Papers in Marine Biology and Oceanography, Suppl. to vol. 3 of Deep-Sea Research, pp. 474-478. 



Coastal currents and the fisheries * 



By C. O'D. IsELiN 

 Woods Hole Oceanographic Institution, Woods Hole, Mass. 



The waters over the continental shelf from Labrador to Cape Hatteras have been 

 examined and described many times over the last 40 years or so. Certain characteris- 

 tic features of the distribution of temperature and sahnity are to be found along this 

 whole stretch of coastUne. Here many clear-cut examples of the basic problems of 

 the circulation of coastal waters are to be encountered, yet it cannot be claimed that 

 much additional understanding has been gained since the pioneer studies of Bigelow 

 (1927; 1933). 



When the present writer was a graduate student at Harvard, Dr. Bigelow had 

 reached the conclusion that significant advancements in marine biology would in all 

 probabihty have to await a much better understanding of the circulation of the 

 ocean. He planned his own work, and also the early programme of the Woods Hole 

 Oceanographic Institution, accordingly. After 25 years, the best that can be said is 

 that we have gained a clearer picture of the deep-water current systems, and that 

 there is hope that a corresponding advance will soon take place in the more difficult 

 problems of coastal currents. The purposes of this paper are to discuss briefly the 

 present status of the shallow-water circulation problem, to point out the more pro- 

 mising Unes of attack, and especially to give fisheries biologists some hope that physi- 

 cal oceanographers will soon be able to answer some of their more pressing questions 

 concerning fluctuations in the physical and chemical environment of coastal fisheries. 



Certain general rules of coastal circulation have become well established. In the 

 northern hemisphere the average flow is paraUel to the shore line with the land on 

 the right hand side. It follows that the average motion of the surface water is anti- 

 clockwise in a bay or gulf and clockwise around a bank. The reverse is of course true 

 in the southern hemisphere. There is a tendency for the most pronounced surface 

 current to be located near the 100 fathom curve, but it is also characteristic to find a 

 second — somewhat shallower and fresher — band of current near the beach. Unfor- 

 tunately, from the biological standpoint it is the rates of cross-current transport at 

 different levels that are important, and these we know very httle about as yet. 



It is typical of coastal waters that sahnity everywhere increases with depth. The 

 only exceptions to this statement are to be found briefly in winter where convection 

 has extended downward to the bottom, or where tidal stirring is especially vigorous. 

 In any case, whether or not vertical stabihty is present, sahnity increases gradually 

 across the continental shelf and then increases more suddenly near the 100 fathom 

 curve, where the contact between the relatively fresh coastal water and the more 

 saUne oceanic waters is usually located. It is also characteristic of coastal waters 

 from the higher latitudes on the westward margin of an ocean that, throughout much 

 of the year, there is a temperature minimum at mid-depths. The reason for this is 

 that there is an inshore component to the flow of the bottom water, and an offshore 



* Contribution No. 792 of the Woods Hole Oceanographic Institution. 



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