FEEDING HABITS OF MACKEREL. 395 



the stations E5 and E6 (the only two falling within the fishing area), 

 published in the Bulletins Conseil International pour I'cxploration de la 

 Mer, it would appear that these several changes in the nature of the 

 food of the mackerel, viz. from phytoplankton to general zooplankton, 

 and from thence to zooplankton of a larger character and animal food 

 generally, e.g. young fishes and material offered as bait, is largely, if 

 not entirely, due to seasonal changes in the nature of the food supply 

 itself. Yet, as it has been abundantly shown that certain planktonic 

 organisms are of sporadic distribution, and that there is some fair 

 evidence to show that others occur in shoals of varying extent, it 

 remains to be seen how far the mackerel, exercising these two distinct 

 methods of nutrition, is capable of discriminating between varying 

 types of food during the period that it is feeding by filtration. 



The question arises, to begin with, whether in adopting the one 

 system the fish is incapable of using the other, and two important 

 points in this connection may be considered. In the first place, the 

 Newlyn drifters state emphatically that it is impossible to take 

 mackerel, when closely shoaling, upon a hand-line ; and secondly, it is 

 a matter of common knowledge amongst fishermen that, when late 

 in the summer mackerel are caught (in drift nets), when feeding 

 heavily upon copepods, they are very liable to rapid decomposition in 

 the region of the stomach, and that this is due to the " soft " condition 

 of the fish themselves, and not to the increased warmth of the 

 atmosphere. Xow, with respect to this matter it may be stated that 

 the present writer, in making examination of the stomach contents 

 of several hundreds of mackerel taken at various times of the year, 

 has invariably found that the walls of the stomach appeared to be 

 contracted and thickened in cases where phytoplankton formed the 

 bulk of the food, whereas the organ in question was distended and 

 the w^alls extremely thin when zooplankton occurred. Moreover, 

 although no careful histological comparison was made between the two 

 types, it was found possible to distend, by means of an air-pump, to a 

 great extent an empty stomach of the latter type without undue 

 pressure, whereas one of the thickened type would not respond to 

 such treatment. Again, throughout an observation extending over 

 nearly six years upon fish in many cases from not exactly determined 

 sources, the writer has found that, with very few exceptions, the 

 thinning of the stomach walls is developed to the greatest extent as 

 the western drift fishery approaches its highest point of productive- 

 ness, viz. in May and June in the case of large mackerel, and in June 

 and July with those of smaller size, which latter, as Dr. Allen* has 



* 0}). cit., p. 25. 



