CHITTENDEN; DYNAMICS OF AMERICAN SHAD RUNS 



1966 males, suggesting that the 1966 and 1968 runs 

 were small-even before the fish reached the 

 Philadelphia area. 



The higher proportion of males in 1962 than in 

 1961 indicates that the 1962 run was larger than 

 the 1961 run. 



Comparative Year-Class Strengths, 

 1956-64 



I here define year-class strength as the numbers 

 of young which exist at some constant point in 

 time-say 1 January-after the entire year class 

 has "passed" seaward through the grossly polluted 

 Philadelphia area. Year-class strength is 

 influenced by spawning success. However, the 

 dominant factor, by far, in setting year-class 

 strength in the Delaware River is the success with 

 which the young pass seaward through the 

 Philadelphia area in summer and fall (Chittenden 

 1969). 



Comparative year-class strengths can be es- 

 timated from the age and sex compositions from 

 1963 to 1965 (Tables 1, 3) supported by estimates of 

 comparative run sizes. Males usually first migrate 

 upstream at age IV and females at age V, so that 

 the size of a run chiefly reflects the strength of 

 year classes produced 4 and 5 yr earlier assuming 

 constant survival at sea. Therefore, American 

 shad runs from 1961 to 1968 chiefly reflect year- 

 class strengths from 1956 to 1964. The largest year 

 classes as defined were produced from 1957 to 1960 

 because the largest runs were from 1962 to 1964. 



The 1963 American shad run was the largest in 

 this period and primarily reflects the 1958 and 1959 

 year classes. I captured 90 age V fish at Lambert- 

 ville in 1963 and 98 in 1964 with approximately 

 equal effort, suggesting that the 1958 and 1959 

 year classes were similar. This agrees with com- 

 parative magnitude estimates for the 1962 and 

 1963 runs. The 1963 run was based on two large 

 year classes and was larger than the 1962 run 

 which was based on one large and one smaller year 

 class. 



The 1960 and 1959 year classes can be compared. 

 I collected 180 age IV fish at Lambertville in 1963 

 but only 60 age IV's in 1964. This suggests that the 

 1959 and, by inference, 1958 year classes were 

 much larger than that of 1960. This deduction is 

 supported by the shift in sex composition from 

 1963 to 1964, which indicates the age V year class 

 from 1959 was much stronger than the age IV year 

 class from 1960. The 1963 run was based on two 



large year classes and was larger than the 1964 run 

 which was based on one large and one smaller year 

 class. 



The 1957 and 1958 year classes can be compared. 

 At Lambertville, 36 and 21 age VI fish were cap- 

 tured in 1964 and 1963, respectively, suggesting 

 that the 1958 and, by inference, 195*9 year classes 

 were stronger than that of 1957. This is supported 

 by mean sizes of males collected during the Tri- 

 state Sur\'eys of 1961 and 1962 (Table 2). The 1961 

 fish (mean = 401 mm = 15.8 inches) were sig- 

 nificantly smaller than the 1962 fish (mean = 414 

 mm = 16.3 inches). The obsen-ed mean fork length 

 of age IV males in 1963 and 1964 was about 422 mm 

 (16.6 inches) (Chittenden 1969, table 17), suggest- 

 ing that age III fish from 1958 were important in 

 the 1961 run. 



The 1961 and 1960 year classes can be compared. 

 Sex composition and catch data show that the 1964 

 run was smaller than the 1965 run. Age structures 

 were almost identical, however, and this indicates 

 that the 1961 year class was smaller than that of 

 1960. 



The strength of the 1962, 1963, and 1964 year 

 classes as defined must have been small, probably 

 being no larger than the 1961 year class, because 

 the 1966 to 1968 runs were very small. 



In summary, the 1958 and 1959 year classes were 

 extremely large in comparison to the year classes 

 produced in the period 1961-64. The 1957 and 1960 

 year classes were much smaller than those of 1958 

 and 1959, but were larger than the 1962, 1963, and 

 1964 year classes. 



GENERAL DISCUSSION 



Delaware River American shad runs in the 

 1960's showed great shifts in the proportion of 

 male fish. This variation was due, in large part, to 

 fluctuations in year-class strength. The propor- 

 tions in 1961 (0^86) and 1962 (0.99) are extreme. 

 They are based upon summer collections and are 

 biased if females tend to return seaward earlier 

 than males do. However, these proportions also 

 probably primarily reflect the temporary resur- 

 gence of the Delaware River American shad runs 

 in the early 1960's reported by Chittenden (1974). 

 The magnitude of American shad runs in the 

 Delaware River was at a very low level prior to 

 1961, increased in 1961 and 1962, and in 1963 the 

 magnitude was such that it ranked among the 

 largest runs in the last 45 >t or more. Because 

 Delaware River males tend to undertake their first 



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