Acanthurus triostegus sandvicensis — Randall 
Experiments on Salinity Tolerance 
Salinity tolerance experiments on manini were 
carried out in a 30 gal. aquarium. All fish were 
retained in aquaria at least 24 hr. before use in 
experiments. The fish were not exposed to the 
test salinity suddenly. Instead the salinity was 
gradually changed over a period of 2 hr. either 
by the removal of aquarium water and replace- 
ment with tap water (previously allowed to 
stand for at least 12 hr.) or the addition of salt 
from evaporated sea water. Fish were held at the 
test salinity for a maximum of 24 hr. The long 
period of exposure to salinity extremes was 
chosen because manini entering brackish areas 
may be subjected to water of low salt content for 
at least this length of time. 
The results are given in Table 2. Omitted 
are trials run at chlorinity values greater than 
1.4%o, none of which caused any deaths. No 
trials were run at chlorinities greater than 38.25 
foo. 
Although more data are needed to determine 
with accuracy the minimum salinity which ma- 
nini can withstand, it seems evident that manini 
in tide pools can tolerate a greater range in 
salinity than they normally experience. Since 
they cannot live in water of extremely low sa- 
linity, they probably do not enter fresh-water 
habitats. 
PREDATORS 
Predation on the manini is probably most 
acute during the early stages of the life history, 
but I have no information on the identity of 
219 
the many pelagic animals that must feed on the 
eggs and small larvae. The large number of post- 
larval stages of acanthurids found in the stom- 
achs of adult yellowfin tuna has been discussed. 
Only a few instances of predation on juvenile 
manini have been encountered incidentally and 
are presented here briefly. Juvenile manini have 
been found in the stomachs of moray eels (Mu- 
raenidae). A small lizard fish (Synodidae) was 
observed to catch a juvenile manini in its jaws 
in shallow water in Kaneohe Bay, Oahu. The ma- 
nini was too large to be swallowed, and it even- 
tually escaped. A 50 mm. specimen of Anten- 
narius was placed in an aquarium with six ju- 
venile manini. A half hour later it had eaten 
one of them ( length, 3 1 mm. ) . 
In the Society Islands a juvenile Caranx mel- 
ampygus about 90 mm. long was observed from 
shore to capture a small Acanthurus triostegus 
triostegus 25-30 mm. in length. This carangid 
occurs in the Hawaiian Islands where it prob- 
ably feeds in part on manini, as may other spe- 
cies of Caranx. 
Also in the Society Islands, the young Acan- 
thurus triostegus triostegus have been found in 
the stomachs of the groupers Epinephelus merra 
and Cephalopholis argus and the snapper Lut- 
janus vaigiensis (Randall and Brock, I960). 
It is believed that predation (man excluded) 
on the juvenile stage of the manini in the Ha- 
waiian Islands is much more pronounced than 
on the adult stage. In addition to the fishes men- 
tioned above, holocentrids, scorpaenids, cirrhi- 
tids, and sphyraenids, and possibly also certain 
TABLE 2 
Salinity Tolerance of Acanthurus triostegus sandvicensis 
NO. OF FISH 
STANDARD 
LENGTH (mm.) 
CHLORINITY 
(0/00) 
NO. SURVIVING 
24 HR. 
maximum hr. 
SURVIVED 
3 
26-29 
1.4 
3 
4 
26-29 
0.7 
0 
5 
3 
27-29 
0.1 
0 
less than 5 
1 
92 
1.4 
1 
1 
89 
0.7 
0 
8 
2 
86-90 
0.4 
0 
between 3 and 15 
3 
27-29 
34.8 
2 
4 
26-28 
35.7 
2 
3 
28-29 
37.4 
2 
4 
26-29 
38.25 
0 
5 
1 
83 
38.25 
0 
17.5 
