Weight for it

This article, which was written by Dave Cooling, former webmaster, originally appeared in the old website and also featured as a letter in BASS magazine 102, summer 2002. It has been resurrected here, because measuring fish rather than weighing them has become more popular. A practice which we think should be encouraged since it is less stressful for the fish and much easier for the angler to do. Unless you require an exact weight, the results achieved by measuring are reasonably accurate, certainly for average fish.

Weight for it — the original article

A new member, Peter Thornley, sent a most helpful letter about the website. One of his ideas was a length to weight conversion table to supplement the existing graph. Helpfully, he sent in a sample table, based on the simple method used by freshwater anglers for the Mona scale for pike and the Sturdy scale for salmon.

Having access to the database of BASS members catches, with information on over 5,000 fish, I thought that a scientific approach could do much better. (If there are still any other members who have been around as long as me, you may remember we have been here before.)

Actually, the 'scientific' approach gave almost identical results to Peter's simpler methods – for the average fish. But, based on length alone, individual fish can vary by as much as plus or minus 30% of that average!

Several members, notably Donovan Kelley MBE and Nigel Hester, have demonstrated that these predictions can be refined considerably if you include the girth as well as the length into the calculation. Not surprisingly, a fat fish will probably weigh more than the average for its length, and a skinny fish less.

However, the real thrust of Peter's letter was to provide a conversion scale to encourage members to measure fish and return them alive – in the best interest of conservation, and for most people, one measurement is quite enough of a chore.

Sadly, this is where the 'little grey cells' started working.

Weight is a pretty poor way of recording the size of a fish. It is difficult enough to weigh a fish accurately in the field anyway – sand on the shore and waves afloat play havoc with any set of scales, and it is fiddly to weigh a live fish without harming it. Furthermore a fish can lose up to 5% of its weight within a few hours of death – before you can get it 'officially' weighed.

Also the weight can vary dramatically – the amount of food, fat and gonad in a fish can vary enormously over the course of just a few days – and is it really a 'bigger' fish just because it has a full stomach?

Length is not only an easier thing to measure on a live fish, but is also a much better record of the real 'size' and age of a fish – and isn't this really what we mean by 'bigger'?

I know that anglers around the world have always used weight as a measure of how big a fish is – and invariably ended up in heated arguments about how accurate these weights are. Actually, that's not entirely true – nowadays, American trout anglers, who are some of the most radical conservationists in the angling world, have turned largely to length as a way of recording their catch-and-return successes.

So where does that leave us, as a Society which is firmly committed to conservation? We are not involved in competitions or trophy schemes determined by the rules of other organisations. We can do our own thing! So – should we actually convert our own certificate schemes and trophies to a length only system – or as a half way house, should we have a 'definitive' conversion scheme for those who prefer to measure rather than weigh?

Mind you, that could involve some interesting 'point scoring' – would some members start submitting thin fish by length and fat fish by weight. Ah well!

Weight for length conversion chart

[NB – measurements taken from snout to fork in the tail]

Fork length (ins) Peter weight (lb) Dave weight (lb)
16 1.7 1.8
17 2.1 2.1
18 2.5 2.5
19 2.9 2.9
20 3.4 3.4
21 3.9 3.9
22 4.5 4.5
23 5.1 5.1
24 5.8 5.8
25 6.6 6.5
26 7.4 7.2
27 8.3 8.1
28 9.2 9.0
29 10.2 9.9
30 11.3 10.9
31 12.5 12.0
32 13.8 13.1
33 15.1 14.4
34 16.5 15.6
35 18.0 17.0
36 19.6 18.4

In addition to the table above, which uses measurements taken from the snout to the fork in the tail, we reproduce below another table which you may find helpful. However this table uses measurements taken from the snout to the end of the flattened down tail.

You may wish to download a copy of these tables (NB – to do this you will require Acrobat Reader, a free copy is available from the Adobe website).

[NB – for those of you who still operate in the 'old' scale, to obtain inches divide the number of centimetres by 2.54]

Weight for length conversion chart

[NB – measurements taken from snout to end of the flattened down tail]

Total length (cms) weight (lb) weight (lb)
36 1 lb 1 oz 0.487 kg
50 3 lb 1.348 kg
52 3 lb 6 oz 1.522 kg
54 3 lb 13 oz 1.71 kg
56 4 lb 4 oz 1.914 kg
58 4 lb 12 oz 2.134 kg
60 5 lb 4 oz 2.371 kg
62 5 lb 13 oz 2.624 kg
64 6 lb 7 oz 2.895 kg
66 7 lb 2 oz 3.185 kg
68 7 lb 12 oz 3.493 kg
70 8 lb 8 oz 3.821 kg
72 9 lb 5 oz 4.169 kg
74 10 lb 2 oz 4.539 kg
76 11 lb 4.929 kg
78 11 lb 14 oz 5.342 kg
80 12 lb 14 oz 5.777 kg
82 13 lb 14 oz 6.237 kg
84 15 lb 6.711 kg
86 16 lb 2 oz 7.228 kg
88 17 lb 5 oz 7.761 kg
90 18 lb 9 oz 8.321 kg