of the Fishery Board for Scotland. 



371 



drawn. From the daily observations made at the Bell Rock Light- 

 house and the North Carr Lightship in 1891, 1892, and 1893, on the 

 temperature of the sea, I have compiled the following table of the mean 

 temperature of the surface water on the east coast of Scotland in the 

 various months throughout the spawning season. The temperature 

 varies to some extent in different years, but there are good reasons for 

 believing that such variation retards or accelerates the spawning season, 

 and that each species in a given region only begins to spawn when the 

 temperature has reached a certain point, normal for that species : — 



Jan. Feb. March. April. May. June. Jury. Aug. Sept. 

 40-5 39-2 39-4 41"9 45 : 5 50"2 53'6 54*3 53'8 



From these mean temperatures it will be seen that the surface water 

 is coldest in February and March, and warmest in August and Sep- 

 tember, and that the range from the minimum to the maximum is close 

 upon 15° F. 



The maximum and minimum temperatures of the bottom water on 

 the east coast, off the Firth of Forth, in from 25 to 30 fathoms of 

 water, is not contemporaneous with the maximum and minimum at the 

 surface. The bottom water is colder than the surface water from 

 February or March to October, and warmer in October, November, 

 December, and January \ in March the surface and bottom temperatures 

 are usually subequal. In January the difference rarely exceeds 1° F., 

 and is usually much less ; in February it is still smaller \ in April the 

 bottom water may be nearly 2° F. colder, but is usually less \ in May 

 the difference is a little greater \ in June the surface water may be about 

 5° F. warmer, and after that the difference diminishes until, in Sep- 

 tember, they are subequal. But, while this variation in the temperature 

 between bottom and surface should be kept in view, it can scarcely 

 affect the conclusions respecting the period of development of pelagic • 

 eggs, for they usually float within a few fathoms of the surface in water 

 of good density, i.e., in the open sea, and in the season during which 

 most pelagic eggs are shed, namely March and April, the difference is 

 comparatively small. 



From the experimental observations made by Mr Dannevig at Dunbar 

 in 1895, and which were instituted principally in connection with the 

 present investigation,* it was found that at the temperature of 39*2° — 

 which is the mean natural temperature for the surface water in Feb- 

 ruary — the eggs of the cod and of the haddock took 20 J days to hatch. 

 At a temperature of 41°, which is nearly one degree colder than the 

 mean surface temperature in April, the eggs of the cod took 17 J days to 

 hatch, those of the haddock 17f days, and the eggs of the whiting 15 J 

 days. At a temperature of 42-8°, which corresponds with the mean 

 temperature of the sea about the beginning of May, the eggs of the cod 

 and haddock took 15 J days to hatch, those of the whiting 13 J days, 

 those of the plaice 18 \ days, and those of the flounder 6 J days. At a 

 temperature of 46 ^F., which represents the mean temperature of the 

 surface water, about the end of May and beginning of June, the eggs of 

 the haddock hatched in 13 days, those of the cod in 12f days those of 

 the whiting in 10 J days, those of the plaice in 14J days, and the eggs 

 of the flounder in 5J days. At 50°, which corresponds to the mean 

 June temperature of the water, haddock and cod eggs hatched in about 

 10 \ days, whiting eggs in 8 days, those of the plaice in 12 days, and 

 those of the flounder in 4J days. At a temperature of 53 0, 6, which is 

 the mean temperature for July, the eggs of the cod and haddock took 

 * Thirteenth Annual Report, Part iii., p. 147. 



