PLANKTON OF THE GULF OF MAINE 105 



refused thorn when offered in the aquarium is interesting as suggesting that the mack- 

 erel is rather an exception in feeding on this pteropod. Naked pteropods are never 

 plentiful enough in the Gulf of Maine to be of any importance as food for larger 

 animals. 



Probably all the fishes that eat plankton consume buoyant fish eggs to some 

 extent, the amount taken depending chiefly on the local supply conveniently available. 

 Thus Brook and Calderwood (1SS6) found fish ova more or less prominent in the diet 

 of Scottish herring, according to the varying abundance of the eggs in the plankton, 

 and although fish eggs have not actually been recorded from the stomachs of Gulf of 

 Maine herring there is no reason to doubt that the latter consume them whenever 

 they offer, as is also the case in the English Channel, according to Lebour's (1924a) 

 recent studies. 



Mackerel also are known to take eggs of their own as well as of other species. 

 Fish eggs have been found in small mackerel from the Woods Hole region, to quote a 

 local instance, and in European seas medium-sized specimens of the American 

 pollock {PollacTiius virens) eat considerable amounts of fish eggs among other 

 plankton. 



The only groups of planktonic animals sufficiently plentiful in the Gulf of Maine 

 to be of any importance in its natural economy, but which are not regularly con- 

 sumed by its fishes in as large quantities as the supply allows, are the medusae, 

 siphonophoras, and ctenophores. E. J. Allen (1908) and Goode (1884 and 1884a) 

 record medusas and siphonophores from mackerel stomachs; but this is exceptional, 

 and although they may bite out pieces of large medusas this is probably for the sake 

 of the amphipods (Hyperia) living within the cavities of the latter (Nilsson, 1914). 

 It would not be surprising to find mackerel gorging on Pleurobrachia in the Gulf of 

 Maine at the places and times when this ctenophore swarms, for Andrew Scott 

 (1924) reports mackerel in the Irish Sea full of them during one of their incursions. 



The spiny dogfish {Squalus acanihias) feeds to some extent on ctenophores 

 (Pleurobrachia) in spring, the fish often containing them when they first appear at 

 Woods Hole in May; and in north European waters this troublesome little shark 

 sometimes devours ctenophores in such quantity that their stomachs are full of 

 them (Mortensen, 1912, p. 72, fide Dr. C. G. J. Petersen). The lumpfish likewise 

 feeds regularly on medusas and ctenophores in European waters, hence probably 

 in the Gulf of Maine, and the sunfish (Mola mola), which is only an accidental 

 visitor to the gulf, subsists chiefly on these watery organisms (Bigelow and Welsh, 

 1925, p. 303) ; but so far as is known neither the herring tribe nor any of the gadoids 

 ever eat them — in fact, no Gulf of Maine fishes other than those just mentioned. 



With the young fry of the whole fish population of northern seas dependent 

 for their existence on the supply of plankton, it is but natural that many attempts 

 should have been made to correlate the movements and migrations of the more 

 important food fishes with local and temporal Quotations in the supply, either of 

 the plankton as a whole or of such members of it as serve as the chief diet of the 

 particular species in question, as well as with the far-reaching physical phenomena 

 that may be looked on as the ultimate causes of such fluctuations. Thus, to mention 

 only a couple of examples, Bullen (1908) has established at least a plausible causal 



