[SOS]

For years we have been told that the return rate for smolts/salmon has fell from 30% to 5% or approx. I for one took them at their word as the 30% figure was quoted and printed numerous times by people who should know.Although i did think the 30% figure seemed a bit on the high side i took their word for it and argued and debated it on here with the likes of MCX and Seeking.
Now i have just read a report from the Girnock burn that to me suggests that even at the highest adult return the rate was less than 7% and the lowest was less than 1%. So although the over all trend has saw a decline the figures used ln the past seem to be wayout, I can not be bothered looking into it or doing the sums but does the 30% mean there are 30% less fish entering the burn or is there only 30% of the historical figures returning.
Adult Return Rates

 

[Fruinfisher]

It looks like 30% of the long term average, which sounds about right

 

[MCXFisher]

Fruinfisher is correct. The return rate for the 2014 Dee smolt cohort appears to have been about 30% of the long term average of sone 6%.

You are also wise to be sceptical of that oft-repeated 30% return rate; and also to avoid getting bogged down in precise numbers owing to all the year to year variations and errors in measurement.It is perfectly possible to have return rates to a river that fluctuate widely between years - say 1% minimum and 18% maximum - to wind up with a long term average of about 6-8%.

 

[Sewinboy]

I have read a research programme report from a UK university. They suspected that larger smolts are twice (15%) more likely to return than smaller ones (7%]. I cant remember what they classified as large and small smooth. In turn their probability of returning as adult salmon depends on factors that enable their growth, such as availability of food, etc when in their nurseries.

 

[offshore]

I have read a research programme report from a UK university. They suspected that larger smolts are twice (15%) more likely to return than smaller ones (7%]. I cant remember what they classified as large and small smooth. In turn their probability of returning as adult salmon depends on factors that enable their growth, such as availability of food, etc when in their nurseries.

If more fish are smolting in the first year (as I have read on here), could this explain the lower total (Nett ) returns ? Higher proportion of smaller smolts, lower nett returns.

 

[Loxie]

For years we have been told that the return rate for smolts/salmon has fell from 30% to 5% or approx. I for one took them at their word as the 30% figure was quoted and printed numerous times by people who should know.Although i did think the 30% figure seemed a bit on the high side i took their word for it and argued and debated it on here with the likes of MCX and Seeking.
Now i have just read a report from the Girnock burn that to me suggests that even at the highest adult return the rate was less than 7% and the lowest was less than 1%. So although the over all trend has saw a decline the figures used ln the past seem to be wayout, I can not be bothered looking into it or doing the sums but does the 30% mean there are 30% less fish entering the burn or is there only 30% of the historical figures returning.
Adult Return Rates

There are some caveats to consider. Firstly neither Burn enters saltwater directory so are of little use in any attempt to measure "marine mortality".Not only do the juveniles has a very long way to get to the sea and the adults have a correspondingly long return, but also, I believe, a large number of returnees originated elsewhere and, presumably many juveniles return to other areas.

 

[SOS]

There are some caveats to consider. Firstly neither Burn enters saltwater directory so are of little use in any attempt to measure "marine mortality".Not only do the juveniles has a very long way to get to the sea and the adults have a correspondingly long return, but also, I believe, a large number of returnees originated elsewhere and, presumably many juveniles return to other areas.

I know there are various reasons why some years are better than others water,food, predation etc and males will stray to other parts of the river or even other rivers. But my point is that a drop from 30% to 5% return rate is much more dramatic than a drop from 6% to 1% it is still a drop by a factor of 6 but to get back to that historical high figures there is a difference of 5% instead of 25%.

These figures are ball park figures not exact figures.

 

[ozzyian]

I know there are various reasons why some years are better than others water,food, predation etc and males will stray to other parts of the river or even other rivers. But my point is that a drop from 30% to 5% return rate is much more dramatic than a drop from 6% to 1% it is still a drop by a factor of 6 but to get back to that historical high figures there is a difference of 5% instead of 25%.

These figures are ball park figures not exact figures.

But does this apparent discrepancy apply across all watercourses? If so, in the rivers maintaining a stable (or increasing return) return why does it matter if it's 30% to 5% or 6% to 1%? Or are we back to Nth Esk, Thurso, Naver smolts doing something different to Dee smolts whilst they are in saltwater? FWIW I can't for the life of me imagine how 30% was ever accepted as a reasonable estimate, just a quick sniff test would make you think it's far too high.

