Understanding the Movements of Atlantic Salmon Parr in the River Tamar

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A fish trap is to be installed at Gunnislake to determine whether an autumn migration of  salmon parr (one year old salmon) is a significant event on the River Tamar. This will be followed, if trapping is successful, with an attempt in early December to locate the parr in the estuary.

The fish trap, which will look similar to the ones pictured below, will be positioned just downstream of Gunnislake Weir. It will be put into the water in mid/late October and left in position until early December 2012 at the latest, when it will be removed.

The type of trap to be used is called a Rotary Screw Trap (RST). The trap will consist of a rotating drum powered by the river flow using an internal Archimedes screw suspended between two pontoons. The rotating drum will move fish gently into a live-box at the rear of the trap. Any parr and other fish or creatures caught in the trap will be removed and monitored by Environment Agency staff tending the trap and then released on the downside of the trap, unharmed.

Atlantic salmon have a complex life history that begins in the upper reaches of rivers such as the Tamar, where eggs hatch and juveniles grow through several distinct stages. The first phase is the alevin stage when fish stay in the breeding ground to grow. Next is the fry stage where the fish grow  and subsequently leave the breeding grounds in search of food. The final freshwater stage is when they develop into parr (one-year-old salmon) in which they prepare for the long trek down river to the Atlantic ocean as smolts (two-year-old salmon), which typically occurs  in the spring.

There is, however, growing evidence that substantial numbers of Atlantic salmon parr migrate from their natal areas in the Autumn and relocate to downstream habitat to overwinter before continuing out to the open sea in spring. It is unclear to what extent this might occur on the Tamar.

The Centre for Environment, Fisheries & Aquaculture Science (Cefas) and the Environment Agency aim to establish how widespread this phenomenon is in England and Wales, especially on Index Rivers such as the River Tamar and whether the numbers involved are substantial compared to the overall run size.

Such movements may have significant implications for survival estimates and for habitat management. Fish that overwinter in the lower reaches of the river, possibly even in the estuary may have very different survival rates to those that do not relocate in the autumn.

A future issue of The Valley newsletter will highlight the results of this work.

For more information contact James Wimpress: James.wimpress@environment-agency.gov.uk

 

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