Salmon redd spawning study
Salmon return to our rivers from October to December. © Laurie Campbell
BACKGROUND
Elinor Parry completed her Masters in research last year at Cardiff University, working with the GWCT’s Salmon and Trout Research team at East Stoke, Dorset. She spent six months analysing a long-term dataset on the location of salmon redds on the river Frome and the effect river flow may have on redd distribution.
KEY FINDINGS
The location of Atlantic salmon redds (egg-laying sites) varies across the Frome, with densities increasing until the middle reaches of the river, after which they decline. This pattern in redd density throughout the Frome changed over time, and was affected by flow during the critical spawning migration period (October and December) each year. As flow decreased, the redds became aggregated in the middle reaches. As flow increased from these drought conditions, redd density became more evenly distrib- uted across the river. Under extreme low flows, high redd density may lead to poor salmon recruitment, as juvenile salmon survival is density-dependent.
Elinor Parry Stephen Gregory 16 | GAME & WILDLIFE REVIEW 2016
Whereas the rest of nature seems to be slowing down in autumn, below the river surface one of Britain’s long-distance travellers is busy returning to our waters. Atlantic salmon can travel as far as the coast of Greenland in search of food and each year return to their natal river to breed. Once the females find an area of fast flow and clean river gravels, they spawn within a shallow depression in the gravel which they excavate with their tail. These nests, known as redds, can contain thousands of eggs and the future of the river’s salmon. Few females survive to mate again, so reaching the river and locating suitable spawning grounds is crucial not only for each individual, but the survival of the river population. Over the past 30 years, the Environment Agency has been gathering data on the distribution of redds along the Frome, and our task was to understand how river flow may be affecting this distribution of redds. Understanding where salmon spawn is important for conservation, because the survival of juvenile salmon is highly density dependent. Under extreme low flow, it is possible that salmon may struggle to reach spawning grounds further upstream. This could lead to more redds in the middle reaches of the river, and a more competitive environment for emerging fry. We aimed to answer two questions regarding Atlantic salmon redds and flow:
firstly, is there a pattern in redd distribution over time, and secondly, does this pattern in redd density change with flow. To do this, we created a database of salmon distri- bution along the Frome by collating data between 1980 and 2015 from old maps, river bailiff totals and the modern redd distribution surveys. We then had to decide, crucially, what time of year over which to summarise flow. After discussions with other researchers, it was agreed that we should use average flow during October to December each year because it is the most crucial time period for salmon migration to the spawning grounds.
Using statistical modelling techniques, we were able to reveal both a pattern
in redd distribution and that this pattern was related to river flow. We found that salmon redds gradually increased in number until the middle reaches, after which they decreased. This was expected as good quality habitat, such as ample river gravels and fast flowing riffles, are present in the main river and middle reaches of the Frome. We then analysed how flow may be affecting this distribution. What we found was that under extreme low flows, redd density increased in the middle reaches, and as flow increased the distribution of redds became more uniformly spread across the river.
www.gwct.org.uk
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