FISHERY BULLETIN: VOL. 70, NO. 3 



to 18°C isotherms, resulting in their concentra- 

 tion off southern California and Baja California 

 in late November and December and continuing 

 at least until April. 



The scope of regular migrations that we hy- 

 pothesize as taking place on the Pacific coast 

 (about 2,575 km, Columbia River to Cedros Is- 

 land) is similar to that which is known to occur 

 on the Atlantic coast (about 2,250 km, St. Johns 

 River, Fla., to the Bay of Fundy) . 



DISCUSSION 



Both the marine and freshwater migrations of 

 shad appear to be regulated by water temper- 

 ature. In the four rivers we have considered, 

 the temperatures at which shad appeared in the 

 river and at which peak movement occurred 

 were remarkably consistent in spite of the great 

 distances separating them. In all rivers except 

 the St. Johns in Florida, water temperatures 

 are increasing as the shad proceed upriver. 

 Water temperature in this river is decreasing 

 during the first half of the run, and the peak 

 movement occurs at the lowest annual temper- 

 ature. Nevertheless, the peak occurred in the 

 same temperature range (15.5°-20.0°C) in the 

 St. Johns as in the other rivers studied. 



The precise correlation between temperature 

 and the timing of the spawning migrations of 

 the shad places the maximum number of adults 

 on the spawning grounds when the temperature 

 is optimum for the survival of eggs and young. 

 In the Columbia and Connecticut Rivers, where 

 our studies of shad movement were conducted 

 at points close to the spawning grounds (Bon- 

 neville and Holyoke) , peak migrations occurred 

 at 18.0° and 19.5 °C, respectively. Massmann 

 (1952) working in the York River, Walburg 

 (1960) in the St. Johns River, and Marcy (1969) 

 in the Connecticut River have all reported the 

 median spawning temperature, as shown by 

 abundance of eggs, to be within the temperature 

 range of 16° to 20°C. Leim (1924) and Brad- 

 ford, Miller, and Buss (1964)= found that max- 



^ Bradford, A. D., J. G. Miller, and K. Buss. 1964. 

 Progress report summary on phase B-2 "to determine 

 by bioassay techniques the inherent tolerance of shad 

 during its egg and larval stages to specific environmental 

 factors of the Susquehanna River and its tributaries. 

 Pennsylvania Fish Commission, Benner Spring Fish Re- 

 search Station, Belief onte, Pa. (Unpubl. manuscr.) 



imum hatch and survival of eggs and larvae oc- 

 curred at 15.5° to 26.5°C. Temperatures below 

 15.5°C prolonged the time of hatching and re- 

 duced survival. 



Since these optimum temperatures are reached 

 later in the year at higher latitudes, the timing 

 of the entry of shad into individual rivers must 

 also be seasonally adjusted. Consequently, the 

 earliest run occurs in Florida during the winter, 

 and the latest runs in June and July in northern 

 rivers such as the St. Lawrence and Columbia. 

 This phase of the timing appears to be regulated 

 by the adherence of shad to similar temperature 

 regimes while at sea. As the ocean warms in 

 the spring, the area occupied by water temper- 

 atures in the range 13° to 18°C moves gradually 

 northward. By maintaining themselves in this 

 thermal zone, shad arrive at the mouth of their 

 home river when river temperatures are suit- 

 able for entry. 



Of course, what we have described is a gen- 

 eral migratory pattern of shad that is clearly 

 associated with water temperatures both in the 

 ocean and in the streams into which they run. 

 Occasional shad are caught in the winter in 

 Chesapeake Bay and in the Delaware and Hud- 

 son Rivers in the fall and winter (summarized 

 by Walburg and Nichols, 1967, also recorded in 

 our recoveries of Connecticut River shad shown 

 in Figures 2 A, 2F.) We learned recently that 14 

 shad were caught in a commercial trawl on Feb- 

 ruary 10, 1972 at lat 41°16'N, long 71°39'W 

 where the water temperature was 3.7 °C at the 

 surface and 4.1 °C at the bottom. Nevertheless, 

 as Walburg and Nichols stated: "The vast ma- 

 jority of fish, however, followed a regular mi- 

 gratory pattern." It may very well be that the 

 observed temperatures associated with the pre- 

 sence of the majority of shad are preferred tem- 

 peratures, not required ones, or they may be re- 

 quired only by a majority of fish. A small mi- 

 nority may be able to condition themselves to 

 water outside the range tolerated by the major- 

 ity. Experiments are needed to determine the 

 ui)per and lower units of temperature tolerance 

 of mature and immature shad, as well as their 

 temperature preference. 



It should be noted that we have related aver- 

 age ocean conditions to an average shad migra- 



668 



