Informe Anual de la Comisión Interamericana del Atún Tropical, 19

ANNUAL REPORT 1987
STATUS OF THE TUNA STOCKS IN 1987
AND OUTLOOK FOR 1988
It is the responsibility of the IATTC staff to monitor the fiuctuations in abundance of the
tropical tunas ofthe eastern Pacific Ocean and to assess the effects of man's activities and natural
factors on their abundance. Each year scientific information gathered by the staff is used to make
recommendations for the management of these species. This is accomplished by presenting
information in background documents and orally at the meetings ofthe Commission. To disseminate
this information to the general public it is summarized in this annual reporto
YELWWFIN
The status of yellowfin in the eastern Pacific Ocean has been assessed by the employment of
age-structured models and production models with data collected by the IATTC staff. These results
will be summarized, followed by a discussion of the condition of the resource in 1987 and
considerations for 1988.
Age-structured models
Age-structured models make use of data on recruitment and on the growth and mortality of
individual fish. Estimates of these parameters are obtained from analyses of data on the lengthfrequency
distributions ofthe fish in the catches, supplemented by tag return data and other data of
various types. The IATTC staffhas collected data on the size composition ofthe catches ofyellowfin in
the eastern Pacific Ocean since 1954, and in addition has had access to the corresponding data
collected during 1951-1954 by the California Department ofFish and Game.
Biomass analyses
Age-structured biomass analyses performed by the IATTC staff have produced information
concerning the abundance and mortality of fish of individual year classes which has been highly
useful for stock assessment. As afirst step, the fish in the catch must be assigned to cohorts. (A cohort
consists of all the fish recruited to the fishery at the same time, such as the X68 cohort, which first
entered the fishery in small numbers during the first half of 1968 and contributed to it significanUy
for the first time during the second half of that year, and the Y68 cohort, which first entered the
fishery in small numbers during the second half of 1967 and contributed to it significanUy for the
first time during the first halfof1968.) The recruitments to the Xand Ycohorts have been estimated,
on the average, to be about the same (Table 19). Then estimates are made ofthe numbers ofthe fish of
each cohort at the time ofrecruitment and at various intervals thereafter and ofthe numbers which
are caught and which die during each interval. These numbers are converted to weights from data on
the average weights of fish of different sizes to calculate the biomass estimates. The information
needed for these calculations includes the numbers and average weights of fish of each cohort in the
catch, an estimate of the natural mortality rate (or estimates of a series of rates if the natural
mortality is not constant for the fish of different ages), and an estimate ofthe fishing mortality rate
for at least one catch interval for each cohort. These are called the "initiating" rates of fishing
mortality. All ofthese estimates are obtained from analyses ofcatch statistics and length-frequency
and length-weight data. The results of these analyses are not as accurate as desired because,
although fish ofdifferent cohorts can be easily distinguished from one another when they are young,
this becomes increasingly difficult as they grow older, so older fish are probably often assigned to the
wrong cohorts. Thus not only are the estimates ofthe catches offish ofeach cohort less accurate than
desired, but the estimates ofthe rnortality rates are somewhat crude. Specifical!y, an estimate ofO.8
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