of the Fishery Board for Scotland. 



117 



the mackerel different individual ova vary in specific gravity within narrow 

 limits. Hensen and Apstein {op cit) say — "Since the specific gravity of the 

 eggs seems to adjust itself to the density of the water, what governs their 

 vertical distribution has yet to be demonstrated. Out of 446 eggs, 104 

 were floating at a depth of 5 metres [2 J fathoms] ; of 148 eggs, 28 

 were found at the same depth, 43 at 20 metres [10 fathoms], and the 

 others — viz., 77 — at still greater depths." In this connection Holt % 

 remarks — "I have no hesitation in saying that the buoyaucy of an egg in 

 ordinary offshore water is subject to fluctuations which are explicable 

 neither by species, degree of development, nor obvious physical and 

 meteorological causes. " It is not surprising that, in a property requiring 

 so exact an adjustment as the specific gravity of the egg, considerable 

 variations should occur. It is well known that considerable varia- 

 tion occurs in the size of the egg; a certain amount of variation in 

 specific gravity must almost necessarily also be present. This fact then 

 offers a sufficient explanation of the fact that the eggs do not all float at 

 one depth. It is very improbable that the specific gravity of an egg will, 

 on its extrusion, change through its contact with sea water. Whether 

 during the development of the embryo a change in specific gravity occurs 

 in consequence of physiological processes may be in the meantime disre- 

 garded. The eggs will rise to levels where the specific gravity of the 

 water is equal to their own, and while the main mass of the eggs will be 

 found floating within certain limits of depth, a considerable number will 

 have remained scattered at various greater depths ; and even if the main 

 mass does not lie very close to the surface, certain examples will have 

 made their way there. This is borne out by the study of the distribution 

 of eggs in Loch Fyne. Gf. also Meek — Report on the Trawling Excur- 

 sions of Northumberland Sea-Fisheries Committee, 1897. 



The ova in the surface stratum of water 15 fathoms deep in Loch Fyne 

 were found distributed at various depths, but the majority were obtained 

 between the surface and 6 fathoms. On certain occasions no eggs were 

 taken at a greater depth than 6 fathoms ; and as a rule the mass of eggs 

 was found to float closer to the surface than 4 fathoms. I have prepared 

 a Table showing diagrammatically the numbers of ova taken in the nets 

 at different depths on each station {vide Plate V.). The unshaded 

 figures refer to pelagic eggs. The breadth of the figure at each depth is 

 proportional to the number of eggs taken at that level. The depth is 

 shown along the vertical margin. Thus at the depth where the figure is 

 broadest the majority of the eggs was taken. The varying breadth indi- 

 cates the unequal dispersion of the ova. The figures are not all made on 

 one standard. In the space at disposal that would have been impossible. 

 Some of the collections contain very few eggs, and in order to have a 

 figure consisting of something more than one line, a larger unit was required 

 than for the large collections. They, however, show at what depth the 

 main mass of eggs was floating, and what proportion that mass bore to the 

 rest, In certain cases the figure indicates that the majority of eggs was 

 within two feet of the surface (0 fathoms in Plate), and as the depth 

 increased trie number of eggs gradually and regularly decreased in numbers, 

 thus giving a regularly tapered outline to the figure. (See Station I., 

 April, May, and July ; Station II., March and July ; and Station III., 

 March.) Others show that the main mass floated a little below the 

 surface — e.g., 1 fathom; and from that level downwards the figures taper 

 gradually. (See Station II., April ; Station III., June ; Station IV., 

 June.) Again the majority may be found at 4 fathoms — e.g., Station I., 

 March ; Station IV., April and May ; Station V., April. There is no 



X Jour. Mar. Biol. Assoc., vol. v., No. 1. 



