LEGGETT and WHITNEY: WATER TEMPERATURE AND SHAD MIGRATIONS 



FiGXJRE 3. — Pacific Ocean average sea-surface temperatures and predicted shad distributions for selected months. 

 (Temperature charts adapted from U.S. Navy Hydrographic Office, 1954.) 



cumstantial evidence suggests a northward mi- 

 gration in summer and a soutiiward one in win- 

 ter as on the Atlantic coast. 



If we assume that the migrations of shad in 

 the Pacific follow the same specific ocean tem- 

 peratures as do the migrations of Atlantic coast 

 populations (an assumption that is supported 

 by the similar relationships between peak move- 

 ments into rivers and water temperatures on the 

 two coasts), it is possible to make predictions 

 concerning the Pacific migrations based on our 

 knowledge of the distribution of 13° to 18°C 

 waters along the Pacific coast. 



During the months of December to April, the 

 13° to 18°C isotherms encompass an area ex- 

 tending from southern California to near the 

 tip of Baja California (Figures 3A, 3B). We 

 predict that the Pacific shad populations will be 

 found to winter in this general area, a prediction 

 strengthened by records of a few shad from San 

 Pedro, Calif., in November and December 

 (Smith, 1896). In May and June a general 



warming trend is noticeable along the coast, and 

 the 13° to 18°C isotherms arch northward off- 

 shore (Figure 3C) . Their northward movement 

 is undoubtedly affected by the upwelling that 

 occurs along the California and Oregon coasts 

 and produces an inshore pool of cool water in 

 spring and summer. We predict that the shad 

 begin their northward movements at this time. 

 If this be so, they are probably directed offshore 

 in the area of this cool water. In July and Au- 

 gust, 13° to 18°C waters occur over a wide range 

 of the Pacific Ocean (Figures 3D, 3E). Shad 

 might be located anjrwhere from Alaska (in some 

 years) to Baja California. The reports, sum- 

 marized by Welander (1940), of rare summer 

 occurrences of shad at Kodiak, Alaska, and by 

 Claussen (1959), off Baja California, are there- 

 fore not surprising. In September, cooling 

 commences and results in a southward movement 

 of suitable water and a contraction of its ex- 

 tent (Figure 3F). During this period we pre- 

 dict that the shad move southward with the 13° 



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