Wednesday 4 March 2020

Estimating the Weight of Salmon

How heavy?
Dalnahoyn Pool
Tomatin House
River Findhorn 2017

Yes, Rory's fish was big, but just how heavy?  Please make your estimate now and then review it at the end of this article.

In this era of near 100% catch and release the whole question of weight has become a vexed issue.  In the old days you knocked it on the head and put it on the kitchen scales (or bathroom if it was truly huge).  It's simple if you've invested £100 in a Maclean weigh net, but that's not an economic proposition for those who only fish one week each year.  I'm fortunate: one of the advantages of having grown up children is that they can afford to be generous and give you a weight net as a birthday present.  Most salmon anglers face the challenge of coming up with a good estimate of the weight of a large fish.  Length alone doesn't cut it: there's no real substitute for pounds and ounces, as AA Milne might have put it, because "Tigger always seemed bigger on account of his bounces". Estimating the weight of salmon is difficult for the inexperienced, especially when they're tiggerly excited and bouncing about on the river bank with post-landing euphoria.

Teviot Silver 2008
Of course ghillies have an experienced and sceptical eye.  Most tend to be very accurate with early season fresh fish. Eoin Faigrieve was within 4 ounces with this chap who'd only been in the river a couple of weeks in early May 2008.  However, from June onwards it starts to become more difficult, especially for the less experienced angler.  

Length to weight estimating scales

The usual recourse is to use one of the scales that you can find either via Google or in various publications.  However, this is where the more vigorous (and sometimes vicious) debates on salmon weight begin in earnest.


Edward Sturdy, who gave his name to the scale, fished in Norway on the Vosso system, where large salmon are commonplace.  His scale is based on a formula, with the input in inches and the output in pounds.

(Length x 1.33) x Girth x Girth

The drawback with Sturdy's scale is that on the Vosso he was dealing with fish built like South Devon bullocks - big and massively broad and deep.  Those Norwegian characteristics are what made the genetic strain so popular with the salmon farmers when choosing their brood stock.  Traditionally Dee fish were viewed as being heavily built.  The squaring of the girth places more emphasis on that feature than the length.  That is appropriate for very big fish, when every extra inch of length sees a disproportionate increase in girth and weight.

Sparkling fresh
Helmsdale in April
The Sturdy Scale seems to work quite well for hefty fresh-run spring fish up until early June.

You can see the high girth to length proportion of this lovely example of an average 2SW spring fish from the Helmsdale.

Same river, same week, different shape
But even on the same river you get variations in shape and hence weight to length ratios.

Sturdy does allow for this in spring fish at least.  However, I suggest that his scale starts to run into problems later in the season and thus must be allied to judgement.

River Ure
Early October 2015

If you apply Sturdy to the dimensions of the Beast of Wensleydale - 42" long, 24" girth - you arrive at a highly improbable weight of 40 lbs.  Clearly something's wrong: I shall explain why shortly.

It was the biggest salmon of my life, but I'd be the first to say it was nowhere near that heavy.

General Scale

What I call the 'General' scale on account of its very wide circulation is probably probably based on blending Sturdy's formula with empirical experience of British fish.  I've seen it reproduced in T&S, on blogs and in fishery board leaflets.  You will find a nice clear reproduction on Richard Donkin's website.  The copy on the desk beside me as I type is printed on water-resistant paper and appears to have been cut out of a fishery board pamphlet.

Like Sturdy it's pretty good on normally shaped fresh run fish until early June.  From that point onwards things start to go awry.  Look at the changing profile of fresh fish as the months advance.  The August fish is much the biggest of the sample, but it may have been hanging around in the North Sea for a few months before running the river, and it came 130 miles to the point of capture, which might explain its reduced profile.

 Scale about right

Scale overestimates by 1 lb


Scale overestimates by 2.5 lbs

So what's the problem with length to weight scales?

Actually there are 5 problems:
  • From the moment they convert to fresh water salmon start to lose weight because they are living on stored energy.  The rate of loss accelerates as the months pass.
  • Grilse and smaller salmon are less efficient swimmers than big fish and so burn off a higher percentage of their stored energy during migration.  They are also more mobile and erratic while running.
  • Hen fish are better at energy conservation than their testosterone-laden brothers, especially in the later summer and autumn.  They do, however, convert a great deal of flesh protein into eggs and the process is not a straight exchange of mass.
  • Very large fish are the most efficient swimmers and once in the river display extreme levels of energy conservation.  In a secure lie they seem able to go into a state of limbo, with all their body systems just ticking over.
  • And cock fish grow their kypes, which serve to exaggerate their length.  This is a major source of error in estimating their weight unless the extra length is discounted.

How much weight do they lose?

