308 TRANSACTIONS LIVERPOOL BIOLOGICAL SOCIETY. 



There is more information with regard to the winter 

 plaice fishery in the Bed Wharf Bay area, but this has 

 been derived almost entirely from the measurements of 

 plaice made there by the S.S. "James Fletcher," and 

 from the samples of fish sent by the officers of that vessel 

 to. the Laboratory. Unfortunately little or nothing is 

 known as to what proportion of the larger fish caught 

 in this area during the latter part of the year are 

 sexually mature, or whether or not they actually spawn 

 there. It is, however, unlikely that they do, so that in 

 this case the migrations appear to be governed by the 

 temperature-adaptation factor. 



It is assumed here, as a working hypothesis, that 

 changes in the productivity of the fisheries from year to 

 year are due entirely to differences in the periodic 

 migratory movements made by the fishes themselves. 

 It can hardly be believed that the cause of a bad plaice 

 fishery in the Irish Sea (say) during a certain winter is 

 that the fish are actually less abundant over the entire 

 area than they were during the previous winter, when 

 the fishery was a good one. It may be the case that the 

 actual quantity of fish inhabiting the whole area is 

 decreasing slowly from year to year, although there is 

 no indubitable statistical evidence in favour of this 

 statement. But the actual difference between the total 

 catch of plaice, in a certain fishing area, in two successive 

 years, may be far too great to admit of the conclusion 

 that the whole area had become depleted to such an 

 extent. What does actually occur is that there are 

 differences from year to year in the degree of segregation 

 of the fish on definite fishing grounds. These differences 

 of segregation are, in turn, due to differences in the 

 migration paths taken by the fish as the result of 



