POLLUTION 



OVERFISHING 



Industrial and domestic pollution has been 

 cited by many workers as a major factor in 

 the decline of shad abundance (fig. 29). The 

 decline in the shad fishery in the Saco River 

 in Maine was attributed to the pouring of dyes 

 from cotton and woolen mills into the river 

 (Atkins, 1887). Industrial pollution from tex- 

 tile and paper mills destroyed the fishery in 

 the lower Androscoggin River in Maine as 

 early as 1884 (Taylor, C.C. 1951). Stevenson 

 (1889) cited many examples of individual river 

 populations destroyed or severely reduced by 

 pollution prior to 1896. In the lower Delaware 

 River, the dissolved oxygen in autumn was too 

 low for safe passage of downstream migrant 

 juveniles (Ellis, Westfall, Meyer, and Platner, 

 1947). Later information of Sykes and Lehman 

 (1957) proved that oxygen content of the lower 

 Delaware River during both summer and fall 

 remained low because of continued pollution; 

 safe passage of juvenile fish to the sea de- 

 pended on river flow sufficient to dilute pol- 

 lution. Heavy pollution in the lower Delaware 

 during the migration of adults into the river 

 for spawning resulted in high mortality. 



Pollution of the Hudson River has offered 

 a serious problem and perhaps has been an 

 important factor of shad abundance (Talbot, 

 1954). Unfortunately, lack of records to show 

 changes in pollution in the spawning and nurs- 

 ery grounds prevents any possible demon- 

 stration of a cause-and- effect relation. 



It is certain that pollution has increased 

 over the years in most streams. No exact 

 measure of it exists that might be studied 

 along with population statistics; hence no 

 definite conclusions can be drawn for most 

 streams. 



Figure 29. — Thousands of dead young American shad, 

 spawned in the spring of 1954, in the Anacostia River 

 near Sousa Bridge, Washington, D.C. (Photograph cour- 

 tesy of Charles Del Vecchio, Washington Post-Times 

 Herald) 



For many years, decline in shad stocks has 

 been attributed to overfishing. Numerous ex- 

 amples have been cited where overfishing was 

 blamed for decreased catch even before 1850 

 (Stevenson, 1899). Stevenson theorized that in 

 1896 heavy fishing in river mouths which pre- 

 vented fish from reaching their spawning 

 grounds was a major cause of depletion. 

 Although overfishing has been nanned as a 

 major cause for decline, proof of this allega- 

 tion in specific areas was not forthcoming 

 until recent years. 



Shad must migrate up rivers enroute to 

 spawning grounds and are particularly vul- 

 nerable to fishing. Rivers can be so heavily 

 fished that much of the migrating population 

 is captured. A certain minimum number of 

 fish must escape capture and spawn to re- 

 place those taken by the fishery, or depletion 

 follows. 



Most shad mature and spawn at an age of 

 4 or 5 yr., and fish native to streams from 

 Virginia northward may spawn more than 

 once. The run in any year, therefor e, generally 

 is composed of fish hatched 4 or 5yr. earlier, 

 plus those surviving from the previous spawn- 

 ing or spawnings. If appropriate population 

 statistics are available on a fishery for a 

 period of years, statistical methods usually 

 can be used to evaluate the effect of fishing 

 on the population. 



Recent studies on the Connecticut and Hudson 

 Rivers, both of which formerly produced at a 

 high level but have fluctuated greatly, revealed 

 that over 80 percent of the annual variation in 

 population size was caused by changes in the 

 size of the spawning escapement (Fredin, 

 1954; Talbot, 1954). These studies clearly in- 

 dicated that overfishing caused the decline of 

 these shad populations, at least during recent 

 years. A decrease in fishing effort led to an 

 increase in abundance in both rivers. Since 

 catch and effort statistics were available for 

 only these two rivers, the effect of fishing on 

 the entire Atlantic coast population could not 

 be evaluated. It can be suspected strongly, 

 however, that abundance of shad populations 

 in the other rivers could be influenced by 

 changes in fishing effort. 



NATURAL CYCLES IN ABUNDANCE 



Populations of marine animals fluctuate 

 widely in abundance; but causes for these 

 changes are poorly understood. They may be 

 due to the effects of man, changing environ- 

 ment, or natural factors that operate in- 

 dependently of others. It has been suggested 

 that cyclical fluctuations in abundance of shad 

 stocks have occurred which may not be re- 

 lated to man's activity and that the period is 



92 