Ure Kelt 2011

The simple answer is an awful lot.  The mended kelt in the picture died during the fight and I couldn't revive it.  It was just over 36 inches in length, so it's reasonable to suggest that when it had entered the Humber a year earlier it was some 20 lbs in weight.  At death it weighed less than 12 lbs.

About 70% of a salmon's weight is flesh.  The other 30% comprises head, bones, skin and internal organs.  So during the last year of its life, this specimen lost more than 50% of its flesh mass and 40% of its original overall weight.  Those figures provide an objective anchor for the bottom of any weight loss graph.

Mr Angry
a colourful shadow of a former 14-15 pounder

As I said earlier, most cock fish lose weight faster.  This chap is a good example of the effects of the testosterone-driven territorial behaviour that is prevalent in the autumn.  He was the 'Pool Boss' of the tail of Flesh Dub: every time another male fish ran through his patch he would launch into an energy-sapping aggressive display.  Indeed, he was so punchy that he came after a small Ally Shrimp being stripped quickly from a range of 15 or more feet: the visual spectacle and tension was like something out of Jaws.  He was 35" long including his kype, but you can see how much body mass has already gone.  By spawning time he would have been knackered and down to a weight of 9-10 lbs.  That's why many fewer cocks than hens survive the spawning process.  it's quite possible that Mr Angry didn't make it to the spawning start-line.

Compare his body profile and mass with a hen fish caught a fortnight earlier 400 yards upstream.

Compare him also to this much bigger chap caught at exactly the same point of the season and place.  It's not a good photo, but make no mistake this was a very good fish.

He's a 37" 3SW who clearly hasn't wasted his energy on pointless behaviour.  But he most certainly isn't the 22 lbs that the scale might suggest on first inspection.  Not only has he lost 2-3 lbs in mass wastage, but also he's grown by 1.5" with the development of his kype.  He's actually somewhere around 15 lbs.

Definitely not a pretty fish
Tomatin 2005
Smaller fish also lose a larger proportion of their flesh mass.  Most grilse are male, so they are caught by the double trap of inefficiency and hyper-activity.  This especially ugly exemplar was my second-ever Findhorn fish on the fly, taken in very low warm water.

I was very much a novice then and delighted to have caught the fish after blank years in 2002 and 2003.  The sharp-eyed tackle geeks will spot an Aircel double-taper line and the Fibatube 13 footer I inherited from my father.  Those were indeed the early days.

This graph, based on an estimate for two identical 15 pound fish that entered the Ure in May, attempts to show with the blue line how the cocks' weight loss accelerates as the testosterone rises.  The main weight loss for the hen in Nov/Dec is the loss of egg mass at spawning: the milt mass in cock fish is significantly less.

Large Ure hen
3rd week of October

The visible effects of flesh mass loss in hen fish is often concealed by the 'plumping' effect of egg development.  You can see the well-stuffed appeared of this very large early October hen fish.  The sharp-eyed readers will spot the figure on the tape measure - 38" - which suggests that in May she was around 24 lbs.  Unfortunately, with flesh loss she was probably nearer 19 lbs by this time.

Note also the very short distance from the from the front edge of her eyeball to her nose - just 2.5" - which is germane to the next heading.

I got into a complete mess landing this fish, which explains why she's covered in sand.  Someone else had my landing net; just as I got hold of her tail in shallow water she went berserk; I fell over; and in the melee between us we broke the top section of my rod.  What a shambles!

How much does the kype extend?

Kype growth is one of the most important factors that lead to the overestimation of the weight of late summer and autumn cock salmon.  It is essential to grasp that the kype is not an integral part of the salmon's skeleton.  The hormone-driven growth of the cartilaginous extension is a temporary addition to the fish's body.  And of course, both estimating scales are based on skeletal length, not overall length.

The easy answer to this question is that it all depends.  Not all kypes are equal, nor do they grow at the same rate.  The first variable is whether the fish has run a river before (note the 'a river' - it could be a strayer).  If the fish survives spawning, its hormones subside and the kype shrinks as the kelt mends, but the nose doesn't return to its start state.  There's always an extra bit of bone and cartilaginous material that remains.  That gives it a head start on the next occasion.  Thus you can have 2 fish of the same birth age, one a 3SW first-timer and the other a former grilse, similar in most respects apart from the rate and extent of kype growth.  You can of course then play all the various permutations, but it won't necessarily give you a consistent explanation.  Then the equation is further complicated by latitude and when the salmon start spawning: as a general rule Findhorn and other northern Scottish cock fish are 4-5 weeks ahead of their Yorkshire cousins in testosterone levels and kype growth.

