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  1. #1

    Default The effect of water temperature on smolt production

    Having read through the forum posts over a long while I have noticed that there doesn't seem to be much reference made to the affects river water temperature may have on smolt production of a river.

    My understanding from trout fish farmers is that trout grow very little over the winter months when water temperatures are below 5 degc, and at 8degc that is when you start too see a change in growth, with 10-15 deg being the optimum water temperature for fish growth.

    I would imagine salmon parr wouldnt be much different.

    So, if there was a long cold winter (and spring) or consecutive long cold winters and springs, it could be possible that parr wouldn't grow as quickly as normal, and therefore stay longer in the river, until it was time for them to start their downward migration.

    This in turn would leave them open to increased predation from piscivorous birds such as cormorants, mergansers, etc.

    Furthermore, any increase in parr numbers would result in an unsustainable number of parr in the river - above it's natural carrying capacity and due to lack of food, these additional parr may die.

    With 2013 and 2014 being poor years, could this be a contributing factor?

    Does anyone have any weather data prior to these years that would back this up?

    Interested to hear peoples thoughts?.....
    Last edited by Gamekeeper; 25-03-2015 at 11:13 AM.

  2. #2
    Join Date
    Mar 2013
    North Yorks

    Default Water Temperature & Smolts

    Thank you for a thought-provoking post.

    There are several factors wrapped together here. Starting with the wider context (i.e. not year to year variations), northern rivers tend to be colder, causing parr to develop more slowly and spend more time in the river. There are research records of dwell times of 4 years and more. Then there is the associated issue of invertebrate food supply, where more acid rivers tend to be less productive, leading to smaller, longer-dwelling smolts. In contrast, southerly alkaline rivers with plentiful food supplies tend to produce bigger smolts in much less time. And the size of the smolt is the key to its survival and initial success as a predator at sea, as the speed at which it can swim is directly related to its body length. But most of the casualties occur in the river, owing to its confined space and profusion of predators (trout, herons etc), so the less time spent there achieving migration size, the better for survival. Of course the size of the predator population is closely linked to the river's productivity.

    The effects of year to year variations might be much harder to assess owing to the number of variable in play. A really long hard winter like 1962-63 might indeed cause some postponement - in some areas rivers were frozen over from 27 December to 28 March, and there were ice floes coming downstream well into April. With a few exceptions, most of what we've had since then has been relatively short duration - e.g. -20 for 10 days - and its translation into longer-term water temperatures has probably not been measured. On the other hand, the succession of cold dry springs we've had in Yorkshire in the past decade have all held back fly hatches, indicating an impact on invertebrate development and hence food supply. But again, the impact on parr development is hard to assess, as they'd probably catch up rapidly from May onwards.

    So, the bottom line is that I don't have a clear answer, but thanks for making me think as I await my first day out next week.

  3. #3


    Interesting. I wonder how salmon and trout parr differ. Browns and rainbows are different I think as the latter cannot breed here because the water is too warm. Or maybe I am making that up?

  4. #4


    Great point

    The factors affecting smolt production are IMHO quite poorly understood and / or measured. This is one of them. As MCX highlights it's hard to assess the impact of temperature because it has an indirect effect on parr by limiting the food supply available.

    I think the available research backs the idea that periods of extended cold lead to longer time to smolting (but potentially bigger smolts) and a greater proportion of 3 year old smolts vs 2 or even 1 year old.

    I think the extremely cold winters of 09-10 and 10-11 should be considered in the relatively poorer numbers of returning fish in 2013 and 2014. In no way is will this be the only factor and it may well not be the most important in any particular river; but it is likely to be one of them.

    I know it's often colder elsewhere (Russia / Iceland / Norway) but that's not really the point. These Salmon populations have adapted to low temperatures over a long period of time. We can expect fish to adapt to a stable environment be that hot or cold, swift changes are much more difficult to accommodate. i.e. if every year 90% of smolts are
    4 years old then your runs are stable within the bounds of natural variation. if you usually have 80% 2 year olds but have only 20% 2 year olds in one or 2 years you will see a difference in the run patterns and numbers.

