186 TRANSACTIONS LIVEEPOOL BIOLOGICAL SOCIETY. 



When large mussels are abundant plaice are scarce 

 because of the relative scarcity of mussel fry. On the 

 other hand, when large mussels are scarce or absent, the 

 scars are usually stocked with very small shell-fish on 

 which the plaice concentrate to feed. Apart from such 

 local concentrations, the plaice population of Morecambe 

 Bay inshore grounds is a stationary one, and such statistics 

 as we possess show that the fish do not migrate out from the 

 Bay until they are over three years of* age. Conversely, 

 there is little influx of fish from other grounds into the 

 Bay, with the exception of some slight immigration from 

 the South during the winter months. 



The rate of growth shown by the Barrow Channel 

 plaice is therefore a normal one, and is undisturbed by the 

 immigration of fish from other grounds. When we com- 

 pare the rate of growth of the Nelson Buoy plaice with 

 that of the Morecambe fish, the only reasonable explana- 

 tion of the anomalous growth-rate on the former grounds 

 seems to be that we are dealing with a population which 

 is being recruited from the nurseries along the Lancashire 

 coast line from Blackpool South to the Mersey Estuary. 

 Fish of one and two years of age migrate out during the 

 months of April to August to the ground round Nelson 

 Buoy, and from there towards Liverpool Bar. The condi- 

 tions on the deeper grounds are presumably more favour- 

 able than they are on the nurseries — first of all, the 

 population on the latter grounds is a much denser one; 

 then food is probably less abundant, absolutely and 

 relatively; and finally (as I have suggested in last year's 

 report), the rising temperature on the nursery grounds 

 affects the fish in some way, so that they move offshore into 

 colder water where the natural physical conditions more 

 nearly approach those which are optimal for fish of this 

 range of ages. We may suppose that the growth-rate is 



