YELLOWTAIL FLOUNDER OFF NEW ENGLAND 



211 



Between the second year and third after tagging 



s= — , a=0.76. The value of a calculated in this 



way indicates the total mortality — fishing mortal- 

 ity and natural mortality occurring simultane- 

 ously. 



Table 33. — Numbers of tagged yellowtail returned in 

 successive years after release 



These estimates of mortality are subject to sev- 

 eral tagging difficulties, as well as to changes in 

 fishing pressure. First, as has been pointed out, 

 we probably experienced some mortality among 

 the tagged fish shortly after the fish were released. 

 Whenever the tagged fish were exposed immedi- 

 ately to a heavy fishery, as was usually the case, 

 undoubtedly some that would have died soon 

 were caught, thus tending to give a larger number 

 of returns in the first year than would be experi- 

 enced in the second from the same rate of fish- 

 ing. The result of this would be an estimate of 

 the annual expectation of death greater than the 

 actual value. Secondly, loss of tags through cor- 

 rosion of the pins probably took place somewhat 

 after the immediate tagging mortality; but we 

 judge that our losses from this cause were small 

 (see p. 180) , although we cannot accurately evalu- 

 ate them. Lastly, changes in fishing pressure in- 

 fluenced the number of returns. This cannot be 

 accurately evaluated because we do not know the 

 amount of fishing pressure on each population. 

 The fishing pressure on the southern New England 



stock (table 24) declined from 6,264 days in 1942 

 to 2,859 in 1945, increased to 4,698 days in 1948 and 

 dropped to 2,843 days in 1949. Except for 1949, 

 it did not change more than 30 percent in any 

 year. Since most of our sums of returns in suc- 

 cessive years include experiments conducted dur- 

 ing periods of both declining and increasing fish- 

 ing effort, we have chosen not to adjust our re- 

 turn data by the amount of fishing. 



The estimates of mortality in the southern New 

 England stock are lower than similar estimates in 

 the Georges Bank stock for which (table 33) al- 

 most all of the tags were returned during the first 

 year. In the Georges Bank stock, the survival 

 rate from the first year to the second was only 



— , or an annual expectation of death of 0.96. 



The proportion of returns in successive years was 

 similar in all lots. Reference to table 4 indicates 

 that in two of the releases off Georges Bank, lots 

 No. 7 and No. 8, a great number of returns were 

 experienced in the first week after tagging, but 

 in lot No. 14 the returns were well scattered 

 through the year after tagging, and yet no differ- 

 ent proportion was obtained in successive years. 



The mortality rate computed for the Cape Cod 

 stock is lowest of all (table 33). Here we obtain 

 the values for the annual expectation of death of 

 0.77 between the first year and the second after 

 tagging and of 0.62 between the second year and 

 the third after tagging. These values perhaps 

 should be even lower than this because we have 

 included lot No. 3, which was tagged under ex- 

 tremely difficult weather conditions and showed 

 no returns after the first year. If we consider 

 only lot Nos. 9 and 13, we find an annual expecta- 

 tion of death of 0.50 between the first year and 

 the second after tagging. 



When we associate these mortality rates with 

 the trends in the yellowtail fishery we find a rather 

 confusing relationship. As would be expected, 

 the lowest mortality rate occurred in the Cape Cod 

 stock where production was relatively stable, but 

 the higher rates occurred, in one instance, when 

 production was rapidly increasing and, in the 

 other cases, when production was seriously de- 

 clining. We have no explanation for this, but it 

 is clear that a high mortality rate from such com- 

 putations is not evidence per se of a dangerous 

 fishing rate. 



