[35] BIOLOGY OF THE RHINE SALMON. 461 
between maximum [and IT, with the males of 1879, is neither 1:1 nor 2:1, 
but nearer 3:2 than either of the two former proportions. This decid- 
edly favors the supposition that, with most males, the interval between 
the first and second spawning period is two, and with most females 
three years; the interval between the second and third spawning period 
being two years. It is also probable that the agé at which the first 
immigration takes place differs greatly. 
With the females the numerical majority of the third immigration 
over the second, and—considering the small number of female St. Jacob’s 
salmon—probably also over the first, is very, marked: 
Number of fish 
of the— To every 100 of 
the second curve 
l the third curve 
j Second | Third | contains— 
curve. | curve. | 
FER eke ee ee eet 642 | 86 | 748.0 
64 
In 1878 not less than 277 fish, or 58 per cent. of all the fish of that 
season, are found included in the narrow limits of length of 861 to 900 
millimeters (average weight on the 12th October about 8.0 to 9.3 
kilograms). 
Is thiseonly caused by the circumstance that the majority of these 
medium-sized fish do not go as far up the Rhine as Basel? My numer- 
ous measurements of Dutch salmon, taken at different seasons of the 
year during three years, do not seem to favor such a supposition. 
Although on two different occasions I found a small number of Dutch 
salmon measuring 700 and 800 millimeters, the large females were, next 
to the St. Jacob’s salmon, decidedly in the majority. Mr. Glaser, whose 
experience as an old importer of Dutch salmon gives considerable 
weight to his opinion on this subject, is prepared to corroborate my 
view. Until we obtain more exact data regarding the salmon of the 
Lower Rhine, therefore, I consider it highly probable that many of the 
females of the third curve (measuring 800 to 960 millimeters in length) 
had only migrated once before, or even were now visiting the Rhine for 
the first time. Such fish will, of course, grow into the limits of the third 
curve at an earlier age, because they escaped the disturbing influence 
(to their growth) of one or both of the former migrations. What they 
gain thereby will not, however, be sufficient in many cases. Some of 
them stop at this intermediary stage, and thus the lack of a deep de- 
pression between Curves IT and III (making it almost appear as if the 
sharp lines between the two main curves were obliterated by the inter- 
polation of a third and smaller curve) is explained by the supposition 
of such an immigration which has been retarded. one year. If these 
same fish immigrate once more, we shall find them in the right lower 
portion of Curve III. 
