212 



FISHERY BULLETIN OF THE FISH AND WILDLIFE SERVICE 



Mortality and Apparent Abundance of Age Groups 



The basic data for our third method of estimat- 

 ing total mortality are to be found in tables 23 

 and 29, which show the percentage age composi- 

 tion and the catch per day in numbers of fish of 

 the southern New England stock. These data, 

 when combined in table 34, provide estimates of 

 the catch per day in numbers of each age group 

 in each quarter from the fourth quarter of 1942 

 through 1947. From these data we shall select 

 the apparent abundance of homologous groups or 

 age classes in successive years, and this may be 

 done more easily if the data are reorganized to 

 show the abundance of each year class in each year 

 (table 35). 



Table 34. — Catch per day in numbers of yellowtail, of each 

 age from the southern New England stock, 4th quarter of 

 1942 through 1947 



[Based on tables 23 and 29] 



Year and quarter 



Year 1942: 



4th quarter.. 

 Year 1943: 



1st quarter.. 



2d quarter... 



3d quarter... 



4th quarter.. 

 Year 1944: 



1st quarter. . 



2d quarter... 



3d quarter... 



4th quarter.. 

 Year 1946: 



1st quarter. . 



2d quarter... 



3d quarter... 



4th quarter.. 

 Year 1946: 



1st quarter. . 



2d quarter.. 



3d quarter.. 



4th quarter. 

 Year 1947: 



1st quarter. . 



2d quarter.. 



3d quarter.. 



4th quarter. 



Number of fish having- 



1 an 

 nulus 



387 

 364 



34 



206 



232 



487 

 ~~26 



2 an- 

 nul! 



4,800 



1,793 



762 



1,909 



1,677 



1,555 

 275 

 976 

 838 



1,083 



426 



5,551 



3,003 



2,275 

 2,268 

 5,630 

 2,793 



1,868 



827 



4,646 



2,049 



3 an- 4 an- 

 nuli null 



4,413 



1,596 

 3,068 

 6,293 

 2,205 



2,177 

 5,776 

 6,836 

 1,795 



1,585 

 2,010 

 2,511 

 1,569 



1,671 



1,936 



834 



1,097 



1,435 

 2,025 

 4,646 

 1,620 



1,164 



1,008 

 1,193 

 3,323 

 1,173 



1,244 

 385 



1,674 

 735 



1,324 

 2,071 

 3,380 

 2,657 



1,012 

 2,904 

 2,078 

 1,032 



650 



662 



1,211 



776 



5 an- 

 nul! 



921 



504 

 565 

 424 

 475 



311 



275 

 488 

 326 



623 



609 



756 



1,224 



566 



968 



1,508 



1.1 



568 

 1,120 

 1,186 



616 



6 an- 

 nul! 



7+ 

 annnJI 



534 



599 

 519 

 482 



194 



178 



AU 

 flsh' 



12, 413 



5,742 

 6,090 

 12, 373 

 5,612 



6,786 

 6,821 

 10,043 

 4,171 



5,347 

 6,091 

 12, 613 

 9,133 



6,007 

 8,066 

 10,369 

 7,187 



5,415 

 5,412 

 12, 332 



5,650 



1 Slight discrepancies occur due to rounding off of the figures. 



We have computed the apparent survival, s, 12 

 between age groups as an average of the several 

 years during which we observed the fishery by 

 summing the catch per day for each year class 

 that appears in each age group, and then deter- 

 mining the ratio between successive age groups 

 (table 36). For example, in the first quarter we 



Table 35. — Catch per day in numbers of yellowtail, by year 

 class and quarter, from the southern New England stock, 

 1937-44 



have computed the ratio 1.024 between age groups 

 2 and 3 as follows : 



2^/040 1 . . . t/o 43 



2' 2i i0 + . . . C2i3 

 in which C3 40 indicates the catch per day of 

 3-annuli fish of a 1940 year class, and so on. The 

 data have been kept by quarters because of our 

 previous observations that different populations 

 tended to be available in different quarters. 



Table 36. — Mean apparent survival between age groups of 

 yellowtail in the southern New England stock 



(Computed from abundance Indexes for fourth quarter of 1942 through 1947] 



u We computed the apparent survival to avoid difficulties with 

 apparent minus mortalities resulting from changing availability. 



Several anomalies occur in the apparent survival 

 data. The ratios greater than 1 between age- 

 groups 2 and 3 are doubtless due to increasing 



