PLANKTON OF THE GULF OF MAINE 101 



The menhaden has no rival among the fishes of the gulf in its utilization of this 

 pelagic vegetable pasture (indeed, Peck (1894) so noted) ; nor is any other local species 

 possessed of a filtering apparatus comparable to that of the menhaden (fig. 42a) for 

 fineness and efficiency, though in European waters its relative, the sardine (Clupea 

 pilcTiardus) , feeds equally on microscopic plankton as well as on copepods". The 

 Pacific anchovy also feeds on diatoms and peridinians as well as on zooplankton 

 (W. E. Allen, 1921, p. 54). 48 



Among clupeoids, as among whalebone whales, a direct relationship obtains 

 between the fineness of the sieve through which the water taken in through the 

 mouth is strained — in this case the gillrakers — and the minimum size of the organisms 

 that can be retained and utilized; everything smaller passes through. Even the 

 menhaden (though most of its food is microscopic) is unable to capture the very 

 smallest organisms, such as coccolithophorids and infusoria; and the herring and 

 alewife, with coarser sieves (fig. 42b), subsist chiefly on organisms with a longest 

 dimension of at least 0.5 millimeter (copepods or larger animals), which they select 

 individually and not by swimming open-mouthed, as the menhaden does 49 (Bigelow 

 and Welsh, 1925, p. 103). 



Experience with the tow net shows that if diatoms are plentiful enough they 

 will be picked up by a coarse mesh, and the mackerel, which carries broadly spaced 

 spines on the long rakers on the foremost gill arch (figs. 42c and 42d) consumes more 

 or less pelagic plants, and especially the diatom genera Lauderia and Chsetoceros, in 

 British waters in winter when the fish are in deep water (Bullen, 1908 and 1912). 

 I know of no direct evidence, however, that mackerel ever feed on diatoms or peri- 

 dinians in the Gulf of Maine unless taken accidentally along with other plankton. 



Pelagic Crustacea of one kind or another form the major part of the diet of the 

 adults of all plankton-feeding fishes other than the menhaden in the Gulf of Maine 

 and in northern seas generally, and of the fry of all Gulf of Maine fishes, the sundry 

 crustacean members of the plankton appearing in the lists of stomach contents with 

 monotonous regularity. For most species of fish, indeed, this is true from the 

 earlier larval stages onward, as just noted. In fact, Lebour (1920 and 1924) found 

 that herring, and others as well, devour larval mollusks, small Crustacea, etc., even 

 before the yolk sac is absorbed. Thereafter the diet of all the species of fish which 

 she studied consisted chiefly of the latter, most frequently of copepods, adult and 

 larval, and of Cladocera, with decapod and other larvae playing a secondary role and 

 microscopic plants taken only vicariously, except that some larval herring had fed to 

 some extent on unicellular organisms. 



Perhaps the most interesting result of Lebour's work, apart from her general 

 conclusion (1920, p. 262) that copepods, other Entomostraca, and molluscan larvae 

 are the chief food of nearly all young sea fish, is that "usually each species of fish 

 selects its own favorite food, to which it keeps, indiscriminate feeding seldom or never 

 taking place." 



It would not be safe to postulate the precise larval food of any of the Gulf of 

 Maine flounders from that of their European congeners, so widely do the latter 



'» Mullets also subsist largely on unicellular plants, but they are only accidental visitors to the cool waters ef the Gulf of Maine. 

 " It is easy to watch them doing so in the aquarium. 



