FEEDING HABITS OF MACKEREL. 399 



To what extent this power, on the part of the fish, to discriminate 

 between food of a wholesome character and that which is otherwise — 

 what we have termed " selective feeding " — is carried, may now be 

 considered. 



The writer, in a previous paper,* has remarked upon the finding of 

 alternating layers of certain species of zooplankton, in ahnost pure 

 condition, in stomach material. The same thing has-been recorded in 

 the case of the Irish mackerel. f 



The explanation of this condition originally suggested was to the 

 effect that the fish might have been feeding first in a shoal of plankton 

 organisms of one type and then in another, or else in different layers 

 of water. 



Prof. Ehrenbaum, of Heligoland, upon hearing of this theory, in- 

 formed the writer that he had on certain occasions observed herring 

 and mackerel, which were kept alive in tanks, showing unmistakable 

 signs of feeding by selection upon individual copepods and other 

 plankton organisms.; 



Further, it may be mentioned that the present writer, in making 

 exhaustive examination of several hundreds of stomachs of mackerel 

 and pilchard (of the former for the purposes of the paper already cited)* 

 was never able to detect any sign of the presence of medusae. Dr. 

 Allen, however,§ in summarizing published information regarding the 

 food of the mackerel, states : " In the first place, it feeds upon the 

 smaller forms of the plankton, e.g. copepods and other crustaceans, larvae 

 of crustaceans, molluscs, echinoderms and worms, diatoms and even 

 siphonophorcs and medusae.''^ \\ That this may be food of a somewhat 

 exceptional character is evidenced by the fact that mackerel and 



* Op. cit. , p. 274. 



t Farran, Eep. on Sea and Inland Fisheries, Ireland, 1901, Pt. II, p. 122. 



% In reply to an enquiry respecting this statement Prof. Ehrenbaum wrote: "With 

 reference to your enquiry, I desire to state that in the Heligoland aquarium mackerel, and 

 at certain times also young herring, are kept alive for months, and both species have often 

 been observed feeding in the manner described. 



"Personally (and in this statement I think I have the support of many biologists), I 

 consider that it cannot be doubted that all fish, which prey upon plankton, /ccf? by selection. 

 This can easily be demonstrated by an investigation of the stomach contents. Such 

 material is never found to consist of all the various plankton components occurring within 

 the area from whence the fish have been taken, but includes only certain species, which 

 have been selected by the feeding fish. 



"In my opinion it is a fallacy that certain fish, e.g. the mackerel, habitually swim 

 round with an open mouth. The filtering apparatus of the gill arches is not intended to 

 collect the plankton about to be swallowed, but serves to protect the tender gill-leaflets 

 from possible damage caused by spurious forms of the plankton, which might occasionally 

 be taken in the act of breathing into the mouth." 



§ Op. cit. , p. 9. 



II The italics have been inserted. 



