SEA-FISHERIES LABORATORY. 223 



cannot be explained satisfactorily, for the final stage of matura- 

 tion of the ova probably consists mainly in the imbibition of 

 water from the general circulating fluids of the body. 



We have also to notice the very high proportion of fat in 

 these herrings. In July over one-third of the wet weight of 

 muscle from a female fish consisted of fat. The percentage of 

 fat was 34-9, that of water was 46-6, and if we take Milroy's 

 average value of protein in the muscles of female herrings as 

 18 % these numbers will add up to 99-5 %. Comparing 

 Table I with other published analyses of herrings we find the 

 fat percentages very high, and this statement applies, to a 

 less extent, also to the Welsh winter-caught herrings. Even 

 the famous Loch Fyne summer herrings are less rich in fat 

 than are these Manx ones, and one's own experience in merely 

 handling the fish agrees with these analytical results. 



The reason that the Loch Fyne herrings are less oily 

 than the Manx ones is not that there is a corresponding 

 difference in the planktonic food present in these sea areas. 

 The summer crustacean plankton of Loch Fyne appears to be 

 much denser than that round the south end of Isle of Man. 

 The difference appears to depend on the fact that the spawning 

 period of the Manx herrings occurs very shortly after the 

 maximum of sea-temperature, so that the two factors, the 

 increase of metabolism due to rise of temperature, and that 

 due to the ripening genital products, work together in the 

 same direction. Herring do spawn in Loch Fyne, but this 

 appears to be rather exceptional — the shoals migrate out 

 from the Loch in order to spawn. Only the temperature factor 

 therefore operates in augmenting the fat-contents, and the 

 temperature of Loch Fyne is, throughout the summer and 

 autumn, lower than that of the sea to the south of Isle of Man. 



Some other points of interest in relation to the morphology 

 and bio-chemistry of the fat tissues are being investigated, but 

 the opportunity has not yet occurred to complete this work. 

 P 



