PLANKTON OF THE GULF OF MAINE 111 



the naked peridinians have repeatedly been found to contain other peridinians, 

 Phssocystis, and occasionally a diatom. 53 



It is a question of moment in the economy of the sea, and of practical bearing 

 on the fisheries problems of the gulf, to what extent the sundry carnivorous mem- 

 bers of its plankton menace the survival of the stocks of larval fishes that are produced 

 there. 



The preceding pages contain sundry instances of planktonic animals eating 

 young fish, which could be multiplied many-fold from published reports, were this 

 worth while. In the Gidf of Maine it is probable that the most deadly enemies of 

 newly-hatched fishes are the medusas, ctenophores, and Sagittas. The rapacity of 

 Mertensia and Pleurobrachia in this respect has been mentioned; when and where 

 the latter are abundant (as is so often the case on German Bank) it is hard to see how 

 any larval fishes can escape their constant fishing. Pleurobrachia is also known to 

 devour buoyant fish eggs of various species. In view of its local abundance, this 

 ctenophore must be a serious enemy to the propagation of cod and haddock over the 

 banks to the south and west of Cape Sable. Lebour (1925) has also reported Bolin- 

 opsis, another ctenophore plentiful in the gulf (p. 372), as devouring larval goosefish 

 (Lophius) in the aquarium; no doubt it accepts a fish diet equally in nature. 



The two medusas which are most abundant in the open waters of the gulf — ■ 

 Aurelia and Phialidum — are also proven fish eaters, as are others plentiful in the 

 coastal zone, 51 and the swarms of both of these which we have frequently encountered 

 (pp. 350, 362) must take heavy toll of the little fishes that cross their paths. 



With Sagitta elegans so plentiful and so widespread in the gulf, it, too, must de- 

 stroy great numbers of young fish; must, then, be as serious a menace to the stock 

 of herring, etc., in the Gulf of Maine as Lebour (1923) has found it in the English 

 Channel. It may, perhaps, be named the most effective check among all the plank- 

 tonic category to the local propagation of such fishes as pass through a prolonged 

 planktonic stage, and this incudes most of the important food-species of the gulf. 

 I have found no published record and have seen no actual instance of the amphipod 

 genus Euthemisto eating fish; but in view of its known rapacity it is likely to do so 

 when occasion offers. Decapod larva? certainly do (p. 107), and these are abundant 

 locally near shore at certain seasons. Euphausiids also eat fish to some extent, 

 though probably it is a minor article in their dietary (p. 108). 



It is fortunate, indeed, that the copepod species which so usually dominates the 

 plankton of the gulf (Calanus finmarcliicus) is not a fish eater (at least, it is not 

 known to eat fish). Were the blue copepod Auomalocera as plentiful as Calanus, 

 hardly a young fish could survive. As it is, few can "run the gauntlet" of the 

 medusas, ctenophores, Sagittas, and crustaceans that prey upon them; and so many 

 species (and these plentiful in the gulf) of these groups are now known to prey on 

 fish larvae that they are almost certainly the most effective check on the survival of 

 the countless myriads of young fish that are yearly produced in the gulf. There is 

 good reason, then, to believe that the fluctuations known to occur from year to year 



<* Lebour (1922) has recently given a considerable diet list for Amphidinium and Gymnodiniun). 



« Lebour (1923, 1924) found Aurelia, Phialidium, Aequorea, Obelia, Laodicea, Rathkea, and Bougainvillea feeding on young 

 fish; likewise several other medusa; and Pleurobrachia. 



75898—26 S 



