SEA-FISHERIES LABORATORY. 297 



" accidental " ; that is, they are due to the operation of 

 a multitude of small causes. 



It is hardly possible to attempt to correlate changes 

 in the nature and abundance of the plankton with 

 the aperiodic migrations or segregations of plaice, or 

 still less with the larger migrations which experience 

 shows are repeated with a certain regularity from year 

 to year. It would, indeed, be possible to trace a possible 

 connection between the abundance of vegetable plankton 

 and the local abundance of small shell-fish on particular 

 parts of the sea bottom, but it seems clear that the 

 demonstration of this relationship is impossible by 

 existing methods of investigation. It is not yet certain 

 what is the kind of food on which these small shell-fish 

 subsist, whether it is organised food in the shape of phyto- 

 plankton, whether it is dissolved organic matter in the 

 sea water, or organic detritus in the sea-bottom deposits. 

 Until this question has been fully investigated, it seems 

 impossible to expect that the food of the plaice and other 

 bottom-living fishes feeding on benthic animals can be 

 traced back to the phyto-plankton ; and changes in the 

 habitat of food, and fishes eating it, related to changes in 

 the plankton contents of the sea. 



It is probable that salinity changes may be factors 

 of significance in determining the nature and times of 

 the migrations made by such fishes as plaice. But this 

 question is not discussed in the present paper. The 

 salinity changes that occur from time to time, and from 

 place to place are, indeed, fairly large in the Irish Sea, 

 though they are less in value, and far less easily observed 

 than the corresponding temperature changes. It is, 

 nevertheless, far more difficult to suggest a hypothetical 

 relationship between salinity changes and changes in the 

 metabolism of fishes, than it is in the case of the tern- 



