SEA-FISHERIES LABORATORY. 157 



South of the " head of the tide," but, on the backing of the 

 wind again to West and South, they would be carried back 

 again into the Solway and South Scottish coast. 



It is reasonable to conclude, therefore, that plaice eggs 

 spawned on the Solway " Slaughter " ground will be carried 

 mostly into the Solway Firth and into Wigton and Luce Bays 

 (on the Scottish coast). The shallow water grounds here are 

 pre-eminently " small plaice " grounds, or nurseries, to at least 

 as great an extent as are those off the Lancashire and Cheshire 

 coasts (see the tables, "Solway Firth"). The northern part 

 of the Irish Sea is therefore supplied with small plaice, which 

 are spawned and reared in the same sea region. 



Hatching and Transformation Stages. 



The plaice eggs found in the tow-nets are, from our 

 experience, always fertilised. Now an unfertilised plaice egg 

 will generally remain alive and buoyant, floating at the surface 

 of ordinary sea-water for about a week. If they were present 

 in notable numbers in the plankton they would certainly have 

 been observed, but there is no doubt that such unfertilised 

 plaice ova are very rare. It is fairly certain, then, that there 

 is definite pairing in the sea, or at least that ripe males and 

 females come together in the same local shoals, at the time of 

 spawning. Thus we account for the absence of unfertilised eggs. 



The period of incubation varies from about three weeks, 

 at the beginning of the hatching time, to about ten days, in 

 April. The sea-temperature in the region between Morecambe 

 Bay Light Vessel and the Lancashire coast rises about 4° C. 

 (from 5°C. to about 9° C.) during the period 1st March to 

 30th April. But the differences are considerable in the various 

 regions : thus the temperature at 1st March may be one or 

 two degrees lower at the Solway Light Vessel and at least a 

 degree higher at Carnarvon Bay Light Vessel. There are also 

 considerable differences from year to year, and there are minor 



