OCEANOGRAPHY IN THE UNITED STATES 373 



Some fishes, like cod, usually stay below the thermocline, and their 

 location on the fishing grounds may be determined by the depth of 

 this layer. All fishes have their preferred temperature limits, and 

 this may cause them to become trapped at certain times, as for 

 example, when w^inter cooling of surface water sandwiches a layer 

 of relatively warm water between cooler layers above and below. 



SALINITY 



We know also in a general way that the amount of salt in the water 

 has profound influences on marine animals. Some, like salmon, 

 striped bass, shad, and others that spawn in fresh water have a re- 

 markable capacity to adjust to the change from salt to fresh and back 

 to salt water again. This capacity is even more remarkable in men- 

 haden, shrimp, and other species that spawn in the ocean, but move into 

 brackish or even fresh water while they are still very small and 

 delicate. Brackish water affords protection to animals such as oysters, 

 since certain of their enemies, like oyster drills and certain oyster 

 diseases, cannot tolerate salinities quite as low. Heavy rains in early 

 summer favor survival of shrimp in the estuaries. 



In the area of the Hawaiian Islands salinity as well as temperature 

 can be used to predict spring and summer skipjack catches. The 

 islands come within the influence of two major currents, the relatively 

 warm, high salinity extension of the Kuroshio in summer, and the 

 cooler, less saline extension of the California current in winter. The 

 time at which the Kuroshio will shift to the northward can be forecast 

 from water salinity and temperatui'e conditions in February and early 

 March. On this basis the poor skipjack season of 1958 was predicted, 

 but the forecast for 1959 was goocl. This has been borne out already 

 by catch statistics for May 1959, when almost two million pounds of 

 skipjack were caught — 10 times the catch in May 1958. 



OCEANOGRAPHY AND FISHERIES 



Among the most universal cliaracteristics of marine animals are 

 the great fluctuations in abundance that occur from time to time. 

 These fluctuations are not confined to those commercial or recrational 

 resources exploited by man, although popular opinion often seems to 

 favor this view. We have found in the Pacific, for example, that the 

 abundance of plankton (organisms of microscopic size that drift with 

 the currents) decreased considerably in 1957 and 1958, and that in 

 general the amount of plankton is correlated inversely with water 

 temperature. 



Fluctuations in commercial fishery resources, however, are more 

 obvious to man because they affect him economically. One of the 

 most disastrous was the recent collapse of the Pacific sardine fishery, 

 once the greatest fishery in the world in terms of annual landings, from 

 a peak catch of nearly 800,000 tons in 1936, to less than 5,000 tons in 

 1953. This completely wiped out the fisheries in the Pacific Northwest 

 and the San Francisco region, leaving a relatively minor and irregu- 

 lar fisliery oft" southern California and in Mexican wat-ers. 



Since 1949 we have been conducting an intensive fishery oceano- 

 graphic survey of the waters occupied by the sardine, from which we 

 have learned a great deal about the sardine and about many other 



