Poor runs of wild salmon predicted following low survival of young salmon


The low number of juvenile salmon (called smolts) leaving the River Frome in Dorset for their feeding grounds in the North Atlantic in the spring of 2017 (called the smolt run) quantified by the Game and Wildlife Conservation Trust in collaboration with Cefas and the Environment Agency is likely to result in a decline in the numbers of adult salmon returning to our rivers in the next few years. This work forms part of a project called SAMARCH part funded by the EU’s Interreg Channel Programme.

In fact, the estimated numbers of smolts leaving the River Frome in spring 2017, 4,381 ±670 was the worst on record and less than half of 10-year average (9,689) and 2,000 fewer than the second worst year on record (2012; see Figure 1).

Dr Rasmus Lauridsen, Head of Fisheries Research for the Trust, said: “The number of young salmon leaving our rivers has a direct effect on the number of adults returning to spawn and, with adult numbers already at an all-time low, this is worrying data for an iconic and economically important species.”


Figure 1: Estimate of number of salmon smolts
emigrating from the River Frome with 95% confidence intervals

Juvenile salmon (also known as “parr”) grow fast in southern English chalk streams and >95% smoltify and leave the River Frome at one year old. On rivers, in the rest of the UK, most salmon smolt leave for the sea at 2-3 years old. The poor 2017 smolt run on River Frome was, therefore, a result of recruitment from the adult fish that spawned in the winter of 2015/16.

Low numbers of young salmon were already apparent during our annual parr tagging campaign in late summer of 2016, for which the catchment population was estimated to be 35,151, which is substantially fewer than the 10-year average (91,353).

Poor recruitment of salmon from the 2015/16 spawning season has been observed in many rivers across England and Wales. See, for example, this press release from Natural Resources Wales. This suggests that it was a national, rather than a local, phenomenon.

In 2015, we recorded 822 adults entering the River Frome for the 2015/16 spawning season, which was slightly more than the 10-year average 746 (see Figure 2). This suggests that the problem affecting recruitment must have occurred sometime between spawning in the winter of 2015/2016 and the late summer of 2016 rather than fewer than normal spawning fish.


Figure 2 Number of adult salmon recorded returning to the River Frome

Why are the numbers so low?

We noted that the weather was particularly warm during the early part of the winter of 2015/16, i.e., when the adult salmon were laying their eggs in the river. The high air temperature was reflected in high River Frome water temperatures: the average water temperature for December 2015 was 11.2°C, which is the warmest December temperature recorded this millennium, 3.3°C warmer than the average December temperature and 2.0°C warmer than the second warmest in this millennium.

We speculate that the high water temperature during spawning and early egg incubation had a negative impact on egg survival and subsequent parr recruitment as this has previously been observed in temperature controlled experiments.

Although we cannot exclude other factors, we believe the floods affecting parts of the UK during the winter of 2015/16 are unlikely to have caused “egg washout” in a low energy system, such as the River Frome.

River Frome acting as an early warning system

As a result of the small 2017 smolt run, we are expecting fewer adults to return to River Frome in 2018 and 2019, i.e., salmon that spent 1 and 2 years at sea, respectively. Furthermore, we would expect fewer than average adult returns in other affected rivers throughout the UK, albeit that the effects might be seen in later years depending on the age that their smolts go to sea.

Thanks to the unique data collected by the GWCT, Cefas and the Environment Agency, we can view the River Frome as an early warning system that affords us an opportunity to take action before the horse has bolted.

In contrast to the 2017 smolt run, the lowest number of spawners recorded entering the River Frome since 1973 were observed in 2013 and 2014. Yet, these low numbers of spawners resulted in larger numbers of smolt than estimated in 2017 (see Figure 1).

In recent years, there has been considerable focus on understanding factors affecting the marine survival of salmon, which is the focus of the SAMARCH project 2, and while their marine survival appears to be low, the poor recruitment from the 2015/16 spawning season is a stark reminder that we must not forget the importance of the freshwater stage the salmon lifecycle and it’s potential to affect stocks in subsequent years.

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Is it too late

at 11:02 on 07/04/2018 by Paul Bowman

After being a salmon angler for 35years I now face MANDATORY catch and release the decrease in salmon and sewin stocks started in the 80's just when sprays and dishwashers were being more widely used. The National Rivers Authorities have dropped us in the S*** they refused to check the silts in a local pond saying that the water quality was good. But the silts are where the poisons collect. Back in the 1970's there used to be thousands of shelducks in our estuary (Milford Haven) now there are none. Look in the fields and you see a monoculture with no other weeds or plants. How long do chemicals stay active in the soil for and where do the spray residues go? straight into the silts on our rivers and estuaries. Of course our governments would not take on Monsanto for the sprays or Proctor and Gamble for detergents would they. Maybe its time for some truths and bring an end to a lot of this chemical usage. Sorry farmers your fields may already be on the downhill path to destruction by all the sprays you use. Paul


at 19:18 on 08/03/2018 by Peter Mullins

Hi while I agree with the above report on smolts numbers in the Frome being effected by temperatures and river flow. I however would like to point out the increased numbers of cormorants on the river Frome during the winter of 2016/17 as a riverkeeper on the upper part of the Frome last winter I witnessed up to 20 cormorants on the roost these birds need to eat 1lb fish per day to survive that's got to be made up primarily of Salmon parr and smolts (plus brown trout). As the upper Frome is not stocked above Wracklford this must have an effect on salmon juvenile fish! It's such a tragedy that just as salmon can now at last access the upper parts of the Frome these visiting cormorants are wiping them out! I support a cull licence to cover the Frome and Piddle with research done on the resulting changes to smolt numbers? We need to save the Frome salmon before its gone.

Salmon runs

at 9:38 on 30/08/2017 by Prof. Terry Langford

This work is among the most important on our rivers today. So good to see it continuing. I will have a look at my records of juvenile sea-trout on the Highland Water over the last 20 years to see if there are any similar trends in the freshwater stages. The longer your records run, the more they need continuous funding .Congratulations all round on the hard work and results. All very best wishes. Terry Langford. (Home e-mail:- terry.langford@btinternet.com).

Frome Salmon

at 15:15 on 29/08/2017 by Tony Medley

The above article does not bode well for the future. But the team seem to be catching plenty of juveniles to tag this month, so lets be optermystic!!! Tony

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