The Ecology of Pelagic Amphipoda, I 
Species Accounts, Vertical Zonation and Migration of Amphipoda from the 
Waters off Southern California 
Gary J. Brusca 1 
A series of midwater trawls in the waters off 
the coast of southern California has revealed 
large numbers of pelagic amphipods. A sam- 
pling program employing an Issacs-Kidd Mid- 
water Trawl (Issacs and Kidd, 1953: 1-21) 
was conducted in the waters of the Outer Santa 
Barbara Passage, in the area of the Santa Cata- 
lina Basin. 
The purposes of this study were (1) to de- 
termine the constituents of the local pelagic 
amphipod fauna, (2) to examine the vertical 
distributional and migrational patterns of the 
abundant species, and (3) to analyze some of 
the hydrographic conditions of the study area 
and relate this information to the ecology of 
the amphipods. 
This is the first in a planned series of papers. 
In view of the forthcoming works, such data 
as size distributions, reproductive conditions, 
and density fluctuations are omitted from this 
present paper. 
All of the collections considered in this 
study were obtained through the use of the 
R/V "Velero IV” of the Allan Hancock Foun- 
dation, University of Southern California. Par- 
tial support for this work was furnished by 
grants from the National Science Foundation 
(G-10691 and G-23467). 
METHODS AND MATERIALS 
An Issacs-Kidd Midwater Trawl (ikmwt) 
with a 10 X 10-ft fishing aperture was used 
in all collections discussed here. A few sam- 
ples were taken with a Foxton closing device 
attached to the ikmwt. The Foxton device not 
only takes a sample at a prescribed fishing 
depth but retains, separately, the material col- 
1 Assistant Director, Pacific Marine Station, Diilon 
Beach, Marin County, California. Manuscript received 
June 29, 1966. 
lected as the trawl is being lowered and raised. 
Only partial success was attained with this 
device. 
Horizontal tows were taken, and fishing 
depths were determined through the use of a 
pressure depth gauge and by triangulation. 
Although these two methods yielded compa- 
rable measurements, the fishing depths recorded 
here may present some error. The pressure 
gauge records only the greatest depth to which 
the trawl descends, and it is the opinion of 
this author and other workers (Aron et al., 
1964:324-333) that the fishing depth of the 
ikmwt fluctuates while it is being towed. Most 
of the trawls were conducted for 2-hour periods. 
Of the 82 samples used in this study, 58 
were quantitatively analyzed by converting 
counts made in pint aliquots to numbers per 
hour trawling time. Total counts presented in 
this paper indicate the number of individuals 
sorted from these pint aliquots. Although the 
recorded numbers per trawl hour probably are 
not precise, they are used at times to offer 
comparative values of relative population den- 
sities at various depths. The qualitative samples 
were used to determine presence or absence of 
species at particular times and depths. Table 1 
is a record of the day and night hauls taken 
at various depths. Complete station data and 
individual sample analyses are on file with the 
author. 
HYDROGRAPHY OF THE STUDY AREA 
The continental shelf off the coast of south- 
ern California is a complex series of basins, 
troughs, and islands. It has been termed a 
continental borderland (Shepard and Emery, 
1941:9) due to the striking differences be- 
tween its topography and that of typical shelf 
areas. Emery (1960:32-61) offered a detailed 
description of this region. 
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