Salmonid ecology
Electro-fishing
survey
Effective conservation and management of salmonid fish in rivers
depends in large measure on understanding the biological and
environmental processes that together interact to govern the
production of young fish in a given river. The distribution
of juvenile salmon and trout, both migratory and non-migratory, in
streams and rivers depends upon factors affecting both the spawning
behaviour of the adult fish and the growth and survival of the
juveniles. Key factors are the availability and distribution of
suitable habitat for the fish at different stages of their life
cycle. For example, the habitat required for good survival of eggs
and alevins differs from that for parr, and so
the fish must be able to find suitable habitat as their needs
change. Understanding these requirements is fundamental to the
conservation and management of stocks. It is also central to
our use of biological reference points (e.g. conservation limits) to determine the
status of stocks and regulate exploitation, because these values
are related to the capacity of the system to produce fish.
Spawning habitat
Studies undertaken by Cefas have shown that habitat degradation,
partly resulting from changes in land use, can create 'bottlenecks'
in the production of juvenile salmonids in many rivers in England
and Wales. One problem that has increased with the
development of more intensive agricultural practices, is the
deposition of fine sediment ('fines') in salmonid spawning
gravels. Spawning salmon and trout require areas of clean
gravel in which to excavate redds and lay their eggs, in order that
the developing embryos receive a good supply of oxygenated
water. Undue deposition of sediments at the spawning sites
prior to or during spawning and incubation blocks the water flow
and thereby cause partial or complete failure of spawning, and this
might also deter fish from using these spawning sites.
A number of factors, mostly caused by human activities, are
known to contribute to increases in sediment loads in streams, for
example enhanced erosion of land surfaces caused by certain farming
operations, bank erosion by farm livestock and factors that reduce
river flows, such as water abstraction and increased weed
growth.
Salmonids in the sea
Both salmon and migratory trout spend a significant part of
their lives in the sea, but very little is known about the factors
affecting the fish during this phase in their
life-cycles. Estimates of the numbers of Atlantic salmon
thought to be alive before they start being taken by fisheries have
shown a general decline in stocks in both North America and Europe
in recent decades. The patterns of decline in the North East
Atlantic are broadly consistent with the changes in marine survival
observed for a small number of monitored stocks, although there has
been clear variation between different rivers. Most of these
monitored stocks showed a drop in the proportion of smolts
surviving to return as one-sea-winter salmon around the late 1980s
and early 1990s, but while returns rates have remained low for some
stocks, they have increased again for others. Reductions in
marine survival were also recorded for many sea trout stocks during
the late 1980s, but while some stocks recovered quite quickly,
those in north-west Scotland and western Ireland continued to
decline dramatically.