HYDROGRAPHICAL INVESTIGATIONS 199 



changes) and the movements and abundance of 

 fishes is not always apparent ; but nothing is 

 more certain than that such physical phenomena, 

 and others, which we may call purely biological 

 ones, are closely related. There are two main 

 factors which govern the migration and abundance 

 of fishes : the search for suitable foods, and some 

 imperfectly understood conditions leading to the 

 choice of suitable spawning localities. In each of 

 these cases we may be fairly sure that physical 

 events taking place in the sea are the remote causes 

 which we seek. 



Now, though this is well known, yet the exact 

 nature of the connection is not easily seen. Fisheries 

 science is less advanced than agriculture in this 

 respect, but there are some instances that may be 

 given of the manner in which the two sets of 

 events are connected. A temporary abundance of 

 plaice on a certain fishing ground may often be 

 proved to be due to a " strike " of young mussels 

 on the fishing ground in question, and this 

 abundance of mussels can be traced to some par- 

 ticular set of the currents, which has transported 

 the " spat," or larvae, to that particular ground ; 

 and further, the growth of shell-fish may be pro- 

 moted by a rich diatom flora, which again may be 

 due to suitable conditions of weather. Again, it is 

 well known that the spawning seasons of fishes 

 are variable, and it is also known that the begin- 

 ning of the spawning season depends to a consider- 



