The Atlantic
Salmon is the most economically important riverine fish
species in Scotland. It is one of only 7 trully native
species due to the last ice age (altantic salmon, brown
trout, arctic charr, eels, sticklebacks, powan and lamprey).
All other species are later introductions.
Threats come in several forms including predation,
competition and disease and parasite transfers.
Predation
As eggs, alevins, fry parr and smolts salmon
can be predated on by other fish, especially pike,
rainbow trout and perch. Some species such as roach
are thought to have arrived as live bait (now illegal).
The introduction of alien species is now illegal,
but
escapes from
rainbow
trout continue
to be a major
problem. It is estimated that 17 tonnes of rainbow
trout have escaped into Loch Tay in recent years.
Competition
Brown trout and Salmon compete for food.
Foretunately salmon seem to prefer faster waters and
if both species
are present it is likely that salmon will dominate.
Disease and Parasites
Perhaps the most well recognised is Sea Lice. Especially
on the West Coast where there are large numbers of
sea cages concentrated around the mouths of rivers
both salmon and seatrout have declined signigicantly
in numbers. The use of fallowing and chemical applications
has helped, but sea lice continue to be a serious
issue.
Less well know is the threat of the freshwater louse
argulus. Again this is a non native
species and is probably arrive on rainbow trout introduced
into put and take fisheries.
Where ever there are large numbers of fish in high
desnsities (fish farms) these act as a potential reservoir
for disease and parasites which in some cases can be
passed onto salmon. |
