372 OCEANOGRAPHY IN THE UNITED STATES 



survival while there, are controlled at least partly by temperature. 

 In Chesapeake Bay, for example, striped bass sometimes are numbed 

 by sudden temjDerature drops in winter, or congregate in deep holes 

 to escape sudden temperature changes in shallower water. In la- 

 goons bordering the Gulf of Mexico, mass mortalities of fishes and 

 shrimp sometimes are caused by cold weather in winter. White 

 shrimp stop spawning and growth ceases at 68° F. or lower. 



The most striking of all known temperature effects occurred during 

 the warm year 1957 in the Pacific. The underlying cause of these 

 unusual phenomena was a major change in atmospheric circulation 

 over the North Pacific. This caused ice to go out 6 weeks early at 

 Point Barrow, prolonged the rainy seasons along the southern Asian 

 coast and in southern California, brought the first recorded hurricane 

 to Hawaii, caused a disastrous hsh-killing El Niiio off' the coast of 

 Peru, brought many southern fishes to northern waters where they 

 had seldom, if ever, been seen before, and caused the highest water tem- 

 peratures in 26 years off' the California coast. The effects upon ocean 

 temperatures were most striking. Along the west coast of North 

 America and in the Gulf of Alaska, temperatures increased by more 

 than 2° F. and some regions as much as 8° F. In the western 

 Pacific, on the other hand, there were equally large temperature de- 

 creases, associated wdth a weakening of the current systems. Phe- 

 nomenal increases occurred in sportfish catches along the California 

 coast, especially barracuda and yellowtail; the set of Pismo clams 

 was the best in 10 years; the location of major sardine spawning 

 shifted from lower California to southern California for the first 

 time in a decade; and the valuable salmon runs to the Fraser River 

 changed the route of their migrations, coming round the north end 

 of Vancouver Island, instead of through the Straits of Juan de Fuca, 

 as they normally do. In 1958 the unusually warm conditions per- 

 sisted and the sardine fishery recovered to a remarkable degree. Re- 

 cent increases in the frequency of shark attacks upon swimmers in 

 California waters may well have been caused by these oceanographic 

 changes. 



Another important feature of the distribution of ocean tempera- 

 tures is the decline in temperature with depth. Usually there is a 

 layer of more or less uniform temperature at the surface, overlying 

 a body of colder water below. Between the two is a layer in which 

 the temperature drops rapidly with increasing depth, and this region 

 is called the thermocline. The average depth of the thermocline is 

 about 200 feet, but it varies considerably from season to season, being 

 shallow in summer, deeper in winter. Because the seasonal tenq^era- 

 ture change at the surface is greatest at high latitudes, the depth of 

 the thermocline varies much more than it does in the tro])ics. 



Many of our important commercial fishes, like salmon and tuna, 

 remain in the layer above the thermocline, and the depth of the 

 thermocline has important effects upon the hshery. This would ex- 

 plain, for example, why the high-seas gill-net fishery for salmon 

 would be more successful in summer. Along the equator in the Pacific, 

 the thermocline is much deeper on the western side. This is probably 

 why purse seines and live-bait fishing are effective near our coast, but 

 long lines are necessary on the Asian side. 



