Weight/length Curves

These are my weight length graphs for some of the species I like to fish for. They are compiled from data collected by myself and plotted against a logarithmic curve from fry and all the way to British record weight. The figures are as accurate as I can make them and show what the expected weight a fish might be when its length is measured, so a 2lb roach, for instance, should be around 14.3 inches in length on average. That a fish is often shorter or longer than its weight would suggest, or heavier or lighter than its length would suggest, is down to variations in body mass caused by many factors including the fish being ready to spawn and full of eggs or infested with parasites, or conversely, out of condition after spawning or very old indeed & approaching death when the fish will be very lean indeed.

The curve for roach suggests that the record weight is about the maximum any roach could ever be expected to reach in British waters as the curve at the weight for that fish is almost at the point where further growth is not possible, however, it has not quite peaked which suggest that it is quite possible for a roach to exceed the record weight should a fast growing fish living under optimum conditions live long enough to gain the length and mass necessary to surpass it. The curve suggests that it is possible, though not very likely! 

Click on the graphs for a larger & clearer view

Silver Bream
The curve for silver bream seems to suggest that the maximum weight for the species has not yet been attained by an angler* and that a fish exceeding the current record is likely to be living in British waters somewhere. The curve has not peaked at the current record weight and could feasibly continue some distance, suggesting a maximum possible weight for the species at perhaps, three and a half pounds? 

The two linked red dots are for a fish that was caught by Keith Jobling at 2lb 2oz in early April of 2010 and recaptured at the end of the same month at a much reduced weight of 1lb 14oz. Clearly, this was a female heavy with spawn when first caught but one spawned out by the time of her recapture a few weeks later. The considerable differences in its weight measurements are remarkable as she couldn't possibly have shrunk or grown in length by any appreciable amount over such a short time scale.

The red dot for the fish at 1lb 5oz is also interesting as this was a fish that had just spawned when captured. Its overall condition was lacklustre for a silver bream, a fish that in good condition has very bright scales indeed, and its girth was lacking. A week earlier this fish would probably have weighed as much as 1lb 8oz.

* Fish at claimed weights of 3lb 1oz & then 3lb 5oz have been caught since last update of this graph. They are yet to be verified as consecutive British records. 

I have only just begun to compile data for dace and so the curve shown is something of a guess at the moment, but I expect this to change as more fish are caught and plotted on the graph. I have no clear idea of the possible length for a British record dace of 1lb 5oz and so the plot for that fish is pure conjecture and only based on the continuation of the curve derived from my limited data. If it is correct, then it suggests that dace heavier than the current record are possible.

With dace, hen fish full of eggs are significantly heavier than cock fish of similar length caught at the same time. A hen caught in July may show an increase of 30 percent of body weight by March.


  1. Interesting graphs, and ones that, based on the few points displayed, would suggest that it is fairly accurate to make predictions of weight based purely on fish length and "whether the fish looked a litte overweight or otherwise".
    But I do have a couple of problems with the accompanying text. You mentioned a logarithmic graph. I was wondering why logarithmic? Perhaps you might explain this in more detail, as usually logarithmic graphs have one non-linear scale. I had assumed you had plotted a line of best fit, which would make rather more sense to me. Fish growth, if they retained a similar "shape" as they aged (ignoring what you might call middle aged spread) would ideally be a cubic function. Something akin to weight=a constant times the cube of the length. This is very diferent to logarithmic.
    The other point is that I was wondering if you might explain in some detail, how the graphs show whether or not you think a maximum weight had been reached for the species? I am fairly well up mathematically, but cannot really see how you are making such predictions.
    It used to be the case where one might say " The current record fish is pretty damn near the maximum weight for the species." People used to say that about many species in the 60's and indeed increases in records tended to be small...that is until the introduction of high protein baits, at which point most of the old graphs had to be discarded. We may see similar effects once global warming has a marked effect on year average water temperatures. But making predictions from a weight length scale, other than by saying that "the biggest fish listed is.." seems a little dodgy.

    Having said that, I am not meaning to be critical, merely seeking enlightenment, either for myself, or perhaps for you.
    May I also say I am really impressed by the standard of your blog writing, and look at it as a target for my own work. Wonderfully descriptive and I have some way to go to try and match it. Inspirational.

  2. I don't mind critical, JAYZS, in fact I'm glad that someone has been critical of what I have published. I think such work requires it, because it cannot be wholly correct, all of the time.

    I'm no mathematician myself, just analytically minded. I'll not alter the text in any way in the light of your more correct knowledge of graphs, it can stand as published. But, I will put something in that directs readers to the comment you've made, because that would be useful, I think.

    My knowledge of graph types is sketchy, I chose logarithmic as a term to describe them, because the graphs seemed to correspond to those I came across here ~


    I should have been more thorough, and not made assumptions.

    As for predictions about potential maximum weights beyond those of current records, I simply continued the curves onwards, and if they hadn't stalled and still looked to be headed further upward, then my assertion was that the fish the curves represent still could attain even greater weights, given optimum growth conditions.

    Of course a growth chart made for tench, say, back in the 1980's, would predict a maximum weight far below would we now know it can be. I've avoided plotting charts for carp, tench, and even chub. They vary far too much in body length and girth I've found.

    Chub for instance, are almost impossible to work with, a four-pounder from the Wark's Avon often the same length as a seven-pounder from the Dorset stour! However, with fish such as roach and rudd, dace and silver bream, fish who do yield a more reliable dataset across many waters, these types of graphs do seem to mean something and are quite useful, I think.

    For example. My predictions for silver bream have proven accurate since the article was published, with two fish exceeding what was at that time, the record of 2lb 14oz. I stated that 3lb 8oz was possible, and that was a prediction based upon the continuation of the curve past the record weight, and it seems it might well be true, because the two fish were 3lb 1oz, and then 3lb 5oz. They both have to be verified as true weights, and not just claimed weights, of course, but the BRFC will decide on that.

    I wish the captors had measured the fish though! That would have been very useful here. And I think a measurement of such fish as silver bream and roach or any fish that exhibits uniform growth would be a very useful tool for the BRFC.