Coastal currents and the fisheries 475 



component to that of the surface water. Along the whole stretch of coastline under 

 consideration here the bottom water near the 100 fathom curve has the charac- 

 teristics of truly oceanic water. That is to say, salinity approaches 35 , and tem- 

 perature is at least several degrees higher than is to be found over the continental 

 shelf at mid-depths at the latitude in question. 



The temperature-minimum layer normally extends offshore as a tongue a few miles 

 beyond the 100 fathom curve. This cold tongue overlies a maximum-salinity layer 

 that appears to have an inshore component, and an origin well offshore in the slope- 

 water area. The mixing processes at the contact between the coastal water and the 

 oceanic water are by no means clear. As normally drawn from serial observations 

 of temperature and sahnity the isotherms and isohalines cross each other at a con- 

 siderable angle near the extremity of the cold tongue, a pattern which is difficult to 

 explain, if indeed it really exists. On none of the sections so far examined have suffi- 

 ciently closely spaced sahnity observations been secured for one to be certain of the 

 changes in the T-S correlation curve as one moves from typical coastal water to 

 typical slope water. It is evident that vertical mixing plays an important role near 

 the outer limit of the coastal water. 



Not only are the small-scale mixing processes near the 100 fathom curve complex 

 and poorly understood, but also large-scale mixing processes are present. Bubbles 

 or tongues of nearly undiluted coastal water are often encountered just below the 

 surface layer in the slope-water area, even as far offshore as the outermost filament 

 of the Gulf Stream System. Much less frequently has oceanic water been observed 

 invading the area inside the 100 fathom curve, except very near the surface and close 

 to the bottom. While the coastal water near the 100 fathom curve is clearly on the 

 average moving towards the south and west, just beyond it the apparently detached 

 masses of coastal water often seem to have an easterly component. In short, it is as 

 yet impossible to describe a sector near the edge of the continental shelf in three 

 dimensions in a rehable manner. No network of observations has been sufficiently 

 closely spaced to permit only one solution to the contouring of the physical properties. 

 The situation, although on a much smaller scale, may be even more ambiguous than 

 FuGLiSTER (1955) has recently shown to be the case further offshore in the Gulf 

 Stream System. 



The least complex stretch of coastal water in the area under consideration, and the 

 one having the most sluggish circulation, is to be found between Nantucket Shoals 

 and Cape Hatteras. Along this 500 mile stretch of coastline many rivers are con- 

 tinually pouring in fresh water which reaches the sea with a salinity of about 30 

 after having undergone the mixing processes at work in the estuaries. In the further 

 mixing which occurs over the continental shelf the salinity is gradually increased, both 

 in the offshore direction and, as Ketchum has recently shown (1955), also in the 

 downstream direction. Since the flow past Nantucket from the Gulf of Maine is weak 

 and probably intermittent, and since little coastal water passes Cape Hatteras. it one 

 assigns an average salinity to the water leaving the continental shelf ot 33 . and a 

 value of 357oo to the slope water inflow along the bottom, then it is evident that the 

 offshore component must approximate 16 times the volume ot all the land drainage. 

 As previously stated, part of this exchange between coastal and offshore waters is 

 accomplished by small-scale mixing processes, both vertical and horizontal, and part 

 of it by much larger eddy-type movements. The first process probably proceeds at 



