Informe Anual de la Comisión Interamericana del Atún Tropical, 19

ANNUAL REPORT 1987
example, the X83 and Y83 cohorts (diagonallines), which were both relatively strong, were not fished
heavily until1985, when they were in age-group 3and their biomass was high. The Y73 cohort, on the
other hand, was only average, and after having been fished heavily as age group-1 fish in 1973 its
biomass was low in the succeeding years.
In Figure 33 are shown the average quarterly and annual biomass estimates for 1968 through
1987 for aH fish regardless of age and for "large fish" (aH Y-cohort fish which had been in the fishery
more than six quarters plus aH X-cohort fish which had been in the fishery more than eight
quarters). The data for 1964-1967 are not shown because during that period the area offishing in the
CYRA was still expanding and the cohort biomass estimates for those years are biased downward.
During 1968-1971 the total biomass averaged about 350 thousand tons. This was comprised ofalarge
proportion of older fish resulting from the fish of the aboye-average 1966 and 1967 recruitments
which were exploited lightly as young fish. Below-average recruitment, coupled with increasing
exploitation rates during 1969-1972, resulted in a decline in the biomass during 1972 and 1973 of
both the younger and older fish. The lower biomass ofolder fish persisted through 1974, whereas the
abundance of smaHer fish increased in 1974 as a result of an extremely large 1974 year class. That
year class accounted for most ofthe increase in biomass oflarge fish in late 1975, all of1976, and early
1977. The fourth-Iargest catch ofyellowfin inside the CYRA in the history ofthe fishery was made
during 1976. Below-average recruitment during 1976 and 1977, coupled with heavy exploitation of
young fish from 1973 to 1982, resulted in the biomass ofboth young and old fish decreasing in 1982 to
the lowest level observed. Although the 1978 and 1979 recruitments were high, the former exceeded
only by that of 1974, so many of the fish were captured as 1-and 2-year-olds that they did not
contribute heavily to the fishery in 1981 or 1982. The recruitment was average in 1980, slightly aboye
average in 1981 and 1983, below average in 1982, and well aboye average in 1984, 1985, and 1986. Not
only were the 1984-1986 recruitments high, but together they produced the greatest combined
recruitment on record (about 25 percent greater than the average combination ofthree years for the
1967-1987 period). The biomass increased in 1983 (Figure 33), due primarily to lesser fishing effort,
and greatly increased in 1984 and 1985 due to good recruitment and reduced fishing on young fish.
The benefits from the 1984-1986 recruitment continued into 1987, in spite ofthe large increase in the
catch. Preliminary estimates of the 1987 recruitment indicate that it was aboye average.
The annual catches, in short tons, offish ofthe Xand Ycohorts, and ofboth cohorts combined,
during 1968-1987 are shown in Figures 34 and 35. In Figure 34 each panel contains the catches of
fish ofdifferent cohorts in the same year, whereas in Figure 35 each panel contains the catches offish
of the same cohort in different years.
Comparison of Figures 32 (annual estimates of the biomass of the X and Y groups), 33
(quarterly and annual estimates ofthe total biomass and the biomass oflarge fish), and 34 and 35
(catches during each year by age and cohort) reveals the changes which have taken place in the age
composition of the population and the catch. During the 1970s, when the fishing effort was high,
fewer and fewer individuals survived to reach the larger sizes. The fishing effort was reduced in 1982,
1983, and 1984, which permitted many fish which would have been caught while young to grow
larger. The total contributions ofeach Xand Ycohort to the fishery have been estimated for the 1968
through 1983 cohorts, using the data in Table 20, and these are shown in Figure 35. They ranged
from 121 thousand tons for the 1970 cohort to 268 thousand tons for that of1974, with an average for
the 16 groups of161 thousand tons. Ifthe Y74 cohort had not been fished so heavily in its first year the
catches for that cohort would have been even greater. Division ofthese cohort catches (1968-1983) by
the corresponding estimates of recruitment (values in Table 19B adjusted to the initial numbers)
gives yield-per-recruit estimates which varied from 4.2 to 7.6 pounds, (1.9 to 3.4 kg). The initial
recruitment varied from 32 million to 92 million fish, averaging 54 million fish, and produced an
average yield per recruit of 5.9 pounds (2.7 kg).
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