62 
long time; in his paper cited Cotton mentions that whelks go without food for a 
long time. I am inclined to believe, that a very essential part of their food comes 
from the large Lamellibranchs (Ostrea, Cyprina, Modiola, Mytilus, Tapes), which 
owing to their relatively small numbers are only seldom mentioned in the Tables; 
they cannot always open these Lamellibranchs perhaps, before the latter die; but 
then they evidently fall a prey to the whelks; and it is certain that these Lamelli- 
branchs die in large number, as is witnessed by the many empty Mya shells 
with hanging siphons, and the many fresh, connected oyster shells without any 
growths on them. 
Whilst these large Lamellibranchs as detritus-feeders must be included 
among the producers, they are of hut little importance when adult as fish food. 
Only the young Mya are at certain times so numerous, that they are of importance 
— even considerable — in this regard; but in general the large Lamellibranchs in 
connection with the fishes must be regarded as a separate, indifferent group of 
producers. 
Lack of information regarding the normal food-animals of the whelk is 
a great want in this investigation; this will, it is hoped, be removed in the future. 
So far as the cod is concerned, on the other hand, Buccinum is to be 
regarded as a producer of great value; but the cod is not always an abundant 
guest in these waters; it is for this reason probably that Buccinum is so common 
in certain years. 
Whilst Buecinum and Nassa by attacking the captured fish are direct 
competitors of the fishes, I cannot therefore accuse them of being food-compe- 
titors of the fishes; they may perhaps even be of importance as scavengers, as I 
take it for granted that almost all animals of the larger kind end as food for fishes 
or whelks. The Echinoderms however form in the main an exception; they are 
but seldom eaten in the Limfjord; they probably end as food for the bacteria. 
If it is thus difficult to determine in all cases, owing to insufficient biolo- 
gical investigations, what animals are producers and what consumers, it is even 
more difficult to determine, how much the various animals produce and 
consume. The quantity of dry matter of all the animals together seems indeed 
to vary somewhat in Thisted Bredning from one year to another or from spring 
to autumn, but at no time is it very small; these variations evidently show only 
for a small part, what is produced and again used up in the course of the year. 
From a consideration of the mass of the separate species at different 
times, we can on the other hand obtain good information regarding the annual 
production, in a similar manner as Hensen (1887, p. 95) in part determined the 
production of plankton; in this way, however, we only obtain minimum values. 
Abra alba in Table V shows, for example, an increase from spring to 
autumn 1910 of 9.51 gm. per 10 m.?; the whole of this can be credited to the 
annual production. In the same period Nucula and Pectinaria show an increase 
of together 2—3 gm. per 10 m.?; but Solen, Cardium and several others of the 
food-animals of the fishes show a decrease; naturally, not because these rapidly 
growing animals have not produced anything at this time, but because they are 
sought after so much. From the autumn of 1909 to the autumn of 1910 Mya 
shows an increase of no less than 163 gm. per 10 m.? According to this mode of 