Here's a pair of fairly typical late season 2SW cock fish taken on the Ure in early October.  The were probably around 8-9 lbs on entry and by then were down to 7 lbs or so.  I apologise for the poor focus on the top one caused by haste to get a good profile of the kype.  They've both got just over an inch of kype growth.  The one to the right is more interesting, especially if you examine his upper jaw in close-up.

What a fine manly jaw
River Ure
October 2019
Here you can see the mark on the upper jaw showing the underlying bone profile from which it has extended by about 1.25".  It looks more, especially if you focus on the lower jaw, but that's fairly typical.  It will, however, extend further towards 2" before spawning in late November - early December.

But it does suggest that with a 2SW cock fish at this point in the season, or in September at the Findhorn's latitude, you need to knock about 1.5" off the length as well as applying mass depreciation in arriving at an adjusted weight.  But to be frank, we're not much concerned with exact weights at this size.

Let's now look at some bigger fish where the estimates of weight take on a whole new meaning.  We'll start with the 37 inch 3SW you saw above.  He probably turned Spurn Point at a spanking 18-19 lbs, but then he was only 35.5" long, not 37".  He hasn't been in fresh water as long as Mr Angry, so I reckon he's lost about 2-3 lbs of body mass.  So looking at the General Scale at 35" (18.5 lbs) and knocking off 3 lbs leads to my estimate of 15 lbs.

No, he's not one of my sons
Here's Rory's first large salmon, taken at Tomatin in early September 2011 and he's rightly proud of this warrior who was led astray by testosterone and chasing a rapidly stripped fly in the early morning.  See the post 'Morning Glory - Sex and Flies" for an explanation.  The latitude effect is evident.  The kype is much more developed than the bigger fish above.  There's close to 3" of upper jaw extension beyond the growth line.  Leaving aside the effect of the well-advised pose, once you deduct 3" of kype and apply appropriate mass depreciation its much-reduced real weight is exposed.  By such means does a 36" fish shrink from 20 to 12 lbs (it was killed and weighed).

And then you get the exceptions.  This brute was caught on the Deveron the week after I caught the Beast by an acquaintance who has a well-established reputation for catching very big late season fish and the prize rods to prove it.  It measured 41.5" in length and had a girth of 21.5".  This fish was therefore slightly shorter and 2.5" slimmer than the Beast and had a similar upper jaw extension of around 2.5 - 3".  With a net length of 38.5" and adjusted at the rate of mass depreciation you see in mid-teen fish, you'd be thinking of 24 lbs.

In fact it was weighed in front of a reliable witness at 31 lbs 4oz, earning the captor the prize for the biggest Deveron fish of the year.  Incidentally, as his prize he chose a Vision MAG 13', which goes to prove that I'm a better judge of rods than I am of the weight of very large cock salmon.  In any event there's little doubt that very big fish are different: they're highly efficient swimmers; they don't waste energy being stupid; and they're a very different shape.  This means that hypotheses that work for fish in the 14- 20 lbs zone aren't reliable for 30 pounders.

So what?

To get a better estimate of the weight of your salmon apply the following adjustments:

  • Until June use the General scale and apply only minor adjustments for mass loss, based on what you see.  Moderate or increase the mass loss to allow for size above or below 15 lbs/33".
  • After June continue to use the General scale but apply the following adjustments
    • Hen Fish  Measure from the nose to the fork in the tail and correct for mass loss using the orange line in the graph above and a degree of judgement.  Don't be fooled or flattered by the 'plumping' effect.
    • Cock Fish  Measure from the bone line in the upper jaw to tail fork.  Correct for mass loss using the blue line and common sense.  To guide that judgement remember that the girth may still be roughly the same as it was in the spring, but by by September the stomach cavity walls have already lost much of their mass.  And invisibly, most of the heavy high-energy fats interleaved in its flesh have gone.
    • Very big fish are where you can go badly wrong: I certainly did in my early years and even now in the post-fight euphoria my judgement gets seriously frayed.  Measure the length and girth; take a good photo side on; and pick it up with both hands to feel the weight (30 lbs is your wife's checked-in bag for holidays).  Please don't commit to an estimate until you've calmed down.  But do embrace the adrenalin, euphoria and accolades; enjoy the moment and the years of pleasurable glow to come; congratulations!  No matter what the exact weight it was very big and you caught it.

What was your estimate of the weight of Rory's fish at the top of the post?

The correct answer, courtesy of Mr Maclean, is 16.5 lbs.

Looking Ahead

After the wettest February on record - albeit strangely in Swaledale but not Wensleydale - at the end of a wet autumn and winter, there's water everywhere.  Let's hope the river levels are still good on 6th April when the Yorkshire season opens, because this year I really hope to catch Yorkshire silver.  On the other hand I say that every year, don't I?

Come March it will be time to launch into my spring routine.  The anticipation is already climbing.  For you lucky folk who have started the new season, tight lines.