management measures whereby the populations 

 might yield sustained high catches. In general, 

 before factors affecting abundance of this fish 

 could be determined, information on size of 

 runs or populations for a period of years was 

 necessary. Scientific evaluation of population 

 changes and of factors that could affect abun- 

 dance were used to determine if any signifi- 

 cant cause-and- effect relation existed. 



The following discussion of factors of shad 

 abundance includes the most recent informa- 

 tion available. 



PHYSICAL CHANGES 



Decrease in shad abundance on the Atlantic 

 coast has paralleled many physical changes 

 in the river environnnent (natural and man- 

 made) such as channel improvements, changes 

 in stream flow, siltation, and changes in water 

 temperature. Few data existed on which to 

 base a valid evaluation of the effects of these 

 physical changes on the entire coastal pro- 

 duction. One exception was data on the Hudson 

 River, for which catch and effort data for 

 37 yr., plus information on physical changes 

 in the environment, were available (Talbot, 

 1954). 



The Hudson River fishery declined at the 

 turn of the century, recovered beginning in 

 1936, and again declined beginning in 1945. 

 Many factors have been suggested for these 

 changes in the fishery. 



Dredging in the spawning grounds would 

 change the physical condition of the area and 

 possibly render it unsuitable for spawning. 

 Also, careless placing of spoil from disposal 

 of dredged material could affect abundance 

 by reducing the spawning grounds and limiting 

 nursery areas. In the Hudson, one of the most 

 extensively dredged rivers on the Atlantic 

 coast, Talbot ^1954) concluded that these op- 

 erations had no measurable adverse effect 

 on shad abundance from 1915 to 1951. Any 

 changes prior to 1915 could not bedocumented 

 however. 



Variation in stream flow could affect shad 

 abundance directly by influencing survival of 

 eggs and larvae, or indirectly by diluting or 

 flushing pollutants into the river. From studies 

 on the Hudson River, Talbot (1954) concluded 

 that any effect this variable had on fish abun- 

 dance was obscured by other conditions. 



Variation in water temperature during tinne 

 of spawning conceivably could affect survival 

 of eggs and larvae, hence population size in 

 subsequent years. Talbot's ( 1954) investigation 

 of the influence of this factor on abundance of 

 Hudson River shad stocks, 1929-51, indicated no 

 relation. Neither trends in water temperature 

 nor variations in average temperature could 

 be found which might account for the deviations 

 between predicted and calculated population 

 sizes. 



In the Connecticut River water temperature 

 and stream flow exhibited no changes or trends 

 that would account for the changes in the total 

 shad population or the deviations from the 

 predicted populations (Fredin, 1954). 



DAMS 



The construction of dams is recognized as 

 an important factor in the decline of shad on 

 the Atlantic coast. Obstructions built on rivers 

 ascended by shad have been discussed in the 

 geographical section of this report. 



How dam construction affects shad depends 

 on the location of dams in relation to spawning 

 areas. Runs were eliminated in rivers where 

 dams were built immediately above salt water. 

 Dams built within spawning areas probably 

 reduced populations in proportion to the amount 

 of natural spawning area destroyed. Dams built 

 above natural spawning areas probably had 

 little or no effect on fish abundance, provided 

 normal river flows were maintained. 



Most dams were constructed during the 19th 

 century, and they were most common in the 

 New England States, where development of 

 water resources played an important role in 

 industrial growth. The terrain allowed de- 

 velopment of a large amount of waterpower 

 with relatively small structures. Southern 

 rivers were not so suitable because of the 

 flat coastal areas. In recent years, however, 

 dams were built in the south for power, flood 

 control, navigation, and more recently for 

 cooling, particularly for steam- electric plants. 

 The disappearance of shad from such rivers 

 in Maine as the Mousam, Kennebec, Penobscot, 

 and Saint Croix was almost entirely the result 

 of their exclusion from spawning areas by dams 

 (Atkins, 1887; Taylor, C.C, 1951). In 1896 

 dams had closed about 3,700 miles of rivers 

 along the Atlantic coast which shad had for- 

 merly used, and these obstructions were one 

 factor that had helped reduce natural repro- 

 duction almost to insignificance. 



Mansueti and Kolb (1953) stated that dams 

 apparently were not the major factor in the 

 decline of shad production along the Atlantic 

 coast for the past 50 yr. Some areas, however, 

 offered exceptions. The Holtwood and Cono- 

 wingo Dams on the Susquehanna River com- 

 pletely eliminated shad from Pennsylvania 

 waters. Lock and Dam No. 1 on the Cape Fear 

 River in North Carolina closed off more than 

 100 miles of spawning and nursery area. 

 Santee-Cooper Dam on the Santee River in 

 South Carolina blocked more than 200 miles 

 of spawning and nursery area. 



Fish-passage facilities have alleviated in 

 Sonne measure the loss of spawning and nurs- 

 ery areas by dann construction. These facil- 

 ities are discussed in another section of this 

 report. 



91 



