112 
PACIFIC SCIENCE, Vol. XII, April, 1958 
The total plankton data used here were 
reported by King and Demond (1953) ; Crom- 
well (1953, 1954) reported all hydrographic 
observations used here. As King and Demond 
stated, the plankton net descended obliquely 
to 200 meters, then made an oblique ascent 
to the surface. Thus the net strained, during 
approximately 30 minutes, a total of about 
1,000 cubic meters of water. This net was 
described in King and Demond’s report as 
being 1 meter in diameter, with a mesh width 
of 0.65 mm., but I should point out here that 
it was an open net. For this reason, vertical 
distribution of the captured animals is un- 
known. 
All animals larger than about 5 cm. were 
removed from each POFI plankton sample 
(King and Demond, 1953). The sample was 
then divided into two equal portions, one of 
which was not used by POFI personnel. The 
latter portions were used for the present study. 
Therefore, in calculations where comparisons 
were necessary, salp numbers and volumes 
were estimated by doubling the figure ob- 
tained from each half sample. In addition, 
many of the removed salps larger than 5 cm. 
were examined and identified. Number deter- 
minations of salps were made by direct count- 
ing, whereas volume determinations of the 
salps were made for each plankton sample by 
means of water displacement in a graduated 
cylinder. In order to determine the relation 
between salp volume and volume of the other 
plankton in the same sample, the salp volume 
determinations were divided by the amount 
of water strained (taken from King and De- 
mond) and compared with the total volumes 
of zooplankton, less salps, reported by King 
and Demond. 
ABUNDANCE OF SALPS 
Physicochemical Factors of the Environment 
A study of the ecology of salps should 
consider all aspects of their environment such 
as food, enemies, and the distribution of 
physicochemical factors of the waters in which 
they are found. Of possible direct importance 
to them is the distribution of temperature, 
density, currents, salinity, and oxygen. On 
the other hand, distribution of light and of 
nutrient chemicals, factors which influence 
phytoplankton production, is probably only 
indirectly important to salps. Which among 
these are limiting factors for salps is almost 
wholly unknown. 
In the open ocean, salinity is presumed not 
limiting because of its small variation, from 
about 34 to 36 % 0 . Salps seem to be absent 
from areas of very low or high salinity, such 
as may be found in certain waters. For ex- 
ample, Apstein (1906) reported no salps from 
the region off the mouth of the Congo River 
in Africa, a situation apparently related to the 
low salinity (30.4 % 0 ) in the area; and Ihle 
(1935) noted that some species of salps were 
absent from the eastern part of the Mediter- 
ranean Sea, perhaps indicating a relationship 
to the high salinity (up to 40 o / 0o off Syria). 
In regard to temperature, also, little is 
known about limits of tolerance for the va- 
rious species, although all salp species — ex- 
cept an antarctic form, Ihlea magalhanica— 
are found principally in warm regions of the 
ocean. Occasionally they are found in high 
latitudes, presumably carried there by current 
tongues, for example, in the Bering Sea (Met- 
calf, 1918), the Gulf of Maine (Bigelow, 
1926), the North Sea (Fraser, 1949, 1954), 
and waters of southern Australia (Thompson, 
1948). Salps have also been found at great 
depths (more than 1,000 meters) where the 
temperature was low (Apstein, 1906; Michael, 
1918; Sewell, 1926, 1953; Leavitt, 1935, 1938). 
The limits of tolerance for both salinity and 
temperature in the various species are thus 
unknown and can only be inferred from re- 
ports of distribution. 
Factors that influence variation in organic 
productivity must also be considered for a 
clear understanding of variation in salp abun- 
dance. Areas of maximum biological produc- 
tivity are found in coastal waters, in temperate 
and higher latitudes, and in regions of up- 
welling. Productivity in these areas is based 
