Schaefer: Assessment of spawning activity of Katsuwonus pelomis in the eastern Pacific Ocean 
347 
E 
E 
a> 
E 
aj 
TD 
CD 
o 
o 
O 
1.0 - 
0.9 - 
0.8 - 
0.7 - 
0.6 - 
0.5 - 
0.4 - 
0.3 
A 
o 
o 
n =228 
O 
1.0 - 
B 
0.9 - 
O n = 265 
0.8 - 
0.7 - 
0.6 - 
0.5 — 
0.4 - 
0.3 
T 
1 
Quarter 
Figure 3 
Seasonal variation in oocyte diameters from the most advanced modal group 
of oocytes from (A) skipjack tuna collected from north of the equator and (B) 
skipjack tuna collected from south of the equator during 1995. Immature fish 
were excluded from this analysis. The width of the box is proportional to the 
number of samples in the group. The horizontal line within the box is at the 
median. The bottom of the box is at the first quartile (Qp and the top is at the 
third quartile (Q 3 ) value. The lines extend from the top and bottom of the box 
to the adjacent lowest and highest observations within the lower and upper 
limits defined as Q, ±1.5 (Q 3 - Q,). Outliers are points outside the lower and 
upper limits and are plotted as circles. 
Equatorial Countercurrent and the Equatorial Undercur- 
rent (Williams, 1972) are unsubstantiated. 
Numerous tagging studies have also been conducted in 
the EPO to investigate movements of skipjack tuna (Fink 
and Bayliff, 1970; Bayliff, 1984). It appears from these stud- 
ies that skipjack tuna show some consistency of directed 
movement in the nearshore regions off Central America 
and northern South America. In the northern region around 
the Revillagigedo Islands and the west coast of Baja Cali- 
fornia, there is a northern and then southern movement of 
the fish between 20°N and 30°N in response to the season- 
al movements of the 20°C surface isotherm between about 
