286 



BULLETIN OF THE BUREAU OF FISHERIES 



GENERAL DISCUSSION 



In attempting to account for the disappearance of cod eggs and the absence of 

 fry in Massachusetts Bay we must take into account the possibility that the eggs 

 perish before hatching. Possibly the absence of fry may be attributed to a combina- 

 tion of two or even more factors. There are several ways in which the eggs and 

 larvae may be destroyed: 



1. Storms.— Without doubt storms destroy not only large numbers of pelagic 

 eggs and fry, but immense numbers of other planktonic animals as well. As pre- 

 viously stated, Tables 4 and 5 show clearly the disastrous effects of the storms preced- 

 ing trips 4 and 6. There can be little question that the decline in the number of cod 

 eggs taken on trip 6, from 697 (the number taken on trip 5) to 236, and the striking 

 reduction in the number of pollock eggs, from 781 on trip 5 to 2 on trip 6, resulted 

 from unfavorable weather. The fact that the sudden drops in numbers in the middle 

 of the season coincided in each case with bad weather and no other apparent change 

 in the physical conditions affords added evidence of destruction by storms. How- 

 ever, storms in themselves are not sufficient to explain how fry can be completely 

 destroyed in a protected area like Massachusetts Bay and yet survive the terrific 

 gales that sweep across such exposed breeding grounds as Georges Banks and the 

 Grand Banks. Were storms the sole destructive agent, at least a small percentage 

 of the large number of late-embryo stages taken in the Provincetown region would 

 be expected to hatch. 



2. Unsuitable physical conditions. — The possibility that wave action (which in 

 water so shoal woidd extend from the surface to the bottom) might so disturb the 

 fish that they would stop spawning is not applicable in this case, because, although 

 it might account for the absence of newly spawned eggs, it would not explain the 

 disappearance of later development stages as well. Other physical conditions, such 

 as temperature and density, were normal in 1924-25 and rarely, if ever, approached 

 limits critical to cod eggs. 



3. Food. — During the late winter and early spring there is a scarcity of zoo- 

 plankton in the Gulf of Maine, that present being dominated by adult animals 

 (calanids and Sagittse) too large to serve as food for early fiy. About the time of the 

 spring diatom maximum in March and early April, the reproduction season of the 

 calanids occurs, and by April the water teems with the young. As copepod nauplii 

 form a favorate diet of cod larvae, there is no lack of food in the late spring. It is 

 reasonable to conclude, therefore, that although food may prove important in some 

 localities, it is obviously not the determining factor in Massachusetts Bay, for the 

 cod-spawning season at Plymouth extends from the period of scanty zooplankton 

 right through the vernal reproduction period of copepods, and yet no young cod 

 appeared at any time. The eggs vanished just as completely during the time when 

 food was plentiful as when it was scarce. The zooplankton scarcity in Massachusetts 

 Bay is a short-time one and probably does not affect seriously the natural economy 

 of the region. 



4. Enemies. — The destruction of fry, and possibly eggs, by enemies in the 

 plankton is probably an important limiting factor. Recent investigators have 

 emphasized the necessity of considering enemies of this sort. Dannevig (1919) 

 found that cod eggs disappear from the spawning grounds in the waters about Lofoten 



