188 MARINE PRODUCTS OF COMMERCE 



sions, logging, and pollution are perhaps the chief oflFenders. In many cases it is 

 possible to have these industrial and agricultural developments and at the same 

 time preserve the runs of anadromous fish. This can be done by treating the 

 polluting eflBuents, constructing fishways over dams, screening irrigation diversions, 

 and applying modern scientific logging practices. 



In exceptional instances the open sea itself may be affected by the activities 

 of man. For instance, the farming of the "dust bowl" area might result in an ex- 

 ceptionally heavy load of silt in the Mississippi River, which, in turn, might pro- 

 vide more favorable shrimp growing grounds in the GuK of Mexico. In recent 

 years there has been extensive dumping of acid-iron wastes at sea in the vicinity 

 of the mouth of the Hudson River. Many fishermen thought that this would have 

 an adverse effect on fish, but extensive studies by oceanographers and fishery 

 biologists have to date shown no basis for their fears. However, the dumping of 

 oils from the bilges of ships may be harmful to the fisheries, particularly to such 

 shellfish as clams and oysters which are close to shore. In most states the dumping 

 of oil is prohibited in such areas. 



Prediction 



Prediction of fluctuations in the abundance of marine fish is of value, both to 

 the fishery administrators and the fishing industry. By knowing the expected 

 abundance of a given population in a given year, the fishery administrators can 

 calculate how much gear should be allowed to fish, and for how long, to allow 

 the appropriate percentage take determined from biological investigations. The 

 fishermen and fish-processing industry may save considerable money by basing 

 their operations on an accurate prediction of the amount of available raw material 

 rather than on unreliable guessing. Because of the importance attached to this 

 subject it has been studied extensively by fishery investigators in practically 

 every major fish producing nation, and in some instances successful methods of 

 prediction have been developed. The major methods will now be considered 

 separately. 



From Estimates of Fry Abundance. If it is possible in any way to estimate the 

 abundance of young fish, it is, of course, possible to make some forecast of the 

 anticipated catch of the adults resulting therefrom. The accurate estimation of 

 the numbers of juveniles is difficult because of their habit of congregating in 

 schools rather than being uniformly distributed over a large area. Even if accurate 

 estimates of the abundance of fry are obtained, predictions may err greatly be- 

 cause of the variations in survival from the young fish stage to the first stage which 

 enters the commercial fishery. 



From Size or Age Composition. Analysis of the age composition of fish popula- 

 tions often shows a regularity in the relation between the abundance of a given 

 age class in one year and the abundance of the next older age class in the next 

 year. Where this is true, it is obviously possible to make a prediction for all but 

 the youngest age involved in the fishery. For the latter it is necessary to depend 

 on the fry estimates previously mentioned or on some average figure based on 

 previous records. In some fisheries application of this method has led to predic- 

 tions of considerable accuracy, although these occasionally go astray because of 

 deviations from the regular pattern of progress of the age classes through the 

 fishery. 



