An article discussing the factors affecting the rate of growth of brown trout, detailing the research udnertaken at Wray Castle, designed to clarify the influencing factors in different waters and investigate what would be required to increase their size. The article considers factors such as alkalinity, total hardness, the presence of coarse fish in the waterbodies and competition for food. Previous work undertaken on brown trout in Ireland by other authors is reviewed. A figure showing average growth rate of brown trout in five lakes, as determined from their scales (Lough Derg, Windermere, Loch Leven, Ullswater, Haweswater)

Swynnerton, G.H.

Worthington, E.B.

English

Aquatic Commons

T H E G R O W T H O F BROWN T R O U T 
by E. B. WORTHINGTON and G. H. SWYNNERTON. 
The size attained by a fish is dependent on its rate of growth 
and its age. In trout the rate of growth is not constant throughout 
67 
life, and differs greatly according to the characteristics of the habitat. 
The extremes, as far as the British Isles are concerned, are represented 
on one hand by the southern chalk streams where three or four years' 
growth may produce fish of two or three pounds in weight, and on 
the other hand by some mo6rland streams and peaty lochs, where 
fish may reach an age of five or more years without exceeding a quarter 
pound in weight. 
The association of the two types of growth with waters giving 
an alkaline or acid reaction (high and low pH respectively) has been 
suggested for many years, but the first person to produce scientific 
data on the subject was probably the late R. Southern, who showed 
that in Ireland brown trout waters fall into two categories, the first 
being lakes and rivers on limestone rocks in which trout grow rapidly, 
become sexually mature late, and live long, and the second being 
waters on granite, quartzite, sandstone and other non-limestone 
rocks in which the trout are small, become sexually mature early, 
and are short-lived. As types of the two kinds of water Southern 
studied the growth of trout in most detail from Lough Derg in the 
first category and Lough Atorick in the second. This conclusion, 
which has been confirmed by more recent work in Ireland by, Miss W. 
Frost, appears to hold very well as far as that country is concerned, 
but many exceptions have come to notice, especially in Scotland, 
where, as F. T. K. Pentelow has recently pointed out, certain lochs 
in Caithness and Sutherland, on peat and non-calcareous rocks and 
therefore acid in reaction, contain trout of remarkably large size, up 
to three pounds and more. 
Information on this subject is being collected steadily at Wray 
Castle, and, as explained on page 44 forms the first stage in a somewhat 
ambitious programme designed to elucidate the factors which control 
the size attained by trout in different waters and to work out methods 
of increasing that size. Rather fortunately the trout in each lake or 
tarn grow at a standard rate, and relatively few individuals depart far 
from the average. There may be exceptions in the case of large lake 
basins where part of the population is slow-growing in the early 
years because some of the young fish stay longer in the affluent streams 
than others, but we are concerned here only with broad comparisons, 
so this variable factor can be disregarded for the present. The 
principle of a standard growth rate for each body of water implies two 
68 
things : (1) that a growth curve based on the average of a fairly small 
sample of fish (say between 50 and 100) is accurate enough for com-
parative purposes, and (2) that the growth of the whole population in 
a given lake or tarn is being influenced by the same set of factors, 
whether they be the chemical contents of the water, the food supply, 
influence of predators, over- or under-population, or a combination 
of all these. 
In discussing growth rates we cannot disregard the other factor 
influencing size, namely, the age attained, because it is a striking 
fact that, in so far as information is yet available, those waters which 
are notable for large fish have the advantage not only that their inhabi-
tants grow quicker but also that they live longer. This may be in 
part a fallacy resulting from the fact that trout in waters favouring slow 
growth, though they may start fairly well, fall off rapidly in their growth 
rate, so that for several years at the end of their lives they may not 
register any appreciable growth on their scales. The fact is, however, 
that the age of fish from these waters, as determined by examination 
of their scales, is seldom more than five years, whereas in waters 
promoting fast growth, eight, nine, and ten year-olds are quite frequent. 
The methods employed in this study are the usual ones of scale 
reading coupled with accurate measurements and weights of the fish. 
In some cases the fish were returned alive to the water after examination 
and in others samples have been killed for examination of food, genital 
organs, etc. The growth rates for six lakes and tarns differing markedly 
in general characteristics have been established by this means during 
the past year. The scales have all been read by Swynnerton, checked 
at intervals by Worthington and Allen, the work being done with an 
apparatus which incorporates a projecting microscope and measuring 
boards. The length of the fish at the end of each winter ring on 
each scale is ascertained by proportion, so that averages for the early 
years of growth are based on the whole sample of fish from each lake. 
This method is based on the assumption that the proportion of length 
of scale to length of body is constant throughout life. Small errors 
