BUREAU OF FISHERIES 95 



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Lacking suitable standards for the evaluation of ])olluted streams 

 as lisli habitats, research has been directed during the past year to a 

 detailed study of the })hysioIogical ett'ects of various polhiting sub- 

 stances and (lie establiislunent of stanchirds of purit}^ for the mainte- 

 nance of an abundant fish fauna. A manuscript has been completed 

 for publication that establishes these minimum standards for fish 

 as a guide to efforts at stream-pollution abatement. 



In addition to the laboratory studies involved in this work detailed 

 surveys of actual conditions in the stream inimical to fish life because 

 of pollution have been made in widely scattered areas throughout 

 the Mississippi drainage. 



COMMERCIAL FISHERY IN\TESTIGATIONS 



Although the total yield of the commercial fisheries in the United 

 States has been maintained, and even increased during the past quar- 

 ter of a century, many of the important commercial species are un- 

 dergoing progressive depletion. The Bureau's investigations of com- 

 mercial fisheries are organized, therefore, as continuing observations 

 of the condition and trend of the important commercial fisheries, 

 rather than as disconnected and temporary inquiries or surveys. 

 Their purpose is to trace from year to year variations in the supply 

 of food fishes resulting from natural causes, the early detection of 

 the first signs of serious depletion, and the recommendation of legis- 

 lation or control of fishing operations and the correction of abuses to 

 maintain the fisheries on the basis of sustained yield. Such investiga- 

 tions are the sole guide to wise conservation policies. 



Haddock. — In the North and Middle Atlantic sections biological 

 and statistical studies have been continued on the fisheries for had- 

 dock, mackerel, squeteague, scup, and other shore fishes from the 

 offshore Nova Scotian banks to North Carolina. Chief attention 

 has been given to measuring accurately the fluctuations in the sup- 

 ply of haddock available for fishermen. This catch analysis shows 

 that the haddock poi)ulation on the principal northeast banks was 

 at a low level of abundance in 1935. In fact, marketable haddock 

 were not more than one-third as abundant as during the period 

 1926-28. As a result of this scarcity, the United States fleet is 

 now forced to fish on banks off the Nova Scotian coast, a distance 

 upward of 600 miles from their home ports. Numerous biological 

 data on age composition, growth rates, and mass movements of the 

 haddock population indicate that the scarcity is due to intensive 

 commercial fishing which is removing haddock from the sea at a 

 more rapid rate than natural replacement by reproduction and 

 growth. One of the causes of this scarcity is the loss of large 

 quantities of undersized and immature fish destroyed in the course 

 of regular trawling operations. 



Mackerel. — Mackerel investigations have been designed to dis- 

 cover the best possible use of the fluctuating supply that nature 

 provides. One means is to protect the future abundance of mack- 

 erel so that the industry may adjust itself to the expected supply, 

 and the other is to find out if possible whether a more stable and 

 more valuable supply of large mackerel would result from reducing 

 the intensity of fishing on the young. Predictions of the general 

 abundance of mackerel in advance of each season have been made 



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