YELLOWTAIL FLOUNDER OFF NEW ENGLAND 



215 



The 1944 year class was abundant as 3-year-olds 

 but probably not later, because the total yield of 

 the fishery continued to decline. 



With this problem of changing availability, Ave 

 cannot fix the total annual mortality rate or even 

 estimate the proportions due to fishing and 

 natural causes. "We can state that among fish on 

 the grounds completely available to the fishery the 

 total annual fishing rate is very high as indicated 

 by the average ??; of 0.86, which was computed 

 from early tag returns. Also it is certain that the 

 total annual mortality rate of the whole stock was 

 considerably less during the period of study as a 

 result of not being fully available. 



REPRODUCTION 



Early in the yellowtail investigation we col- 

 lected material from the commercial fishery at 

 New Bedford, Mass., to provide information on 

 the breeding habits of the yellowtail. Data were 

 obtained on the age and length at maturity and on 

 the spawning season of the yellowtail landed from 

 the southern New England stock. Attempts to 

 collect information on the juveniles were unsuc- 

 cessful; 14 however, we are able to present data on 

 yellowtail eggs and larvae which resulted from the 

 extensive plankton work undertaken by O. E. Sette 

 in his study of the eggs and larvae of the mackerel. 



Age and Length at Maturity 



At the peak of the spawning season in May 1943, 

 288 yellowtail were obtained at random from the 

 commercial landings at New Bedford, measured, 

 sex and condition of the gonads determined, and 

 scale samples obtained. At this time it was simple 

 to classify the individuals according to stage of. 

 maturity and, in mature females, whether ripe or 

 spent (table 39). 



Determination of the age of these fish revealed 

 that most individuals of both sexes mature dur- 

 ing their second and third years, although a larger 

 percentage of the males mature at a younger age 

 and smaller size. Of the females aged, 52 per- 

 cent were mature at 2 yeai-s, 67 percent at 3 years, 

 and 100 percent at 4 years and older. Of the 

 males, 84 percent were mature at 2 years, 92 per- 



14 Fifty-six tows with a 1%-inch mesh shrimp trawl, at times 

 lined with %-ineh mesh in the cod end, were made inside the 

 20-fathom contour between Nantucket and Long Island at various 

 times from July to October in 1943, 1945. and 1946. No juvenile 

 or larval yellowtail were taken. (Data on file at the Woods Hole 

 laboratory of the D. S. Fish and Wildlife Service.) 



cent at 3 years, and 100 percent at 4 years and 

 older. 



If we extend the results of this sample to obtain 

 an estimate of the proportion of immature in- 

 dividuals in the catch during the spawning sea- 

 sons in the period during which we have studied 

 this fishery, we must assume that the proportion 

 of immature fish found in May 1943 is represent- 

 ative of that obtained in other years. This seems 

 a likely assumption inasmuch as we have already 

 pointed out that only very slight changes in 

 growth rate and in length composition were noted 

 during the period of study, 1942 to 1947. There- 

 fore, if we apply our percentages of maturity to 

 the summarized age composition for the second 

 quarters of the years 1942 to 1947 (appendix table 

 D-2, p. 254), we estimate that during the second 

 quarters 94 percent of the males and 84 percent 

 of the females in the landings would be mature. 

 The same percentages would apply to the land- 

 ings during the spawning season, since (as will 

 be shown in the next section) almost all of the 

 spawning occurs during the second quarter. 



The same data provide us with an estimate of 

 length of the yellowtail at maturity, but since the 

 majority of the males mature before they appear 

 in the commercial catch, it is not possible to relate 

 maturity to length with any precision. For our 

 purpose it is adequate to know that the males do 

 mature before entering the commercial catch and 

 mostly before they attain the length of 26 cm. 

 Females, however, mature after reaching com- 

 merial size and our sample appears adequate for 

 determining the size at which they mature. The 

 most reliable estimates can be obtained by assum- 

 ing that the data form a sigmoid curve and by 

 transforming the data to the probability integral 

 or "probit." 15 



A line fitted to the transformed data resulted 

 in the formula y= — 0.2176 + 0.1631a? in which y 

 equals the estimated probit and x equals the 

 length in centimeters. The estimated probit was 

 then transformed back to a percentage to find the 

 points for the sigmoid curve in figure 23, and to 

 provide the estimates that 50 percent of the female 

 yellowtail in the landings were mature at a length 

 of 31.98 cm. and 90 percent were mature at 40.17 

 cm. We may further compute the variance of the 



15 A discussion of the use of probits for this purpose will be 

 found in appendix F, p. 266. 



