ABUNDANCE OF CONNECTICUT RIVER SHAD 



257 



In 1946, the proportion of 3-year-old males 

 in the sample from Enfield Dam was 21.9 per- 

 cent; the proportion in 1947 was 22.6 percent. 

 These are the only 2 successive years in which 

 the proportionate abundance of 3-year-old males 

 in the Enfield Dam sample was similar. In 1946, 

 the 6-year-old class predominated in the Enfield 

 Dam samples. The total and extraneous- 

 mortality rates among this group of fish for 1 

 year will be determined. 



Where 



P 6 = the proportion of 6-year-old fish in the 



sample from Enfield Dam in 1946, 

 P 7 =the proportion of 7-year-old fish in the 



sample in 1947, 

 £"1946= the estimated total escapement in 



1946, 

 E m7 =ihe estimated total escapement in 



1947, 



then, 



n t =P i E ma = the number of 6-year-old fish 



that escaped the fishery in 



1946. 

 n 7 =P 7 E 1U7 =the number of 7-year-old fish 



that escaped the fishery in 



1947, 

 and 



— — — =the total mortality rate among 6- 

 716 year-old fish for 1 year. 



During the course of a year, the 6-year-old 

 shad are subject to mortalities from two sources: 

 extraneous mortality and fishing mortality in the 

 river. Extraneous mortality occurs prior to the 

 following season's fishing mortality in the river; 

 the number of deaths by extraneous causes is 

 assumed to be negligible during the fishing season. 

 Where M z is the extraneous mortality rate and 

 M f is the river fishing mortality rate, the total 

 number of deaths among the 1946 6-year-old 

 group is: 



M x n & +M,(n & — M I 7i 6 )=n 6 — n 7 ; (8) 



further, 



~n 9 — n 7 



M, 



=[^-*,>a- 



M,y 



(9) 



The total and extraneous mortality rates of the 

 6-year-old shad escaping the fishery in 1946 are 

 estimated as follows: 



■E'i946= 66,253. 

 £ 1947 =51,368. 



■^,,1947 = 0.810. 



K , = the number of fish sampled atEnfieldDamin 

 1946 = 1,183. 



-K" 2 =the number of fish sampled at Enfield Dam 

 in 1947 = 1,344. 



P„= proportion of 6-year-old fish among K x fish 

 273 



1,183" 



= 0.231. 



P 7 = proportion of 7-year-old fish among K 2 fish 



= 1^4 = - 036 - 



tt 6 =P a tf m6 =0.231 (66,253) = 15,304. 



n 7 =P 7 £' m7 =0.036(51,368) = 1,849. 



n 6 -n 7 = 15,304-l,849 = 0879 = the tQtal mortal . 

 n 6 15,304 



ity rate. 



.879— .810 



M x =- 



1-.810 



=0.363. 



An approximation to the variance of M z is 

 obtained from the expectation of {dM x ) 2 (Deming 

 1943) : 



r P6 2 F 7 (i-p 7 ) p 7 2 p 6 (i-p 8 ) "i 



l * * * J 



[red* (10) 



For the case presented, (VM t ) =0.009195; the 

 standard deviation is approximately 0.096. 



The number of 6-year-old shad that escaped 

 from the fishery in 1946 and died from extraneous 

 causes before returning to the river in 1947 is esti- 

 mated to be 0.363 (15,304), or 5,555 fish. The 

 commercial fishery in the river removed 0.81 

 (15,098-5,345), or 7,897 fish of this age class. 

 Thus, of the total of 13,452 deaths, about 40 

 percent occurred outside the Connecticut River 

 shad fishery, and the effect of the extraneous 

 mortality on the 6-year-old group of shad was 

 nearly as great as the river-fishing mortality. 

 Extent of fluctuation in the extraneous-mortality 

 rate from year to year is not known, but if a 

 standard sampling technique is followed in the 

 collection of scales from the commercial catch, 

 and samples are taken each year for a number of 

 years, the extraneous-mortality rate can be esti- 

 mated each year and changes in it determined. 



It was pointed out earlier that tagging studies 

 have shown that Connecticut River shad are 



