186 MARINE PRODUCTS OF COMMERCE 



Egg Enumeration. If data are available as to the number of eggs laid each 

 season by each female fish and the percentage of female fish in the population, it 

 is possible, of comse, to estimate the total population. The estimation of egg 

 numbers itself constitutes a very important branch of fishery research and requires 

 the use of extensive equipment and personnel. One or more boats are usually 

 sent to sea to collect the floating eggs of pelagic species by means of fine meshed 

 nets. These nets are towed at stations located on a regular pattern or grid covering 

 the total spawning area. It is necessary to separate the eggs of the species to be 

 studied from the rest of the plankton material collected in the nets and enumerate 

 the eggs from each haul. By interpolation and integration, combined with a 

 knowledge of the rate of incubation of the eggs, it is then possible to estimate 

 the total number of eggs laid. With these data at hand a simple arithmetical calcu- 

 lation will transform the number of eggs into number of spawning fish. 



Research Vessel Catches. For fisheries where extensive facilities for research are 

 available it is sometimes possible to estimate the size of fish populations by means 

 of the special catches of research vessels. These usually operate in a manner like 

 commercial fishing vessels, but make their catches in a regular pattern instead of 

 concentrating on the most heavily populated portions of the fishing grounds, as 

 do the commercial fishermen. Catches of these research vessels are enumerated 

 and the fish identified as to species, after which the data are treated in a manner 

 similar to that indicated in the estimation of egg numbers. 



Causes 



The subject of causes of fluctuations has occupied the attention of fishery 

 investigators and fishermen since the beginning of the world's great fisheries. It is 

 easy to see why this is so, since a knowledge of the cause of fluctuations in 

 abundance would either permit their control or their prediction, both of which 

 items of knowledge would be of great value to the conservationists, fishermen, 

 and fish processors alike. The known or suspected causes in the fluctuations in 

 abundance of marine fish may be separated into two general categories, those 

 of natural origin and those of man-made origin. 



Natural Causes. Since marine fish spend all or almost all their lives in the open 

 sea, it is natural to look into the changes in the hydrographic conditions of the 

 oceans for the source of fluctuations in the numbers of fish present. It has been 

 found by means of the age analysis just described that there is a tremendous 

 variation in the size of the broods of many marine fish produced in difi^erent years. 

 When this phenomenon was discovered it was natural to attempt to find some con- 

 dition in the environment which corresponded to the success or failure of these 

 year classes of fish. Many promising leads have been found and have proved re- 

 liable for a period of years, only to cause disappointment when there was a 

 sudden deviation from the previously established correlation. Apparently, the 

 relationships of marine fish to their environment are so complex that few, if any, 

 of the studies to date have adequately accounted for all of them. Some of the 

 ones which have been observed are of interest, and it is worthwhile to consider 

 them. 



As far back as 1908 the British investigator, E. J. Allen, discovered a rather 

 close relation between the amount of bright sunlight recorded during February 

 and March and the catches of mackerel. He theorized that sunlight causes in- 



