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458 REPORT OF COMMISSIONER OF FISH AND FISHERIES. [32] 
In examining the three elevations we are, first of all, struck by their 
very considerable breadth. This points either to very great individual 
differences of growth, especially in youth, or—which appears far more 
plausible—to an unequal age of the fish when they firstimmigrate. Per- 
sons who have raised trout will be better able to decide than I whether 
differences of 20 to 30 per cent. of length and 80 to 100 per cent. of 
weight occur in fish of equal age, or in younger fish of otherwise normal 
conditions of life. It is improbable that these differences are merely 
produced by differences of food, because (according to Mr. Glaser’s and 
my own observations) the smaller St. Jacob’s salmon appear to be well 
fed, to judge from their shape and the condition of their flesh. 
Is it, moreover, merely accidental that both the starting points and 
the maxima of the first curve are at an exactly equal distance from the 
second as the second from the third, and, therefore, show exactly the 
same increase in length? As it can only be a question of whole years, 
either the vertebral column grows between the first and second period 
exactly two or three times quicker or slower than between the second 
and third, or (which seems to me more probable) the increase of length 
of the vertebral column takes place at the same periods, and to the 
same amounts; and this equal distance would then be an indication of 
' equal differences of time between the three fresh-water periods. 
The heights of the curves likewise deserve some attention. The small 
St. Jacob’s salmon of the first curve are of no account in this calculation, 
as they do not come in our neighborhood. On the other hand, it is 
surprising that the third curve corresponds to twice as many fish as the 
second, whilst (by fishing and mortality) one would expect the older fish 
to be much Jess numerous than the younger ones. Is this caused by 
the greater ease with which they can escape from the net? In that case 
the St. Jacob’s salmon must be still more rare. Do not these male fish 
of average size migrate to the Upper Rhine as frequently as the larger 
fish, or does only the smaller portion of the St. Jacob’s salmon of one 
year which have returned to the sea again appear during the next sea- 
son? and does the greater portion come a year later, and perhaps make 
their appearance in the Rhine for the first time? Nothing but further 
observations as to the relative frequency of the different sizes, made on 
the Lower Rhine, can solve this problem. 
As regards the difference of time (always supposing that the same fish 
occur in two successive curves), mt will not be proper to class all fish under 
one and the same category. , for instance, the male fish of the 
spawning season 1878 (middle a November till middle of December) 
could hardly have again reached the Rhine before the beginning or 
middle of January, it is self evident that the exceedingly fat winter 
and spring salmon, with very short nose, which immigrate into Holland 
as early as November, 1878, to March, 1879, and reach Basel from Jan- 
uary to May, must have skipped the spawning season of 1878, This 
possibly also applies to one or two later months. On the other hand, I 
