18 FISHES AND FISHERY METHODS 



catches and the amount of effort expended by the fishing fleet in taking 

 the catches. Used together, these studies provide a convenient measure 

 of year-class strength, i.e., the relative size of different broods produced 

 from year to year. Those species primarily affected by environmental 

 factors rather than fishing often are characterized by wide fluctuations 

 in year-class strength. Among 14 successive year classes of western 

 Atlantic mackerel studied, the largest year class in the series was some- 

 where around 15,000 times as abundant as the smallest year class^. 

 Clearly, fluctuations of such great magnitude must be due to environ- 

 mental causes rather than fishing. 



Some investigators have proposed that the more critical period in sur- 

 vival of a year class may occur during development of the eggs prior to 

 spawning, after the eggs are released into the water, or during the larvae 

 stages. Others believe the critical survival periods occur during the post- 

 larval period when food requirements are stringent or during the period 

 when the young are particularly vulnerable to predators. Irregularities 

 in survival rates probably occur at all stages. However, most evidence 

 suggests that survival through the egg and larval stages is the most 

 critical one in determining year-class strength. 



Diseases and Parasites. Diseases and parasites also affect animal popu- 

 lations in the sea. One of the most striking examples of the effect of 

 parasites on fish is the severe decline in catches of trout in the Great 

 Lakes brought about by an influx of sea lampreys. The influx of lampreys 

 resulted from opening the Great Lakes to navigation by ocean-going 

 vessels. Fortunately, measures were quickly adopted to control the preda- 

 tory sea lampreys. These included treatment of streams wdth a chemical 

 lampricide that kills all generations of developing lamprey buried in the 

 stream beds and installation of electrical barriers that prevent reinfesta- 

 tion of treated streams. 



Diseases often are very destructive to animals and plants on land. One 

 might expect diseases to play an equally important role among animal 

 and plant communities in the sea. The scanty information that is availa- 

 ble suggests this may be true. Fungi have been found to infect many 

 species of fish and animals which serve as food for fish. For example, 

 during severe fungi epidemics many North Atlantic herring have been 

 observed to succumb to this pathogenic organism'. 



Although diseases may be an important cause of fluctuations of marine 

 animals and plants, they have received little study to date. In the future, 

 scientists can be expected to devote increasing attention to the effects 

 of disease-causing viruses, fungi, bacteria, and other groups of organisms 

 on animals and plants in the sea. 



Changes in Ocean Circulation. Temporary vagaries in ocean circulation 



