— 208 — 
investigated attains an age of 6—7 years, and has no pelagic larve, as in the case of 
the mussel. B. undatum does not exhibit so great and rapid fluctuations, in the number of 
individuals from 1 to 7; a decrease in numbers from the first four periods to the last four 
is, however, evident. Such decrease has, moreover, been demonstrated by means of other 
larger dredges constructed for the purpose; the fishermen also believe to have been able 
to observe a decrease of this nature in the occurrence of this organism during the time 
referred to. 
“Abra alba reaches its maximal quantity in the autumn of 1910, whereas Corbula 
gibba is at this time almost at a minimum. Corbula attains its maximum in the spring 
of 1912, about the same time as Solen pellucidus. The maxima and minima of the three 
species do not thus correspond in point of time; as to the conditions, however, which 
favourably or otherwise affect the numerical values of these species, I am not in a posi- 
tion to make any statement at present. 
“Figures illustrating the recruiting of Zchinocardıum cordatum from young stages 
in a single favourable year will be found in the last column of the table, calculated from 
40 samples (of 0.1m?) taken m Nissum Bredning; in one instance, 24-4-1913, only 20 
samples were taken, the contents being multiplied in the table by 2. The figures show 
the rarity of the young stages here in the autumn of 1912, as also the manner in which 
they increase in size, until the largest specimens, in the autumn of 1913, can only with 
approximate accuracy be distinguished from the grown organisms. 
“We may in all probability expect that Zchinocardium will occur in considerable 
numbers in Nissum Bredning in 1914, by which time the great majority of the young 
should be full grown. In other words, Echinocardium takes about two years to attain 
full growth”. 
Great mortality of older stages where cold and warmer currents meet. 
The fluctuations here described by Dr. PETERSEN may probably be attributed to 
causes exactly similar to those which occasion the fluctuations among the fish. It is 
obvious, however, that fluctuations in the numbers of those organisms which to a great 
extent compose the food of the fish must again affect the vital conditions of the fish 
themselves. As far as northern waters are concerned, there would appear to be some 
reason to believe that the older stages, also, of these food organisms may be affected 
by natural conditions, in such a manner as to occasion, from time to time, considerable 
mortality among the grown individuals. I have on a former occasion given a survey 
of what is knonw as to such cases of exceptional mortality among grown organisms, 
especially in waters where cold and warm currents meet, on the eastern coast of the 
American continent, and in the Barents Sea*). The capelan, for instance, (Mallotus 
villosus) may at times suffer so excessively that great expanses of sea are found covered 
with their dead; a fact which might well be of considerable significance to the cod. 
Fluctuations among the earlier stages probably of greater importance. 
However this may be, the information available up to the present, based upon the 
investigations of 1901—1913, distinctly points to the earlier stages as of the greatest 
importance in determining the numerical value of year classes, those year classes which 
*) Depths of the Ocean, pp. 705—708. 
