246 
only at night. New arrivals to inshore areas have 
been collected all through the night, some as 
early as 9 P.M. and others just before dawn. Most 
of the night collecting was done at low tide. It 
is not known whether the influx of acronuri 
is greater at one phase of the tide than another. 
On two occasions before midnight at low tide 
in ankle-deep water along the shore of the Ala 
Wai Yacht Basin near the entrance to the Ala 
Wai Canal, manini acronuri were observed just 
coming into the area from deeper water. They 
did not passively float into the shallow zone but 
swam in rapidly. One crossed several times 
through the beam of a head lamp before it was 
caught. If it is assumed that these fish were in 
deep water beyond the breaker zone before 
nightfall, then they must have actively swum 
into the harbor area and not been carried in by 
any tidal currents. Prior to low tide, tidal cur- 
rents would be flowing out of the yacht basin 
and not into it. This is contrary to the belief of 
Breder ( 1949^: 296) that acronuri of Acan- 
thurus hepatus (= A. chirm gus) are carried 
into shallow water by "vagaries of current.” 
Breder reported observing transforming speci- 
mens of A. chirm gus at sizes from 23 to 29 mm., 
and found juveniles from 10 to 20 mm. in 
length in tide pools at Bimini. He inferred that 
late postlarval surgeonfish reach a certain size 
in the plankton at which transformation is pos- 
sible but continue to grow. The size at trans- 
formation is dependent on the size of the acron- 
urus when it reaches shallow water. Breder s 
explanation of the cause of this variability in 
transformation size is plausible, although a range 
of at least 10 to 29 mm. for one species seems 
high. 
From April 11 to October 4, 1953, a total of 
175 manini were found in shallow water on 
Oahu in their first day of transformation to the 
juvenile stage. Their standard lengths (meas- 
ured to nearest 0.5 mm.) ranged from 22 to 
29.5 mm. Even this would seem, a priori, to be 
more variation in length than would be expected 
from mere growth variation alone in the pelagic 
habitat. A small amount of this variability in 
Hawaii is due to the change in temperature at 
which development occurs during the season 
(Fig. 16 and discussion below), but consider- 
able variation can be seen in the transformation 
PACIFIC SCIENCE, Vol. XV, April 1961 
size of acronuri taken within the period of a 
single month, and therefore cannot be attributed 
to seasonal temperature differences. 
The fluctuation in abundance of incoming 
young manini has provided an opportunity to 
test Breder’s hypothesis. If the variation in size 
at transformation were entirely due to normal 
variation in growth, then no difference should 
be apparent in the size of manini which are 
transforming during the time of the month when 
a big influx is occurring and the period when 
the recruitment to inshore areas is minimal. If, 
however, more extremes in size were found 
when the recruitment is at a minimum, it could 
be assumed that growth of the manini occurs 
in the plankton after transformation is possible 
and that members of a modal group which ar- 
rive inshore and transform earlier or later than 
the majority of the group will be smaller and 
larger, respectively. 
In Figure 15 the lengths of the 116 trans- 
forming manini which were collected within the 
period 5 days before to 5 days after peak tide- 
pool recruitment (taken as 2 days before the 
time of new moon; see p. 234 and Fig. 6) are 
compared to the lengths of the remaining ma- 
nini caught outside this period. Clearly, a higher 
percentage of extremes in length at transforma- 
tion occur during the part of the month when 
the number of acronuri entering shallow water 
to transform is low. 
Fig. 15. Size of Acanthurus trio ste gus sandvicensis 
during the first day of transformation from the acron- 
urus to the juvenile stage. The solid line represents 
the percent of fish at the designated standard lengths 
from samples taken 5 days before to 5 days after peak 
tide pool recruitment (3 days before new moon). The 
dotted line represents the percent of fish from sam- 
ples taken during the rest of the lunar month. The 
solid line is based on a total of 116 fish and the 
dotted line on 59 fish. Samples include all of the first 
day transforming manini which were collected from 
Apr. 11 to Oct. 4, 1953. 
