416 
PACIFIC SCIENCE, Vol. XIX, October 1965 
TABLE 6 
Distribution of Gnathophausia gracilis in Eastern Pacific According to Size 
EASTERN PACIFIC 
NORTH OF TROPIC OF CANCER 
EASTERN PACIFIC 
SOUTH OF TROPIC OF CANCER 
Number of 
Length 
1 
Number of 
Length 
Individuals 
(mm) 
| Percentage 
Individuals 
(mm) 
Percentage 
58 
<80 
59.6 
22 
<80 
75.9 
41 
81-115 
41.4 
7 
81-115 
24.1 
Total 99 
Total 29 
VERTICAL DISTRIBUTION 
The Isaacs-Kidd Midwater Trawl is not 
equipped with a closing device, so that the 
exact depth of capture cannot be determined 
for specimens collected with this sampler. The 
maximum depth, however, is used as the as- 
sumed depth of capture, while recognizing that, 
although the majority of specimens were most 
likely captured at the maximum depth where 
the greatest length of trawling time is spent, 
there is still the possibility of catching strag- 
glers between this depth and the surface as 
the net is raised. All depths of capture re- 
ferred to in this paper have been calculated 
by measuring the amount of wire played out, 
then correcting for the wire angle to compute 
the actual collecting depth of the trawl. 
Of the three predominant species of Gnatho- 
phausia found in the Scripps collections from 
the eastern Pacific, G. gigas is the deepest- 
occurring (average depth of capture, 2100 m). 
Another somewhat less deeply-occurring spe- 
cies is G. gracilis, with an average depth of 
capture in the Scripps collections of 1600 m. 
G. in gens occurs predominantly in more shal- 
low waters, where its average depth of capture 
was 1100 m. A weighted average depth of cap- 
ture for G. ingens was also calculated, taking 
into consideration the number of specimens 
captured at each depth. This weighted average 
depth of 850 m shows that the majority of 
specimens of G. ingens were found at an even 
more shallow average depth. 
Time-depth graphs have been plotted for the 
three predominant species of Gnathophausia 
found in the 1950-53 Scripps collections (Figs. 
11, 12, and 13), in order to detect any possi- 
ble evidence of nocturnal vertical migrations. 
In only one species, G. ingens, is there any evi- 
dence that the animals are found, in general, 
in more shallow water during night collections 
and in deeper water during daytime collections 
The graph shows that during the day practi- 
cally no specimens of G. ingens,„ were taken 
above 650-700 m. On the other hand, the ma- 
jority of captures at night recovered individ- 
uals from the zone between 274 and 650 m. 
All captures at night from deeper than 650-700 
m were from hauls which extended into the 
daylight period; thus there is the possibility that 
the specimens were picked up at these deeper 
levels during the daylight period of towing. 
Similar time-depth graphs for G. gigas and 
G. gracilis do not seem to indicate any con- 
sistent depth differences between daylight and 
dark sampling. In order to study more precisely 
the possible existence of nocturnal vertical mi- 
gration in Gnathophausia it would be neces- 
sary to employ the use of closing nets to deter- 
mine more accurately the exact depths of 
capture. For even more accurate determinations 
a depth telerecording unit has been employed 
(Boden et al., 1955). 
Gnathophausia gigas 
Although G. gigas tends to occur at greater 
depths than the other two species, it was occa- 
sionally collected in more shallow water (less 
than 1000 m) than that which Fage (1941) 
reports as the shallowest depth of occurrence 
of this species for the captures of the Dana 
(2000 m depth, 3000 m of wire out). The 
average depth of capture of G. gigas in the 
Scripps collections was 2100 m, ranging from 
600 to 4400 m. Only 3 (19%) of the 16 sta- 
tions where G. gigas occurred were at less than 
