With the last post being about dams I believe it appropriate
to talk about salmon in this post.
The salmon is an amazing creature, it is born in fresh water
before migrating down to the sea and adapting for a salt water life before then
returning to fresh water to breed and die. This is not a simple process it
requires some serious changes within the physiological structure of the salmon
so as they are capable to maintain the composition of their body fluids. To be
able to cope with these extreme changes in salinity salmon alternate the amount
of fluid they intake, freshwater fish avoid water gain by producing large
amounts of urine which they do not excrete to raise their osmotic potential,
while fish which are leading a marine existence change their habits and consume
more water so as to produce dilute urine leaving their body fluids with a lower
osmotic potential which is then filled by the salt water. In addition to this
process marine fish are capable of secreting excess salt through their gills
while fresh water fish are capable of absorbing salt.
Salmon do not just change their methods of maintaining their
body fluid osmolality they also go through changes in their diets throughout
their lives. Upon hatching the tiny salmon feed on invertebrates and plankton,
once mature however the adult salmon is a large predator located near the top
of the food chain, it is capable of catching larger prey such as squid, shrimp
and other fish. Depending on the sub
species of salmon they can grow from 4 to 110 pounds and vary in length from
20inches to 5ft, with Cherry salmon being the smallest and Chinook salmon being
the largest.
Once salmon reach sexual maturity they begin the long
journey back to the same stream they were born in. This journey marks the start
of the end for a salmon and it is triggered by day length and water temperature.
Once entering fresh water salmon stop feeding and their stomach and the rest of
their digestive tr
act begins to disintegrate so as to maximise the space available
for egg development. Male salmon also undergo physiological changes and their
snouts develop into a longer hooked shape called a kype, they also change
colour and become redder. 
Once the salmon reach the same area in which they were
hatched, which is usually an area of shallow well oxygenated water, they can
breed. A female then attach their eggs to vegetation or rocks before males
release their sperm into the water for external fertilization, this is a milky
fluid and in some areas has been known to fill the streams before dispersing. After
spawning the majority of salmon die within the following few days, some females
do remain alive long enough to guard their nests for a couple of weeks but none
remain alive for the entire gestation period of the eggs which is around four months.
This process of mating and then dying is not uncommon in the
natural world, it is known as semelparity or suicidal reproduction, many plants
as well as insects, frogs, lizards and a handful of mammals exhibit this
behaviour. Semelparity is generally the
outcome due to complete exhaustion and depletion of resources it an attempt to
maximise breeding success, which is the ultimate aim of any organism. The death
of the previous generation is also beneficial for the following generation as
it means a reduction in competition for resources such as food and shelter.
Finally global climate change has already had effects on
migratory fish such as salmon; this is due to the fact that longer hotter and
drier summers mean that river levels are lower and so salmon struggle to reach
their spawning grounds high upstream.
Although many of you
may have already known a lot of this information I hope you found out a few new
things, let me leave you with the fact that the highest recorded salmon jump is
3.7m (12ft) at Orrin Falls.
Thanks for reading
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