Schaefer: Assessment of spawning activity of Kcitsuwonus pelcimis in the eastern Pacific Ocean 
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150 140 130 120 110 100 90 80 70 
Distributions of reproductively-active (solid squares) and inactive (open squares) 
female skipjack tuna sampled during 1995. 
Spatiotemporal patterns in spawning 
Reproductive activity is certain for the females with ad- 
vanced yolked oocytes that are equal to or greater than 
0.55 mm in diameter. Of these, 429 (28%) were classified 
as reproductively active, and reproductively active females 
were found in 27 of the samples (42%) (Table 2). Of the 429 
skipjack tuna with advanced yolked oocytes, 232 (54%) 
also had residual hydrated oocytes in the lumen of their 
ovaries, indicative of recent spawning. Based upon the dis- 
tribution of these samples, skipjack tuna spawning in the 
EPO appears to be fairly widespread from around 15°N to 
10°S and from the coast to about 130°W (Fig. 2). Reproduc- 
tively active skipjack tuna were present north of the equa- 
tor throughout the year and south of the equator during 
the first three quarters of the year, and there were no 
apparent seasonal peaks in either stratum (Fig. 3). No 
samples of mature fish were collected during the fourth 
quarter south of the equator. The length-frequency distri- 
bution of the females from the 65 independent samples is 
shown in Figure 4. 
Based upon the SST data collected in conjunction with 
catch information for each of these samples, it appears 
that skipjack tuna are sexually inactive at SSTs less than 
24°C. Of the 65 samples, 20, or 31%, were taken from skip- 
jack tuna captured at SSTs less than or equal to 24°C (Fig. 
5). In other words, just over half of the 38 samples that 
did not contain reproductively active fish were obtained 
from skipjack tuna captured at SSTs below those at which 
spawning occurs. 
Table 1 
Sex ratios of skipjack tuna, Katsuwonus pelamis. 
Number observed 
Percent 
male 
Length (cm) 
Male 
Female 
Chi-square 
50.0-54.9 
873 
687 
56.0 
22. IS 
55.0-59.9 
365 
387 
48.5 
0.64 
60.0-64.9 
252 
285 
46.9 
2.03 
65.0-69.9 
226 
170 
57.0 
7.92 
70.0-74.9 
21 
18 
53.8 
0.23 
Total 
1737 
1547 
52.9 
10.99' 
*=P< 0.05. 
The frequency distributions of the estimated sizes, in 
metric tons, of the skipjack tuna schools sampled, along 
with the subset of those that were classified as spawning, 
are shown in Figure 6. The prominent mode in each dis- 
tribution was around 25 metric tons; the ranges in each 
mode were from about 5 to 270 tons. 
Discussion 
It is evident from the results of this investigation that sig- 
nificant spawning of skipjack tuna, 50 cm or greater in 
