Salmon
Salmon is a common name for many species of fish from the family called Salmonidae. Some of the other fish in this family fall under the name of trout. You can most commonly find salmon swimming in the Atlantic and Pacific Oceans, the Great Lakes and some other land locked areas. Most salmon are anadromous, which means that they are born in fresh water. After they are born they migrate to the ocean and only return to the fresh water to reproduce.
There is no clear method, that we know of, that salmon use to navigate, though we do know that their strong sense of smell might have something to do with it. The salmon that live in the Pacific Ocean die a few days or weeks after they spawn. This is known as semelparity. There are very few salmon that will spawn more then once.
When the female salmon is ready to lay her roe, she uses her tail fin to create a shallow depression, which is called a redd. The redd can hold up to 5,000 eggs. Once the eggs are laid one or more of the males will approach the female while she is in the redd where he then drops his sperm over the roe. Afterwards the female will cover the eggs with gravel and then move on to make another redd. The female salmon can make up to 7 redds or until she has no more eggs left to drop.
When it is time for the eggs to hatch into alevin or sac fry, the fry will quickly develop into parr with camouflaging vertical stripes. The parr stay for one to three years in their natal stream before becoming smolts which are distinguished by their bright silvery colour with scales that are easily rubbed off.
It is estimated that only 10% of all salmon eggs survive long enough to reach this stage. The smolt body chemistry changes, allowing them to live in salt water. Smolts spend a portion of their out-migration time in brackish water, where their body chemistry becomes accustomed to osmoregulation in the ocean.
The salmon spend one to five years (depending on the species) in the open ocean where they will become sexually mature. The adult salmon returns primarily to its natal stream to spawn. When fish return for the first time they are called whitling in the UK and grilse or peel in Ireland. Prior to spawning, depending on the species, the salmon undergoes changes. They may grow a hump, develop canine teeth, develop a kype (a pronounced curvature of the jaws in male salmon).
All will change from the silvery blue of a fresh run fish from the sea to a darker color. Condition tends to deteriorate the longer the fish remain in freshwater, and they then deteriorate further after they spawn becoming known as kelts. Salmon can make amazing journeys, sometimes moving hundreds of miles upstream against strong currents and rapids to reproduce. Chinook and sockeye salmon from central Idaho, for example, travel over 900 miles and climb nearly 7000 feet from the Pacific ocean as they return to spawn.
The age of a salmon can be deduced from the growth rings on its scales, examined under the microscope. Each year, the fish experiences a period of rapid growth, often in summer, and one of slower growth, normally in winter. This results in rings (annuli) analogous to the growth rings visible in a tree trunk. Freshwater growth shows as densely crowded rings, sea growth as widely spaced rings; spawning is marked by significant erosion as body mass is converted into eggs and milt.
Freshwater streams and estuaries provide important habitat for many salmon species. They feed on terrestrial and aquatic insects, amphipods, and other crustaceans while young, and primarily on other fish when older. Eggs are laid in deeper water with larger gravel, and need cool water and good water flow (to supply oxygen) to the developing embryos.
Mortality of salmon in the early life stages is usually high due to natural predation and human induced changes in habitat, such as siltation, high water temperatures, low oxygen conditions, loss of stream cover, and reductions in river flow. Estuaries and their associated wetlands provide vital nursery areas for the salmon prior to their departure to the open ocean. Wetlands not only help buffer the estuary from silt and pollutants, but also provide important feeding and hiding areas.