salmon, like the landlocked Atlantic salmon, stay in fresh water entirely. These are called potamodromous fish. They have evolved to live in lakes and rivers without ever entering the sea. Whether they roam the vast ocean or stay in a single lake, salmon remain a marvel of biology.