CONSERVATION OF MARINE RESOURCES 17 



Fluctuations in Abundance 



Many marine species of fish and shellfish are characterized by large, 

 and often unpredictable, fluctuations in abundance. Since the livelihood 

 of fishermen depends upon an adequate supply of fish for harvesting, 

 these fluctuations are of grave concern. During periods of poor fishing 

 entire communities suffer. 



Fluctuations in abundance occur from natural causes and from man- 

 made causes. A great amount of study has been devoted to evaluating 

 the relative effects of man-made factors and natural factors on the 

 abundance of marine species of fish and shellfish. There is agreement that 

 man's impact upon the stocks has introduced an additional element into 

 the already complex and fluctuating conditions under which a species 

 may exist. Differences of opinion as to the relative weight that should 

 be placed upon man's impact as opposed to changes brought about by 

 natural causes chiefly arise from incomplete knowledge concerning the 

 species in question. 



Natural Causes Changing Availability to Capture. For some species 

 drastic changes in catches occur from year to year as a result of changing 

 availabihty of the fish to capture. This may arise when a population 

 changes its normal migratory route. Wide variations in year-to-year 

 catches of albacore tuna off the states of Oregon and Washington exem- 

 plify this process. In the spring, albacore appear off southern California 

 and shortly thereafter are taken northward along the Pacific Coast. In 

 some years they have been caught as far north as southeastern Alaska; 

 at other times they have not been taken beyond Oregon or Washington. 

 In some years they may enter coastal waters, but in other years they 

 remain offshore. Failure of the California albacore fishery from 1928 to 

 1934 is generally attributed to low availabihty of the fish. The albacore 

 population probably was as large as usual, but did not enter the range 

 of the fishing fleet. 



Varying Survival Rates. Fluctuations in abundance (and catches) also 

 result from varying survival rates due to changes in the natural environ- 

 ment. In some years conditions in the ocean may favor survival, while 

 in other years adverse conditions, such as abnormally high or low water 

 temperatures or scarcity of food, may hinder survival. Thus, the number 

 of fish reaching fishable size (or age) each year may vary widely, often 

 without any apparent relation to the number of parents that spawned 

 them. 



The relative numerical sizes of different age groups of fish may be 

 determined from studying their age composition in the commercial 



