FISHES OF THE GULF OF MAINE 195 



The vertical movements of the fish during their summer stay (that is, their 

 appearances on the surface and descents to lower levels) are no doubt governed 

 chiefly by the level at which food is most abundant, and there is no reason to suppose 

 that they ever descend more than a few fathoms during their stay, the supply of 

 small crustaceans on which they feed (p. 201) being invariably richer above than below 

 50 fatho'ms depth in the Gulf of Maine. As yet no attempt has been made to cor- 

 relate, on a definite statistical basis, the local abundance or reverse of the American 

 mackerel and the precise depths at which they swim vrith the supply of available 

 food, but fishermen have long appreciated the fact that mackerel are most apt to 

 be plentiful where there is a good supply of "red feed" (copepods) or other small 

 animal life in the water, and a relationship has been found to hold in the English 

 Channel between the catches of mackerel and the numbers of copepods present in 

 the water ,^^ mackerel being plentiful when there is a plentiful supply of the latter. 

 To go one link further back in the chain of cause and effect, Allen ^° foimd that the 

 more hours of sunshine in February and March (hence the more diatoms to support 

 copepods), the more mackerel were caught off Plymouth, England, over a period 

 of six years, but suggestive though these data are, much more of the same tenor is 

 needed before the parallelism can be proved to be actually a causal one. 



No feature in the natural history of the mackerel has attracted more attention 

 than its habit of gathering in dense schools. It is not knowTi how long these schools 

 hold together, but the general opinion of fishermen is that they do so throughout 

 the migrations at least, and although the mackerel may scatter and the schools 

 mix more or less, especially when they are feeding on the larger and more active 

 members of the free-floating fauna, as is said to be the case in British waters, they 

 usually run very even in size. As a rule mackerel school by themselves. At times, 

 however, they are found mingled with herring, alewives, or shad, as Kendall (1910, 

 p. 287) has described. How the mackerel hold together, whether by sight or by 

 some other sense, is yet to be learned, and various explanations have been proposed 

 to account for the schooling habit, such as that it is advantageous for feeding, that 

 it is a concomitant of spawning (this would not explain its persistence out of the 

 spawning season, however, or the fact that even at spawning time any given school 

 is apt to contain green and spent as well as ripe fish) , or that it affords protection 

 from enemies (which is just the reverse from the truth) ; but when all is said the 

 instinct prompting it remains so mysterious that we can classify it no better than as 

 a sort of sociability such as prompts so many species of birds to gather in flocks. 



Autumnal migration. — As autumn draws on the fish that summer along the 

 coast of Maine evidently work back toward Cape Cod, and of old, good fishing was 

 had successively off Portland, near Boon Island, and oflf Cape Ann. Some time 

 in September or October, in good years, the large mackerel reappear in abtmdance 

 in Massachusetts Bay, and on many occasions schools have been reported and 

 actually followed swimming on the sm-face southward across the mouth of the Bay 



" Bullen. Journal, Marine Biological Association of the United Kingdom, Vol. VIII, New Series, No. 3, Oct., 1908, p. 269, 

 302. Plymouth. 



>« Allen, Ibid, p. 394-406. 



