116 



Part III. — Seventeenth Annual Report 



May, and Station II. showed a corresponding diminution in the quantity 

 of eggs. The remaining two stations decreased slightly, III. as regards 

 Copepods and V. as regards eggs. The ova increased in June on Stations 

 III. and V., but remained at the other stations fairly constant in 

 number. July witnessed a large fall in the total, though two of the 

 stations — viz., I. and IV. — showed a slight upward tendency. Iu 

 August the eggs practically vanished, as did the Copepoda. 



The curve formed by the Copepoda is a regular though very steep one. 

 The curve for the eggs shows two peaks, indicating, what has been 

 already noticed, a reduction in spawning during May. 



On the Levels at which Pelagic Eggs and Copepoda Float. 



The question as to what conditions affect the vertical distribution of 

 pelagic eggs and Copepeda is an interesting one. That pelagic ova are not 

 absolutely confined to any one depth is well known, since tow-nets 

 attached to a bottom trawl when in shallow water usually bring up some 

 eggs. While it is of course possible that in all cases some eggs may be 

 taken from the surface by such open nets while descending and ascending, 

 the total number of eggs captured cannot be always accounted for in this 

 way. M'Intosh,* referring to the pelagic eggs obtained by the Garland 

 in St. Andrews Bay in 1891 and 1892, says : — " Generally speaking, the 

 bottom trawl-like tow-nets, or other tow-nets used on the bottom, gave the 

 largest number of ova, and besides, they were most productive of fishes, 

 and these at a more advanced stage." The depth on the stations in St. 

 Andrews Bay varies from 6 to 12 fathoms. In the Firth of Forth at 

 Stations V. and II., where there are depths of 25 and 13 fathoms 

 respectively, a considerable number of eggs were obtained in the bottom 

 net. At Station V., Firth of Forth, a tow-net at 12 fathoms produced 

 about 100 eggs, and a net towed 6 to 10 fathoms under the surface on 

 Cross-section II. is credited with about 1500 eggs. One-fathom tow-nets 

 on Cross-section I. contained large collections of eggs. On the majority 

 of the stations of the " Garland," the depth on which exceeds 20 fathoms, 

 eggs were conspicuous by their absence in the net attached to the trawl. 

 Other collections of eggs described in the reports by Masterman,t 

 Williamson, % Wallace, § Kyle,|| lend additional evidence in support of this 

 statement.^" In the report by Kyle there are collections made in Loch 

 Fyne at various depths up to 6| fathoms. At the latter depth consider- 

 able numbers of eggs were obtained at the head of the loch, and above 

 Otter Spit ; but the net at 1 fathom produced a greater number than 

 either the 6-fathom or 2i-fathom net. The collections made at the latter 

 depth were smaller than those made at 6 fathoms. 



The buoyancy of a pelagic egg is, as Fulton** has proved, due to the 

 "entrance into the ovum of a large quantity of watery fluid of low specific 

 gravity." The same author found " a considerable variability in the 

 specific gravity of the eggs of the same species and of the same female." 

 A batch of fertilised eggs of the plaice in the multicelled stage had a 

 specific gravity ranging from 1024-1026. Cunninghamft found that for 



* Tenth Report of the Fishery Board, Part III., p. 301. 

 + Eleventh Report ,, ,, p. 250. 



X Thirteenth Report „ „ p. 258. 



§ Fourteenth Report ,, ,, p. 223. 



|| Fifteenth Report „ „ p. 246. 



IT Vide also Scott, Sixteenth Report of the Fishery Board. 



** "On the Growth and Maturation of the Ovarian Eggs of Telostean Fishes," Six- 

 teenth Report of the Fisher?/ Board, p. 88. 



ft Journal Mar. Biol. Assoc., i., 1889, p. 25. 



