29 
their effects but they are known for example to divers; these state that masses of 
mud will very quickly (in a few days) cover the oyster banks on which they are 
working, so that they are obliged to give up diving operations for a time; a storm 
with a different current will however soon clean the bank again, and the oysters 
are not necessarily dead, at least not all; the death-rate will obviously depend 
partly upon the thickness of the layer of mud, partly on the length of time it 
covers the oyster bank. Every one who has seen a storm disturb the water in 
the fjord, and that unfortunately there is frequent opportunity to see, will have 
noticed how after the storm everything under the water is covered with a layer 
of mud, unless the current as in the narrow courses or the action of the waves 
on the open coasts in shallow water again carries the layer away, from which we 
know that this mud may also cover the more sheltered oyster banks. Masses of 
sand might also cover the banks in this manner, hut the fine mud is certainly 
the most dangerous. One circumstance comes into consideration here, however, 
namely that wherever a dense growth of Zostera covers the bottom, the mud 
accumulates there the most readily, as it comes to rest there most quickly; this 
circumstance along with the fact that the weed itself contributes greatly to the 
formation of mud on rotting away and through giving support whilst living to a 
quantity of small organisms whose excrement and other remains likewise turn to 
mud, makes the Zostera indirectly one of the greatest enemies of the oyster. The 
Zostera may also be destructive directly, as before it has quite rotted away it 
may lie in the rotting condition over the banks and thus smother the oysters. It 
might also be thought that other sea-weeds as well as the Zostera would be harmful 
to the oysters, and certain algæ may perhaps be so at places. There is unfor- 
tunately much that indicates that the above-mentioned heaping up of the mud in 
the Lim Fjord is increasing; certain parts of the fjord, e.g. Harrevig, are spoken 
of, where oysters lived earlier but where the mud and Zostera now make it diffi- 
cult for the oyster to live; yet I have no fixed opinion on this matter for the 
time being, though it seems difficult to believe that the conditions are not as 
described. Heaping up of mud seems to occur also in most of our other fjords. 
It may be understood from this how it is possible that dredging can remove both 
the mud and the Zostera and again make the ground suitable for laying out 
oysters. That, on the other hand, dredging on an oyster bank where the condi- 
tions are good is harmful to the stock of oysters as well as to the animal life on 
the whole is likewise certain. 
Wherever the bottom is stony without much mud and without vegetation, 
we should there expect to find the most favourable conditions for the success of 
the oysters, and this is certainly in general also the case, hut there are exceptions. 
Dredging was carried on in August 1895 (see Table I No. 15 and 16) at" Hilligsø 
and at Oddesund on hard ground, which consisted chiefly of stones on an average 
from ca. 10 to !/, inch, but nothing else came up in the dredge. As the Table 
shows, extremely little spat occurred on these banks in comparison with what is 
found on many other banks, for example at Bjørndrup and Hanbjerg (Table I No. 
9 and 14). This small quantity of spat is not characteristie of these waters as a 
whole, as the oyster boats hbelonging to Lemvig took up later a considerable 
quantity of spat there in the dredge, and Handbjerg with its small stock of spat 
