274 PLANKTON STUDIES IN RELATION TO 



globosa, the exact proportion of which it was found impossible to 

 gauge. In certain samples, however, owing to the absence of diatoms 

 and other protophyta (Samples 68, 69, 93, 94, Tables III, IV), it was 

 found possible to estimate the amount of Fhacocystis with more 

 probability. 



It may be mentioned that Cunningham,* in describing the early 

 spring food of mackerel, remarks, " In some [stomachs] there occurred 

 a quantity of the green slimy vegetable matter, which was then abun- 

 dant in the sea." 



During the examination of certain series of stomachs, instances 

 have occurred where tlie contained food was deposited in layers 

 (Samples Nos. 94, 120, 121, 123, 124, etc., Tables IV, V). Farran 

 {Report on Sea and Inland Fisheries, Ireland, 1901, Part II, p. 122) 

 records the same thing, and Mr. W. M. Tattersall informed the writer 

 that he has frequently observed a similar condition in mackerel from 

 the west of Ireland. Such a state of the stomach contents is 

 specially obvious where the Pteropod Limacina retrovcrsa (Flem.) 

 occurs together with one or more species of Copepods, the dark colour 

 of the former contrasting sharply with the bright orange tint of the 

 latter. It has often been suggested that certain plankton organisms 

 occur in shoals of varying extent. Now it is interesting to note in 

 this connection, that many fishermen think that shoaling mackerel, 

 when feeding, scarcely move at all, beyond maintaining their position 

 against the current. The theory of the fish feeding, therefore, first 

 in one shoal of plankton organism and then in another as they pass, 

 may be suggested as an explanation of this phenomenon. The fact 

 might also be due to the fish swimming first in one and then in 

 another layer of water. 



By an examination of the Food and Plankton Tables it will be 

 seen that the plankton organisms occurring in the stomach contents 

 are common also to tow-nettings taken on the same position. In 

 a majority of cases also, the relative proportions of individual species 

 are similar in both, or nearly so. Occasionally differences occur. But 

 in the examination of the large mass of material which is generally 

 found in a mackerel stomach when plankton is abundant, it is often 

 difficult to decide the comparative proportion of one species to another. 

 This fact, together with that of the method of treatment already 

 explained (see p. 273), will account for the differences which are 

 occasionally shown between the analyses of stomach contents and 

 those of plankton samples from the same locality. 



Although, however, certain species occurring in the tow-nettings are 



* Marketable Marine Fishes, p. 313. 



