Informe Anual de la Comisión Interamericana del Atún Tropical, 19

ANNUAL REPORT 1987
set fishing, as it did during 1973-1982, CPDF underestimates yellowfin abundance, and when the
fishery switches back to dolphin-set fishing, as it did during 1984-1987, that CPDF overestimates the
abundance.
TUNABIOWGY
Size composition ofthe cateh
Length-frequency samples of yellowfin, skipjack, bluefin, bigeye, and black skipjack,
Euthynnus lineatus, from purse-seine and baitboat catches made in the eastern Pacific Ocean are
collected by IATTC personnel at ports of landing in Ecuador, Mexico, Panama, Peru, the USA
(California and Puerto Rico), and Venezuela. The catches ofyellowfin and skipjack were first sampled
by the IATTC staff in 1954, and sampling has continued to the presento Information obtained from
length-frequency samples has been used for the staffs estimates of growth, mortality, yield per
recruit, and year-class abundance. The results ofthese studies have been reported in several IATTC
Bulletins and in all its Annual Reports since 1954.
The staff collected and processed 911 yeJlowfin, 359 skipjack, 15 bluefin, 13 bigeye, and 23
black skipjack samples from the 1987 catches in the eastern Pacifico Most of these were 50-fish
samples. For both yellowfin and skipjack, the length-frequency samples are stratified by market
measurement areas (Figure 10), months, and gear. Sampling within each stratum is done in two
stages, with a boat "unit" (usually a weJl or pair ofwells) as the first stage and individual fish as the
second stage. The units within strata are sampled randomly and fish selected randomly from each
sampled unit are individually measured. The total number offish in each length group in a sampled
unit is estimated by dividing the total catch in weight in the unit by the average weight of the
sampled fish in the unit and then multiplying this quotient by the fraction ofthe sampled fish in that
length group. The stratum totals, in numbers offish, for each length group are obtained by summing
the totals for each sampled unit and then multiplying this total by the ratio ofthe weight ofthe logged
catch ofthe stratum to the sum ofweights ofthe sampled units. The quarterIy and annual totals are
obtained by summing the data for all the sampled strata for the quarter or year in question. The
quarterIy and annual average weights are obtained by summing over all the length groups in the
quarterIy or annual estimates and dividing the sum into the sum ofthe weights of the catches for all
the sampled strata.
Histograms showing the estimated tons of yellowfin caught, by 2-cm intervals, based on
logged catches, for the market measurement areas of the CYRA (Figure 10) in 1987 are shown in
Figure 11. The areas are arranged approximately from north (top) to south (bottom) in the figure.
Most ofthe catch in Area 1consisted of fish less than 70 cm in length. In Area 8the fish in the catch
ranged from about 40 to 140 cm. There were modes at 48,68, and 90 cm. In Area 2 most ofthe fish
caught ranged between about 50 and 160 cm. In Area 4the fish in the catch ranged from about 40 to
160 cm. There appear to have been several overlapping modal groups, with the most prominent
centered at about 94-98 cm. There are indications ofmodes at about 50,76, and126 cm. InArea 5the
size range ofthe fish in the catch was about the same as in Area 4. There appear to have been several
modal groups, the most prominent of which were at about 104 and 124 cm. In Area 6the size range
was from about 30 to 150 cm. There are major modes at about 62 and 126-128 cm and severallesser
modes. In Area 7 the fish caught ranged from about 40 to 160 cm. The catch in this area was low
compared to those in most other areas, and there appear to be several modes, none of which is very
prominent. Most ofthe fish caught in Area 13 were between about 120 and 150 cm in length. The most
prominent mode was centered at about 140 cm.
Histograms showing the estimated tons of yellowfin caught, by 2-cm intervals, based on
logged catches, in the entire CYRA for each year ofthe 1982-1987 period are shown in Figure 12. The
17