296 TRANSACTIONS LIVERPOOL BIOLOGICAL SOCIETY. 



There can be little doubt that there are optimal con- 

 ditions of temperature for plaice of different ages ; that 

 is, growth occurs most rapidly when the temperature is 

 at a certain value, and this must be the case even when 

 food is abundant, for digestion and assimilation must pro- 

 ceed most rapidly at such an optimal temperature. The 

 scarcity and abundance of food is, perhaps, a factor of 

 less importance than might at first be supposed : on such 

 a coast as that of Lancashire, Cheshire, and North Wales 

 the Crustacea and shell-fish on which plaice of different 

 ages feed are generally very abundant somewhere or other 

 on the fishing grounds. Local scarcity of food may, 

 indeed, lead to migrations, but these are merely the local 

 ones, occurring indiscriminately in many, directions, 

 which are indicated by the marked fish experiments. 

 There are instances of great abundance of plaice on 

 various restricted parts of the sea bottom where food is 

 abundant : such, for instance, was the case near West 

 Hoyle Bank, at the entrance to the Estuary of the Dee 

 in the year 1910 ; or the local abundance of plaice in Rock 

 Channel, off the Estuary of the Mersey, a phenomenon 

 observed many times ; or the local abundance of small 

 plaice at the entrance to Walney Channel in Morecambe 

 Bay. In all these cases food of some kind was particu- 

 larly abundant on the areas mentioned : Mactra and 

 Scrobicularia, in the case of the West Hoyle Bank shoals ; 

 Cardium and Pectinaria in Rock Channel; and small 

 Mytilus in Walney Channel. So also with many more 

 similar instances which might be observed, if particular 

 attention were paid to the question of the association of 

 fish shoaling with changes in the bottom fauna. But all 

 such migrations and segregations are apparently 

 aperiodic ; they do not recur, or if they do, the recurrence 

 cannot be predicted. In a certain sense they are 