 

[SOS]

This is my calculations and may be totally wrong.
2000 smolts at a return rate of 5% =100
2000 smolts at a return rate of 10% =200
2000 smolts at a return rate of 2% =40
2000 smolts at a return rate of 4% =80

Now take a look at page 4 of this document suggesting a fall from 40% to 6% and compare to Girnock returning fish female or both would mean that 2000 smolts at 40% would =800 fish and even 2000 smolts at 6% =120 fish.

http://www.riverdee.org.uk/f/articles/2016-Dee-Stock-assessment.pdf

Miss sold ppi comes to mind

 

[ozzyian]

This is my calculations and may be totally wrong.
2000 smolts at a return rate of 5% =100
2000 smolts at a return rate of 10% =200
2000 smolts at a return rate of 2% =40
2000 smolts at a return rate of 4% =80

Now take a look at page 4 of this document suggesting a fall from 40% to 6% and compare to Girnock returning fish female or both would mean that 2000 smolts at 40% would =800 fish and even 2000 smolts at 6% =120 fish.

http://www.riverdee.org.uk/f/articles/2016-Dee-Stock-assessment.pdf

Miss sold ppi comes to mind

I think I'd need to be convinced that the sample size and methodology that led to these 30 and 40% returns was really of sufficient size and quality to support that sort of conclusion.

It certainly seems to be passed off in this document as a fact. MCX once pointed out to me a danger in the assumption that out of 5000 eggs per paired couple 2 of them will go on to successfully spawn to give your stable population. Certainly that is statistically the case but the probability is that in fact it is more likely that 100 paired couples generate no successful spawners however 1 pair produced 200.

If fitness for purpose via good genetic qualities mean anything then why not? What if that genetic superman and superwoman of the salmon world spawned in the Girnock. How would that affect a small sample size?

 

[SOS]

But does this apparent discrepancy apply across all watercourses? If so, in the rivers maintaining a stable (or increasing return) return why does it matter if it's 30% to 5% or 6% to 1%? Or are we back to Nth Esk, Thurso, Naver smolts doing something different to Dee smolts whilst they are in saltwater? FWIW I can't for the life of me imagine how 30% was ever accepted as a reasonable estimate, just a quick sniff test would make you think it's far too high.

As you can see in the document this is not taken from the dee but from mss studies on the north esk.

 

[MCXFisher]

It certainly seems to be passed off in this document as a fact. MCX once pointed out to me a danger in the assumption that out of 5000 eggs per paired couple 2 of them will go on to successfully spawn to give your stable population. Certainly that is statistically the case but the probability is that in fact it is more likely that 100 paired couples generate no successful spawners however 1 pair produced 200.

Quite so: in one research example from Scandinavia, the progeny of just one hen fish accounted for over 70% of all the smolts exiting one spawning tributary. However, that success owed far more to good fortune in terms of highly variable elements than any genetic superiority. If a hen fish gets exactly the right sort of gravel, in just the right place, digs it just right, secures 100% fertilisation, covers the eggs up just right etc etc then it may beat the average by a very wide margin.

But as I've said before, with each hen producing some 5,000 eggs, to achieve an average outcome of 2 adults returning to spawn requires the majority of returning salmon to fail completely in their reproductive mission by delivering none whatsoever.

 

[SOS]

Quite so: in one research example from Scandinavia, the progeny of just one hen fish accounted for over 70% of all the smolts exiting one spawning tributary. However, that success owed far more to good fortune in terms of highly variable elements than any genetic superiority. If a hen fish gets exactly the right sort of gravel, in just the right place, digs it just right, secures 100% fertilisation, covers the eggs up just right etc etc then it may beat the average by a very wide margin.

But as I've said before, with each hen producing some 5,000 eggs, to achieve an average outcome of 2 adults returning to spawn requires the majority of returning salmon to fail completely in their reproductive mission by delivering none whatsoever.

But would this not lead to inter breeding and in the long run be counter productive?

 

[MCXFisher]

But would this not lead to inter breeding and in the long run be counter productive?

Not necessarily: the indications are that they can identify their siblings by smell.

 

[ozzyian]

As you can see in the document this is not taken from the dee but from mss studies on the north esk.

Yes I take your point but the inference in that document is that what is happening at sea (ie the apparent huge increase in marine mortality) is mostly responsible for the Dees reduced returning stock.

