Pacific Salmon: Amazing Long-Distance Travellers

 

Photo: Al Harvey/Slidefarm

Because of their remarkable drive to reproduce in their place of origin, salmon migrate hundreds of miles from the ocean - where they spend their adult lives - to the fresh water rivers where they spawn.

Their journeys often involve battling upstream against strong currents and rapids.  The exact mechanism of how they navigate is not entirely understood; however, their keen sense of smell is thought to be involved.  Once they have spawned, these individuals die within a few days or weeks.

There are five species of Pacific salmon: Chinook, Chum, Coho, Pink, and Sockeye, which range from northern California to Alaska.  Their flesh tends to be orange to red in colour due to the krill and other crustaceans they eat.

Photo: Karen Miller, Creative Commons

Lifecycle: Each salmon starts out as one of about 5000 eggs laid in a shallow depression in the riverbed gravel known as a redd formed by the female prior to laying eggs which are then fertilized by the male.  After these eggs have hatched into fry and then quickly developed into parr with camouflaging vertical stripes, they remain in these streams for one to three years, depending on the species.  Only about 10% of all salmon eggs survive to this stage.  They then leave the rivers to swim to the oceans at which time their body chemistry changes to allow them to live in salt water.  Again depending on the species, they spend anywhere from one to five years in the ocean as adults.  Once they become sexually mature, they return to their native streams to spawn.

Threats to salmon: Climate change may cause shifts in the ocean range of the salmon.  For example, as waters warm, their feeding grounds may shift further northward, thus increasing the travel time and energy required to return to the southern rivers for spawning.  Additionally, receding glaciers and changing precipitation patterns may affect the flow of cool water in rivers.  River temperature highs are increasing - so much so that before 1930 no fish experienced a temperature above 18 degrees C. However, 6 of the 15 years before 2005 saw highs exceeding 19 degrees, and by 2100 every second year may be warm or warmer.

Other problems for wild salmon originate in fish farms, including the transimission of sea lice and other diseases.  As well, antibiotics and biocides fed to farmed salmon can pollute the oceans and accumulate in the flesh of wild salmon.  The impact of interactions with escaped farmed Atlantic salmon and wild Pacific salmon is not fully known at present although DNA analysis proves that escaped Atlantics are able to successfully reproduce and compete for food and breeding space in the wild.

Photo: Andrew S. Wright

Overfishing by commercial and recreational fisheries can also have a significant impact.  There can be effects even if salmon are not the target per se.  For example, bottom trawling which destroys groundfish habitat can affect productivity and cause shifts in salmon feeding grounds.  As well, overfishing of herring and other forage species can alter ecosystem dynamics.

Another significant threat is habitat destruction of streams and rivers due to a variety of human activities including agriculture, forestry, mining, hydro projects, and urban development.  Any of these can destroy stream- and riverbeds, reducing the salmon’s ability to reproduce.