in our collection area appear in June, rise to 

 greatest abundance about late July, and diminish 

 in numbers thereafter. Greatest losses occur, 

 presumably, as a result of predation and migra- 

 tion from the collection area. 



Although O-group gizzard shad were taken in 

 greatest numbers in late July, none of them had 

 become large enough to be caught by the gill 

 nets and trap nets until August. Subsequently, 

 the size of the collections of the O-group coming 

 into this catchable size range increased regularly 

 until they reached peak abundance toward the 

 end of the calendar year (table 4). They were 

 the dominant age group among fish taken by 

 these nets from August to the end of the year. 

 From January to August of the next year, con- 

 trary to expectation, these fish, as the I group, 

 were not dominant in the catch of gill nets and 

 trap nets. Over this period the II-group fish 

 dominated the catches. After August, however, 

 the I group became more plentiful than the older 

 groups and maintained that abundance thereafter. 



The scarcity of I-group gizzard shad in the gill 

 net and trap net samples from January to August 

 cannot be fully explained. When all the fish 

 captured in January-July are considered (whether 

 they were obtained by dynamite, rotenone, 

 electrocution — data not tabulated here), then the 



Table 4. — Age composition of gizzard shad taken by trap 

 nets and Jf-inch-mesh gill nets, Lake Erie, 1954 



[Percentages of fish in age-groups I-VI given in parentheses— the O-group 

 fish were not considered in the computation of percentages or average age] 



1 Average number of annuli. 



2 Young of the year first became available to the nets in August. 



I group is the dominant group in the collecting 

 area in all the early months except June and 

 July. The June collection especially is note- 

 worthy — the I group was very poorly represented 

 in samples from both the lake and Fishery Bay — 

 especially in the bay where only 4 I-group shad 

 were collected along with 820 II-group fish. 



It is noteworthy that the II-group and older 

 shad were taken principally in April through 

 July. Only during the spawning season (June- 

 July) was I able to capture the Vl-group shad, 

 which were the oldest in the samples. The 

 August-September samples were dominated over- 

 whelmingly by age group I (89.0 to 98.8 percent). 

 The II group made up 1.2 to 10.6 percent, and 

 only four older fish were taken in the 5 months. 



Sampling difficulties, as previously described, 

 prohibited any detailed consideration of fluctua- 

 tions of year-class strength; however, the 1952 

 year class apparently was one of unusual abun- 

 dance. In random samples from commercial trap 

 nets this year class made up 85 percent of the 

 total as age group I, 71 percent as age group II, 

 and 11 percent as age group III. 



Survival Rate 



Although the records (table 4) do not give a 

 fully satisfactory idea of the age composition of 

 the gizzard shad stock, the collections made in 

 April-July probably permit fairly reliable inference 

 as to the ratios between the numbers in successive 

 age groups for II-group and older shad, since these 

 mature fish seem to constitute a homogeneous 

 group during this period. Collections made from 

 August to December are probably reasonably 

 reliable for determining the ratio of the I-group to 

 the II-group shad. From these two sets of data, 

 I have estimated the survival rate of shad from 

 one age group to the next higher one (table 5). 



An estimate (not given in table 5) was made also 

 of the survival of gizzard shad from egg deposition 

 to age-group I. The first step was the calculation, 

 from data on sex ratio (table 15) and fecundity 

 (table 18), of the probable number of eggs de- 

 posited by the gizzard shad of age-group II and 

 older. The females of the 6,049 fish (total of the 

 II-group and older of the right-hand column of 

 table 5) were thus estimated to have deposited 926 

 million eggs. If the 100,000 I-group fish of table 5 

 are held to be the survivors of this number of eggs 

 (it is assumed that the population is relatively 



GIZZARD SHAD IN WESTERN LAKE ERIE 



399 



