FISHES OF THE GULF OF MAINE 201 



Nothing definite is known as to what determines the success or failure of repro- 

 duction of mackerel in any given year, nor what is the most vulnerable, hence critical, 

 stage. It is obvious that there are two major factors concerned. It may be 

 either a question of the number of eggs spawned and of their vitality, which harks 

 back to the physiological condition of the parent fish, or may depend upon the success 

 of the larvfe in surviving the dangers and difficulties of subsistence that confront 

 them. Onslaughts by enemies, abundance and ready availability of food, tempera- 

 ture, salinity, density, and perhaps other physical and chemical conditions of the 

 sea water (e. g., its alkalinity) all react upon the young fish. It may well bo that 

 a favorable environment depends on such a happy combination of all these that it 

 is necessarily a rare event. Study of the composition of the stock of fish in periods 

 of high and low production also suggests that there is a very definite correlation 

 between the number of adult mackerel existing in the sea at any time and the 

 success with which they breed, years of great production always falhng when 

 fish are both scarce and average very large and when, by general report, they 

 are very fat. 



We believe this justifies the working hypothesis that when there are few mack- 

 erel in the sea they grow fast, go into the wnter in excellent condition, and hence 

 are able to produce eggs of high vitality and in abimdance; but when the fish 

 are very plentiful they so deplete the food supply that individually they do not fare 

 as well during the feeding period of late summer and autumn. Hence they neither 

 grow as fast nor emerge from their winter quarters in as good physiological con- 

 dition in spring, and under such circumstances they do not produce as many eggs 

 per female, fertilization is less successful, and such larva?, as hatch are not as strong. 



Food. — We may assume that the diet of the young mackerel is at first much 

 the same in the Gulf of Maine as in the English Channel," namely, copepod larvae 

 and eggs, the smaller adult copepods, and various other minute pelagic Crustacea 

 and small fish larvae. As the young fish grow they depend more and more upon 

 larger prey. Our Gulf of Maine mackerel have repeatedly been seen packed full 

 of Calanus, the " red feed " or " cayenne " of fishermen, as well as with other copepods, 

 so often, indeed (we have examined many in this state), that it would be tedious 

 to quote individual cases. They also feed as greedily on euphausiid shrimps, 

 as do herring (p. 103), especially in the northeastern part of the Gulf where these 

 crustaceans come to the surface in abundance. Various other planktonic animals 

 also enter regularly into the dietary of the mackerel. Thus, Doctor Kendall 

 writes in his field notes that in August, 1896, he found some of the fish caught on 

 the northern part of Georges Bank packed with crab larvae, others full of Sagittse, 

 others, again, of Sagittae and amphipods (Euthemisto) , of small copepods (Temora), 

 or of "red feed" (Calanus), so that even fish of one school had selected the various 

 members of the drifting community in varying proportion. vSimilarly, 1,000 

 mackerel caught near Woods Hole from June to August contained pelagic amphi- 

 pods (Euthemisto), copepods, squid, and launce;*" others taken off No Man's 



<s Lebour (Journal, Marine Biological Association of the United Kingdom, Vol. XII, New Series, No. 2, 1920, p. 306) gives 

 diet lists for 90 larval mackerel ranging from 5 to 13.5 mm. in length, taken in the English Channel. 



*> Nilsson (Publications de Circonstance, Cons6il Perraanent International pour I'Esploration de la Mer, No. 69, 1914) 

 gives a similar list for Swedish waters. 



