98 
PACIFIC SCIENCE, Vol. VIII, January, 1954 
of the growth curve would seem to persist 
for a year or more. It would be difficult to 
obtain an acceptable fit of a growth curve to 
these modes on any other basis. However, 
lacking data regarding the nature of the initial 
accelerating portion of the growth curve, the 
possibility that the entering year-class is more 
than 1 year old, even though deemed un- 
likely, cannot be disproved. 
This interpretation of the length data would 
imply that the aku landed in Hawaii is a short- 
lived, rapidly growing fish and that the fishery 
depends on aku belonging largely to two 
adjacent year-classes and entirely to three. 
This interpretation is not proved here but is 
offered as a plausible hypothesis to account 
for some of the features of the length fre- 
quency data given in Eigure 1. The most di- 
rect method of testing this hypothesis would 
seem to be by the tagging and recovery of 
fish of known size, as there is as yet no reliable 
means of age determination for this species. 
However, the albacore fishery of western 
North America also apparently draws its catch 
from a stock composed of but a few year- 
classes, as described by Brock (1944). 
The appearance of distinct modal groups 
in these length frequencies, if due to year- 
classes, is somewhat surprising when the ap- 
parent rapid growth rate is considered together 
with the long spawning period, the evidence 
for which will be discussed later. The period 
of active spawning is perhaps much shorter 
than the occurrence in the landings of fish 
with fairly large ova would indicate. It is also 
possible that, although the period of spawn- 
ing may be of considerable duration, only the 
hsh which hatch during a limited part of that 
period survive in significant numbers. 
LENGTH SEGREGATION IN SCHOOLS 
As previously mentioned, beginning in the 
summer of 1949 a series of samples were 
taken in such a manner that each represented 
fish captured from a single school. This was 
practical inasmuch as only a single school 
would ordinarily be chummed up by live bait 
and fished at one time. This catch would often ' 
be stored by itself in an empty bait well. The 
fish were sampled aboard the vessel when it | 
reached port, and samples were taken from j 
those bait wells which, according to the cap- 
tain, contained fish caught from a single 
school. 
The '’pure school” samples obtained during 
the summer of 1949 proved of sufficient in- 
terest to justify an increase in this sampling ' 
program. Between May 16 and September 30, I 
1950, some 120 samples with, on the average, j 
34 fish measured in each were obtained. As 
these length data for 1950 were more adequate ; 
and did not differ in the features of interest 
mentioned here from those for 1949, the 1950 
data will be discussed. The 1951 data were, 
in all aspects considered here, also similar to ' 
the 1950 data. 
The most striking feature of these samples ■ 
is the comparatively small range of the length i 
of fish within them which is in contrast to the ' 
range of the length of fish in the landings as 
a whole. For example, the mean range in | 
length for the pure school samples was 11.3 
centimeters, the least range 5 centimeters, and | 
the greatest 21 centimeters. In contrast to i 
these small ranges, the range of the seasonal , 
summary of length measurements for the 
same period is 47 centimeters. It would appear 
that the school is a highly size-selected group ‘ 
in which those causes that select for size may 
well obscure the pattern of size distribution ! 
in the population as a whole. Hence, measur- i 
ing a larger number of fish from a few schools 
would not reliably reveal the pattern of size : 
distribution of the fishable stock; fish taken 
from a large number of schools must be 
measured. The fact that aku appear to school i 
by size must, of course, be considered in the 
design of any program to obtain length sam- 
ples. Although the effect may be less extreme, 
it may be suspected that schooling by size 
occurs in other tuna populations and species. 
The causes of the observed size segregation ; 
by schools is a matter of interest, but they >i 
are not clearly evident from an examination 
