YELLOWTAIL FLOUNDER OFF NEW ENGLAND 



213 



availability, because, as was presented in table 

 30, the 3-year-old yellowtail comprised the largest 

 fraction of the landings. We suspect also that 

 the mean apparent survival ratio of 0.565 between 

 age-groups 3 and 4 may be a little high for the 

 same reason. At any rate, the survival rate seems 

 to level off at 0.470 between age-groups 4 and 5 

 and at 0.451 between age-groups 5 and 6. Beyond 

 age-group 6, the apparent survival ratio jumps 

 again to 0.719; but this is not a good estimate 

 because too few age determinations were used and 

 the age-groups 6 and older were combined in the 

 first quarter and age-groups 7 and older in the 

 other quarters. 



The year-to-year survival rate has been obtained 

 by comparing the catch per day for age-groups 

 3 and older with the same group a year later 

 (table 37). For example, the comparison of 

 1943^14 in the first quarter was made from the 

 following formula: 



_ <74 40 +<753 9 +<76 38 

 S CZ i0 +C4 39 + C5 3S + C6v 

 C3 40 are the 3-annuli fish of the 1940 year class; 

 C6 38 are the 6-annuli fish of the 1938 year class, 

 et cetera. Here we find a low survival rate from 

 1943 to 1944, a high value for the next year, and 

 a decline from 1945 to 1947. If we compare year- 

 classes 1942 and 1943 for the fourth quarters only, 

 we find the survival rate is even lower than from 

 1943 to 1944. 



Table 37. — Mean apparent survival between years of yellow- 

 tail in the southern New England stock 



Survival rates computed from the abundance 

 indexes average substantially higher than rates 

 computed from the tagging returns in successive 

 years. This discrepancy may result from several 

 factors. The tagged yellowtail may have been 

 caught from a group whose migratory habits made 



476995 O— 59 4 



it more available to the fishery and thus actually 

 suffered a higher mortality rate than the average 

 for the stock. Other factors which we believe had 

 only a small effect on the computing of survival 

 rates were the immediate tagging mortality, the 

 continuing loss of tags, and possibly the slightly 

 higher, continuing death rate of tagged fish. The 

 significance of the first factor will be more ob- 

 vious after we examine the relation between fish- 

 ing effort and total mortality. 



We sought an estimate of natural mortality, q, 

 by modifying the method proposed by Silliman 

 (1943), who in effect considered the relation be- 

 tween the total instantaneous mortality rate, i, 

 and fishing effort, /, and then extrapolated to zero 

 fishing to find the natural mortality. We have 

 estimated the total instantaneous mortality rate, i, 

 for yellowtail 3 years and older (table 37), and 

 related it to the appropriate amount of fishing, /, 

 (table 24). For example, i computed for the 

 fourth quarter of 1942 to the fourth quarter of 

 1943, was compared with the amount of fishing 

 from the fourth quarter of 1942 through the 

 fourth quarter of 1943. For the succeeding annual 

 averages, the corresponding fishing effort was con- 



Table 38. — Relation of total mortality rate, i, to amount of 

 fishing effort, X 



(The total mortality rate, i, has been computed from the relative apparent 

 abundance of 3-year old and older fish in quarter N and the 4-year old 

 and older fish in quarter N+4. The fishing effort X has been computed 

 for various periods as follows: Jfi=effort In quarter N, Xi In quarters N 

 and N+l, X, in quarters N, N+l, and N+2, X, in quarters iV, N+l, 

 N+2, and iV+3] 



Year and quarter 



1942-43: 4th quarter. 



1943-44: 



1st quarter 



2d quarter 



3d quarter 



4th quarter 



1944-45: 



1st quarter 



2d quarter 



3d quarter 



4th quarter _ 



1945-46: 



1st quarter 



2d quarter 



3d quarter _ 



4th quarter 



1946-47: 



1st quarter 



2d quarter _ 



3d quarter 



4th quarter 



X, 



6.128 



5,453 

 5,219 

 4,890 

 4,260 



3,927 

 3,059 

 2,467 

 2,465 



2.859 

 2,944 

 2,994 

 2.950 



3,369 

 3.302 

 3.737 

 3.861 



CORRELATION COEFFICIENTS 



i Jfi=0.37 

 i X,=0A3 

 i Jfi=0.55 



i x,=o.m 



Regression: 



i= -0.397+0.000312 X, 