You actually highlight that if as reported in this document mortality at sea is conservatively 5 fold it once was (Nth Esk numbers) how come the Nth Esk is in rude health. If it were the case (given that the marine stage is the most proximate to the counted returning adult stage) surely a 5 fold increase in marine mortality would equate to (as near as makes no difference) a five fold decrease in adults over the counters? There might be a 5 fold decrease in stocks on some rivers, not on the Nth Esk though.

 

[ozzyian]

But would this not lead to inter breeding and in the long run be counter productive?

It would only be counter productive for one cycle

 

[SOS]

Just to square the circle the north esk catches in 2017 fell by aprox 50% on the 10 year average catches and the counter recorded a decrease of aprox 18% of the 5 year average.
But my point is that a decline from 30% to 5% is harder to adjust than a decline of 6% to 1% in case you have not noticed i have totally changed my view this is not to do with sea or in river this is to do with the numbers we have been given.

 

[ozzyian]

Just to square the circle the north esk catches in 2017 fell by aprox 50% on the 10 year average catches and the counter recorded a decrease of aprox 18% of the 5 year average.
But my point is that a decline from 30% to 5% is harder to adjust than a decline of 6% to 1% in case you have not noticed i have totally changed my view this is not to do with sea or in river this is to do with the numbers we have been given.

I did notice! Welcome aboard

I think it was Loxie saying a reasonable way of looking at averages is maybe 10 yrs with the 1 or 2 highest and same lowest removed as outliers. Pretty much necessary I think with something as totally random as catch records year to year on small rivers (any actually) Otherwise what do we do when faced with a repeat of the 2010/2011 results - close down the conservation industry?

 

[SOS]

I did notice! Welcome aboard

I think it was Loxie saying a reasonable way of looking at averages is maybe 10 yrs with the 1 or 2 highest and same lowest removed as outliers. Pretty much necessary I think with something as totally random as catch records year to year on small rivers (any actually) Otherwise what do we do when faced with a repeat of the 2010/2011 results - close down the conservation industry?

No i think you need at least a 3 year increase to say things are on the button and then year on year.

 

[Guest]

Just to square the circle the north esk catches in 2017 fell by aprox 50% on the 10 year average catches and the counter recorded a decrease of aprox 18% of the 5 year average.
But my point is that a decline from 30% to 5% is harder to adjust than a decline of 6% to 1% in case you have not noticed i have totally changed my view this is not to do with sea or in river this is to do with the numbers we have been given.

Instead of focusing on the 30% number(s) from early data, ignore those and look at the return rate data over the following years. Most of it shows a decrease in the long run.
If, as some assume, smolt output is relatively stable i.e. limited by the production capacity of the river, then any long term reduction in return rate will result in a reduction of returning spawners (what we fish for)
If smolt output is not stable i.e. rivers are more polluted these days, any reduction in return rate will further decrease numbers of returning spawners or if you like, return rate would need to increase for no reduction in returning spawners.
Then consider that over the long run, netting has reduced on average about 9% per year since the 60's then this should increase returning spawners if river smolt production is stable and marine mortality is stable.
Given the huge reduction in declared netting of half a million fish on the Scottish coast alone (roughly half a million fish enter Scottish rivers nowadays) it's easy to see why estimates of PFA show a roughly 70% decrease since the days of widespread declared and undeclared netting. It's also not surprising that counter numbers can remain stable in the long term in such a situation i.e. a huge reduction in pre fishery abundance can be offset by a huge reduction in fishery activity before the fish are counted in-river.

I'm talking long term effects of netting, not year on year looking for significant changes in highly variable data. I am also not saying that reduced netting will lead to more smolt output. It won't, that much is obvious.

What I am saying is that if all else is stable (smolt production and marine mortality) then massively reducing netting in the long term should have led to more fish in our rivers. It hasn't, therefore either marine mortality has increased, smolt output has decreased or both.

So take a look at all the data and you will see stable smolt output in Girnock and Baddoch burns and slowly reducing return rates. Something is happening between burn and return to keep counter numbers relatively stable in the face of massively reduced fishery activity before fish are counted over the long run.

If I am right, now that most nets are off and there can be no more significant reduction in netting to offset counter numbers then for the first time, counters will actually be measuring pre fishery abundance. It will be interesting to see how counter numbers fare over the next 20 years in the post netting era.