    Have a look at the Dee stock component report 2013 for the smolt counts out of the Girnock and Baddoch burns to see how their smolts vary between 2 and 3 years old.

  5. #5
    Join Date
    Aug 2008
    Yorkshire (were there a god it'd be god's own country) & Afrique


    Not made up T7, though introduced rainbows can survive in warmer and less oxygenated settings (e.g. Africa) than brownies, the blighters do breed in the Wye (Derbys) and even the Don (Yorks).

    Back to the OP - MCX has summed up some of the problems.

    I'm not sure that predation is easily explainable. Mother nature is a funny old thing. A really hard winter also will do for the goosanders and mergansers so predation may be much less. Lots of variables. Density dependence etc. Do cold winters affect run timings? Do increased synthetic inputs into rivers change smolt behaviour? No baselines and lots of variables.

    Also whilst a year may be rubbish for fishing, it may be good for breeding - this may be why a really good year for fishing e.g. 2008 may follow a really poor year for fishing - in this case 2003. Counters may tell a different tale (as did the N Esk and Tyne in 2014 when numbers were stable despite the parlous rod catches). Then again the bumper 2004 was followed by a poor 2009...

    Far too many variables to deal with IMHO. The data from the N Esk on smolt monitoring shows that year-on-year variation of smolt output estimates varies by up to over 400% (lowest a shade over 100k, highest almost 430k) and trying to link that to climate variation is particularly difficult.

    Perhaps see also:

    Crisp's book (Trout and Salmon Ecology) is a good read about this and other related matters if you've a library copy nearby.
    "...hooking mortality is higher than you'd expect: further evidence that as a numbers game, catch-and-release fishing isn't always as straightforward as it seems"
    John Gierach

    Fed up of debating C&R - see Hidden Content

    Unless otherwise stated, data used in any graph/figure/table are Crown copyright, used with the permission of MSS and/or EA and/or ICES. MSS / EA / ICES are not responsible for interpretation of these data by third parties

  6. #6
    Join Date
    Feb 2008


    I don't know how it works in the wild but commercial smolt production, as part of the salmon farming industry, light is certainly a factor and, as I understand it, the trigger to smolt production.

  7. #7
    Join Date
    Feb 2008


    Certainly 2009/2010 were very cold and long winters and may very well have had an effect on Alevin/fry, and smolts to a lesser degree.

    Be interesting to see how this year ends with regard to returning salmon.

  8. #8


    A lot of good points made above.

    I was thinking along the lines that if a salmon parr was in a particular river system/area/altitude/distance from sea, and was destined to smolt after 2 years in freshwater under normal winter conditions, but due to a longer than normal colder winter/spring, the parr may miss the window due to poor growth and end up having to wait a further year, ie becoming a 3yr smolt, then In this case it's possible it's chances of survival in river would be lessened.

    If this happened to large numbers of parr, I wonder how much this would affect the returning adult numbers - if there was so many parr that didn't survive the extra year.
    Last edited by Gamekeeper; 25-03-2015 at 07:53 PM.

  9. #9
    Join Date
    Mar 2008
    Paisley strathclyde.


    Can only talk about brown trout as they are all I have some knowledge of.
    A cold hard winter can be good for browns as it makes them sort of stationary. There is very little food for them to eat at that time of year.
    Think back to the hard winters we had in the seventy's and the eighty's and the fishing was good despite U.D.N. a few years before.
    The same would apply to parr one would think. Am sure the biggest danger to small parr is trout and other parr. Nature planned things far better than man.

  10. #10
    Join Date
    Nov 2009
    In a cooling North Atlantic...


    Quote Originally Posted by bradan View Post
    I don't know how it works in the wild but commercial smolt production, as part of the salmon farming industry, light is certainly a factor and, as I understand it, the trigger to smolt production.
    As far as I understand (not much), light (insolation) levels have been shown to be highly instrumental in smoltification. Many studies on latitudinal variation apparently confirm this.

    Re OP: one of the great missing added-value possibilities that I believe SEPA has been investigating are temperature readings. I'm not aware of any available records on temps tho, but the Dee Fisheries Trust (I think) has some info on the temp/smolt link.

    You're probably aware of it but this site might be useful if only for links:

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