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Salmonid ecology

electro-fishing surveyElectro-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.

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Last Modified: 27 April 2014