are probably involved, especially after individual fish have spawned 
and their scales have been subject to partial resorption (" erosion "), 
but in the present state of knowledge of trout scales no correction 
can be applied, and the errors are not considered to invalidate the 
broad comparisons between the growth rates from different localities, 
provided always that the sample of fish examined is larger than about 50. 
F I G U R E 12. The average growth rate of brown trout in five lakes, as 
determined from their scales. The number of fish examined from each 
lake is indicated in brackets. 
[Reproduced by permission from Country L i f e . ] 
The full results will be published in due course, but meanwhile 
the average growth rate of three waters, namely Loch Leven, Ullswater 
and Haweswater are shown graphically in figure 12, on which has 
been added the average growth rate for Windermere, from Allen's 
paper mentioned on page 37, and for Lough Derg, from Southern's 
results. 
69 
L. Derg. L. Atorick 
Hawes-
water Ullswater 
Winder-
mere L. Leven 
pH 8.0-8.4 6.8 7.1 7 . 2 6.6-6.9 7.6-7.8 
Total 
hardness 
(CaC03) 176 30 23 2 0 28 (57)* 
70 
A proper discussion of these and similar results cannot be included 
in this article, but attention may be drawn to a few significant facts. 
Firstly, referring to Southern's distinction into alkaline and non-
alkaline waters, the following table shows typical figures for pH and 
total hardness, expressed as CaC03 , for the lakes considered here. 
It should be noted that total hardness is a better indication of degree 
of alkalinity than pH, especially in the soft unbuffered waters of the 
type found throughout the Lake District. Thus little significance 
can be attached to the fact that Haweswater and Ullswater have a 
higher pH than Windermere. 
From these figures it is obvious that all the waters of the Lake 
District fall quite definitely into the second category, but as shown by 
figure 12, the growth rates of their trout (Windermere, Ullswater and 
Haweswater) are by no means identical. The growth rate of trout in 
Lough Atorick is similar to that in Haweswater but is a little more 
rapid during the first two years and subsequently shows a more abrupt 
arrest. The conclusion therefore is that the relative alkalinity is not a 
sufficient explanation of the differences in growth rate found. 
Another factor to which Southern attributes importance is the 
presence or absence of pike and other coarse fish. Lough Derg for 
example has many pike accompanying its fast-growing trout, while 
Lough Atorick has none. With this the results from the Lake District 
comply, Windermere having an abundant supply of pike and perch, 
Ullswater and Haweswater having no pike and but few other fish such 
as skelly (Coregonus clupeoides) and perch. In this connection Loch 
Leven is particularly instructive. The growth rate of its trout, which 
*Carbonate hardness by titration. 
71 
in their first three years is almost identical with that of Lough Derg, 
falls off later, so that the fish have a remarkably standard size, averaging 
a little under a pound. In recent years pike have been practically 
exterminated from Loch Leven, but the history of the fishery shows 
that in past years there was a large number of pike sharing the lake 
with trout and perch; in 1903, when 1,400 pounds weight of pike 
were removed, only 2,000 trout were captured, compared with 58,000 
in 1938, but their average weight was 1.38 pounds compared with 
0.81 pounds in 1938. 
Looked at from another point of view, it is a striking fact that the 
order of lakes according to the rate of growth of their trout is the same 
as the order of their classification according to the amount of change 
which has taken place in them since the Ice Age. Thus, of those 
included in figure 12, Haweswater has much less sediment and a much 
less advanced vegetation than Windermere, while Ullswater occupies 
an intermediate position. Loch Leven and Lough Derg are certainly 
at a still higher level of general production. A higher rate of production 
implies, of course, a larger food supply for fish and hence faster trout-
growth, but it was pointed out many years ago by W. H. Pearsall 
that not only do the associations of plants change with the steady 
development of the freshwater environment, but those of fish do 
likewise, the change in the Lake District being from a fish association 
of trout, char and skelly to one of perch and pike. Thus, it appears 
that Windermere and probably also Lough Derg have already passed 
the optimum conditions for trout, because, although their trout 
grow large, they are unable to compete in numbers with coarse fish. 
Loch Leven would have done so likewise had not the fishery there 
been carefully managed by a ruthless war on pike to the point of their 
extermination, and considerable netting of perch. 
In considering growth rates of fish, competition for food must: 
be a factor of importance. It is significant, for example, that records 
from big trout fisheries such as those in Loch Leven and Blagdon 
show a striking inverse relation between number of fish captured and 
average size; usually in years when abnormally large catches have been 
recorded the average weight was relatively low, and vice versa. Although 
little is said about the food factor in this article, data on the food of 
trout and other fish are being accumulated, and results on that subject 
should be ready for publication before long. 


The growth of brown trout

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The growth of brown trout

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