Fishes of the Western Nortli Atlantic 479 



Following construction of a hydroelectric plant on the Shannon River, Ireland, 

 there was a marked change in the composition of the incoming runs (138: 789). Taken 

 as representative of the pre- and post-period of hydroelectric operation, about ay.^^/o 

 of the total run occurred in June of 1928 compared with 70 "/o that month in 1941 ; 

 in 1928, 24.7 */o of the run consisted of grilse, and in 1941, 75*/o. The age of the fish 

 in 1 94 1 averaged approximately one year less than in 1928, and of course the average 

 size was smaller. This change, due to a proportionate rather than an actual increase in 

 the number of grilse, arose from a reduction in the number of age groups spending 

 two or more years feeding at sea. Hydroelectric operations resulted in a reduction of 

 the Atlantic Salmon stocks in the river to about 30—40 "/o of those of prehydroelectric 

 days. The Mulcair River, which was the major spawning ground for the smaller fish, 

 became the major spawning tributary for both large and small fish. 



In the Miramichi, regulations to restrict fishing to the early run and encourage 

 reproduction of the late run (natural as well as artificial) have not succeeded materially 

 in either reducing the early run or increasing the late run, although this unconscious 

 experiment had been carried on for more than 80 years (^8). 



Length of Life and Spawning Survival. During all stages, from egg to fry, fry to 

 smolt, smolt to adult, and after spawning, there are wide differences in the number of 

 Atlantic Salmon that survive, not only from river to river but in the same river from 

 year to year, depending on meteorological conditions, population density of Atlantic 

 Salmon as well as other species, abundance of predators, disease, and man's activities. 

 In considering these differences, factors responsible for discrepancies in estimates of 

 the composition of the runs should be kept in mind (ftn. 1 2). 



Kerswill has found in experiments in progress on the Pollett River, N. B., that 

 survival of fry from egg deposition (three light seedings) had averaged 6 "/o in July 

 (1957 Mss.). "Under favourable conditions, each 100 square yards of stream is cap- 

 able of producing 5 or 6 smolts .... These will arise from 10 or 12 large parr, and 

 they in turn from 30 to 40 hatchery fingerlings planted in late summer." About 200 

 eggs, it was estimated, should produce this number of fish (5: 49). Another estimate of 

 survival from fingerling to smolt stage, based on observations for five successive years, 

 is 8 «/„ (^o). 



Percentage survival from smolts to returning adults is difficult to determine; the 

 following figures are not all directly comparable. For example, it has been estimated 

 from the counting and marking of descending smolts and from the trapping and 

 counting of returning adults in several rivers of the Maritime Provinces that "about 

 8 "/o of the smolts survive the rigors of life in the sea and return to fisheries and 

 rivers as mature salmon" {42: 25; gi: 825). On the other hand, the contribution to 

 various fisheries by the rivers was found to be about 2 "/^ of the marked descending 

 smolts (5). 



The following figures on smolt survival have been published: Little Codroy 

 River, Newfoundland, 3.3 "/o {lOj); Pollett River, 0.50/0 (88)\ Great Britain, i "/o 

 {ll6)\ Tay River, Scotland, nearly 2 "/p {loo)\ Sweden, 5-10 «/o (j2). 



