56 



Atomic Radiation and Oceanography and Fisheries 



Historically, a few studies of the vertical 

 migration of zooplankton had been made prior 

 to the war. Great impetus was given these 

 studies when a false bottom was repeatedly 

 observed on echo sounding recorders (Dietz, 

 1948; Hersey and Moore, 1948). This has 

 been called the scattering layer. Although there 

 is still controversy as to which organisms are 

 the principal scatterers in the sea, it has been 

 established that one or more layers are com- 

 monly found which migrate vertically over a 

 depth of as much as 800 meters, being at or 

 near the surface at night and at great depths 

 at mid-day. 



No observations of the changes of elements 

 involved in the biological cycle which may be 

 associated with vertical migrations have been 

 made. Most of our analytical techniques are 

 too insensitive to detect the day to day changes 

 which might be expected in biologically active 

 elements if our present evaluation of the density 

 of the populations and their respiration and 

 excretion rates is correct. It is known, however, 

 that direct assimilation of some elements is 

 possible by invertebrate forms and vertical trans- 

 port of radioisotopes might be expected to re- 

 sult. Indeed, the transport of radioisotopes 

 might prove an excellent tool for the study 

 of vertical migrations if a source were provided 

 at one depth within the range of the migration. 



Ecologically the following effects might be 

 expected as a result of vertical migration. The 

 zooplankton are certainly in the area of the 

 most dense concentration of their food, the 

 phytoplankton, when they are at the surface at 

 night. During the hours of darkness they may 

 therefore be expected to consume the living 

 material in the water, and some of this, at least, 

 would be excreted or passed as faecal pellets at 

 depth in the day time. This process would thus 

 augment the effects of gravity on those elements 

 incorporated in the biological system. There 

 is also evidence that the zooplankton can as- 

 similate dissolved elements from sea water. If 

 elements were assimilated at depth they might 

 be excreted or exchanged near the surface and 

 thus directly modify the vertical distribution 

 in the sea. 



It should not be neglected that larger or- 

 ganisms can certainly migrate vertically over 

 greater distances than we have discussed above. 

 Certainly whales, tuna and sharks, and pre- 

 sumably the smaller forms upon which they 



feed are known to go to considerable depths 

 in the ocean. Quantitatively, of course, these 

 members high on the food chain are propor- 

 tionally small compared to the plankton or- 

 ganisms. However, their effects on vertical dis- 

 tribution of materials may not be negligible 

 over periods of several decades. 



Horizontal migrations of organisms may also 

 result in the transport of material involved in 

 the biological cycle and are also independent 

 of the currents of the ocean. Here again man 

 does not know enough to assess these quantita- 

 tively, but their possible effects should not be 

 ignored. 



The migrations of pelagic fishes may be of 

 considerable interest in this regard. The salmon 

 for example reach maturity in the open sea, 

 then migrate in enormous numbers to coastal 

 areas to breed. Such a horizontal migration 

 could transport radioisotopes, since the salmon 

 could accumulate materials from large volumes 

 of the sea and, by their migration, concentrate 

 them many thousand-fold in the rivers and 

 estuaries. 



Many other fish also exhibit extensive migra- 

 tions. Even though some of these do not enter 

 the rivers to breed, they may enter the areas 

 where they are available for commercial cap- 

 ture, thus becoming some of the food supply 

 of the nation. Unfortunately, in many of these 

 species we do not know the complete life his- 

 tory and most of our information concerning 

 their occurrences and migrations is obtained 

 only during the period of year when they are 

 caught. The Atlantic tuna, for example, are 

 caught in the early spring in the Caribbean and 

 off the Bahama Banks. As spring and summer 

 progresses they migrate northward along the 

 coast, and maximum catches occur in New 

 England in late summer and early fall. The 

 winter habitat and breeding area of these large 

 and important food fish is largely unknown, 

 though preliminary data suggest that they prac- 

 tically circumnavigate the North Atlantic Ocean 

 (Mather and Day, 1954). Similarly the mack- 

 erel catches are first concentrated in the south- 

 ern part of the Atlantic coastline in the late 

 spring and early summer. The large catches 

 off New England occur in August and Septem- 

 ber. This species breeds on the Atlantic con- 

 tinental shelf during its summer northward 

 migration (Sette, 1943, 1950). 



Additional examples of mass migrations into 



