186 TRANSACTIONS LIVERPOOL BIOLOGICAL SOCIETY. 
which seems to me to be supported by the study of the 
fish migrations. 
On the other hand, we have to deal with a water 
basin in which seasonal salinity changes are relatively 
insignificant, and are dependent almost entirely on the 
amount of land drainage; in which tidal streams are so 
strong that they lead to an intense mixing up of the water. 
The latter is very uniform in composition, except within 
the twenty-fathom line, which may be regarded as the 
true coastal limit. Temperature changes are rapid and 
well-marked, and the positions of the isotherms depend, 
to a great extent, upon the course of the tidal streams. 
Apart from tidal streams there is only one “ current,” 
and that is the general drift of water from South to North. 
Probably fish migrations are determined almost 
entirely by the temperature of the water, that is, they are 
to be associated with the shifting of the isotherms. It 
is quite likely that in the close study of the latter, com- 
bined, of course, with the study of the regional distribu- 
tion of salinity, is to be sought the proximate causes of 
those fish migrations which lead to the establishment of 
“fishery periods.” The latter are well marked in the 
Irish Sea: the summer mackerel and bass invasions; the 
winter cod fishery; the winter plaice fishery on the shoal 
grounds between Isle of Man and Cumberland; the 
summer plaice fishery in Liverpool Bay between More- 
cambe Bay and Liverpool North-west Light Ships; the 
Liverpool Bay sole fishery in the summer and autumn; 
and the ‘‘ back-end” plaice fishery between Liverpool 
North-west Light Ship and the coast of Anglesey, are 
instances. It ought to be possible to associate these 
periodic fisheries with the changes in the isotherms, 
perhaps, also with the distribution of salinity. At any 
rate the extension of the hydrographic observations at 
