0.947 mm. The total netting aperture ratio was 1:11. The neuston 
net was towed at a speed of 5 kn for 10 min. Length of the bridle 
was adjusted so that the net usually fished the upper 0.5 m of 
water. Sea conditions greatly affected the depth fished. Towing 
speed together with warmth of the water caused significant 
damage to the specimens, thus complicating identification. 
The 0.505 mm bongo samples and neuston samples were sorted 
manually for fish eggs, larvae, and juveniles. The 0.333 mm 
bongo samples were used for volumetric determination of 
zooplankton abundance. Neither the egg data nor volumetric data 
are presented in this report. 
To compare abundances of fish larvae in bongo tows, the 
volume of water filtered was determined for each tow, and this 
value was divided into the product of the depth of tow multiplied 
by 10 to yield a standard haul factor. This haul factor, when 
multiplied by the number of larvae in a net tow, yields the number 
of larvae under 10 m? of sea surface. All of the neuston tow data 
are expressed as number of larvae per tow. 
Collections were made when the ship arrived on station, 
regardless of the time of day; consequently, collection times 
varied (Table 2). 
Observations made at each station included an XBT (expend- 
able bathythermograph) cast. I reviewed the XBT traces and 
abstracted information which is presented in the water 
temperatures section below. 
I identified larvae to the lowest taxon possible, with the excep- 
tion of the following: All leptocephali were identified by D. G. 
Smith, University of Texas Medical School, Galveston, Tex.; 
some myctophid larvae were identified by E. H. Ahlstrom and H. 
G. Moser, NMFS Southwest Fisheries Center; belonids and some 
hemiramphids from neuston collections were identified by B. B. 
Collette, NMFS Northeast Fisheries Center; myctophid juveniles 
of the genus Diaphus were identified by B. G. Nafpaktitis, 
University of Southern California; diodontids from neuston col- 
lections were identified by J. M. Leis, University of Hawaii; and 
clupeids and Bregmaceros were identified by E. D. Houde, 
University of Miami. I used literature sources and some original 
studies for the larvae that I identified. Larvae that could not be 
identified were placed in two categories: 1) Damaged larvae whose 
identification was questionable, and 2) larvae in good condition 
but not identified. 
RESULTS 
Overall standardized abundances were greater at night than day 
(Table 3). Numbers of larvae caught near sunrise and sunset were 
intermediate. Mean standardized number per tow was 1.48 
greater at night in summer and 1.54 greater at night in winter. 
Ahlstrom (1971, 1972) found greater differences in both 
EASTROPAC | and II data, with 2.76 more night-caught larvae 
in the first cruise and 2.27 more night-caught larvae in the second 
cruise. His sunrise-sunset collections were intermediate between 
day and night cruises. 
Day-night numbers for 23 selected families showed some in- 
teresting differences (Table 3). In comparing families with dif- 
ferent mean standardized numbers per haul, the families Myc- 
tophidae, Gonostomatidae, Gobiidae, Scaridae, and 
Bregmacerotidae were more prevalent at night, whereas the re- 
maining families exhibited no differences based on inspection of 
the data. Small sample sizes may account for this lack of dif- 
ference, because the families demonstrating clear differences were 
the most abundant ones. Also, there were some strong differences 
between cruises. With the myctophids there were 2.13 times more 
night- than day-caught larvae on the summer cruise compared 
with 1.43 more night- than day-caught larvae on the winter cruise. 
This is also reflected in the gonostomatids, 1.57 at night compared 
with 1.28 during the day; gobiids, 3.53 compared with 1.51; and 
the scarids, 2.44 compared with 1.20. With the Bregmacerotidae, 
the opposite was true; the greater day-night difference occurred 
on the winter cruise which reflects a very large sample size from a 
single station. Differences in day-night collections probably were 
due to the larvae’s visual reaction to the net, or lack thereof, or to 
vertical migrations below the sampling depth of the net. I believe 
the first reason to be the major factor for the differences or lack 
of differences seen, because the net presumably sampled the entire 
vertical range of the larvae. 
Number of Fish Larvae Obtained 
Fish larvae were obtained in every bongo net tow made on each 
cruise; actual counts of larvae per haul ranged from 24 to 267 on 
cruise OTP I and from 13 to 435 on OTP II. Relative numbers per 
haul are given in Table 4 together with average numbers per haul. 
Mean standardized abundance of larvae per haul is shown in 
Figure 1. There was no apparent geographic or hydrographic rela- 
tionship between abundance of larvae and station location. On 
cruise OTP I, six stations had > 1,000 larvae under 10 m? of sea 
surface, with two of these near the Virgin Islands, one east of the 
Antilles Islands, one south of Hispaniola, one in the Yucatan 
Basin, and one between Cuba and the Bahama Islands. There 
were no trends in latitude or proximity to land masses. One cruise 
OTP II, only twe stations had > 1,000 larvae under 10m? of seas 
surface, and these were off the coast of Venuzuela in an area of 
strong upwelling. Also on OTP II, two stations had < 100 larvae 
under 10 m?, whereas on OTP I, no stations had <100 larvae 
under 10 m’. On OTP II, other than the two stations mentioned, 
there were no apparent features affecting the number of larvae. 
Unfortunately, information is lacking on the depth distribution 
of these larvae, particularly in relation to thermocline depth. I do 
not know whether thermocline depth or depth of the mixed sur- 
face layer is related to abundance. 
Comparisons with Ahlstrom’s (1971, 1972) catches cannot be 
made directly since he reported his catches in number per tow. 
However, in his appendix tables, he gives standardized haul fac- 
tors which I averaged and multiplied by his average catch per tow. 
His catches under 10 m? of sea surface were for EASTROPAC I: 
399.1 (Argo), 614.7 (Jordan), 675.0 (Rockaway), and 828.56 
(Alaminos); for EASTROPAC II: 754.7 (Washington), 1,249.1 
(Undaunted), and 1,367.74 (Rockaway). Comparing these values 
with the Oregon II data (see Table 4) indicates that catches at 
EASTROPAC I stations were within the same order of 
magnitude as in the Atlantic, but that two EASTROPAC II 
regions yielded much higher values. During OTP II, two stations 
had values > 1,000 larvae under 10 m? of sea surface, comparable 
to EASTROPAC II conditions (Fig. 1). 
Water Temperatures 
Water temperatures at depth were available for each station 
from XBT casts. For each cast I looked at the temperature of the 
surface, depth of the mixed surface layer (MSL), temperature of 
the MSL, depth of the 24°C isotherm, depth of the 20°C isotherm, 
and temperature at 200 m depth. I chose the 24° and 20°C 
