A Comparison of Euphausiid Shrimp Collections Made with a 
Micronekton Net and a One-Meter Plankton Net 1 
Charles W. Jerde 
In an evaluation of variable factors affecting 
the apparent geographic range and estimated 
abundances of euphausiids, Brinton (1962) 
compared euphausiid catching ability of a 1-m 
diameter net, made principally of 0.65 mm 
mesh, with a 45 -cm diameter net made of 0.33 
mm mesh. He found that adult and juvenile 
euphausiids were taken by the larger net in 
numbers as great or greater than were obtained 
with the 45-cm net, but that only about half as 
many larvae were taken with the coarser meshed 
meter net as with the 45-cm net. Collections 
with the 45-cm net contained almost as many 
species as the collections with the 1-m net, which 
filtered a volume of water 5 times as great 
(Brinton, 1962). 
On Scripps Tuna Oceanography Research 
cruises 64-1 and 64-2 (off southern Baja Cali- 
fornia) an attempt was made to sample con- 
secutively to the same depth with a micronekton 
net and a 1-m diameter plankton net, in order 
to compare euphausiid catches between the two 
nets. This paper is an evaluation of the euphau- 
siid catching ability of the two nets. 
The author is indebted to Dr. Edward Brin- 
ton for his assistance in the identification of the 
euphausiids. The constructive advice of Dr. 
Maurice Blackburn, Dr. E. W. Fager, Dr. Mil- 
ner B. Schaefer, and Dr. Paul Smith was grate- 
fully received. 
METHODS 
A description and figures of the micronekton 
net are found in Blackburn and associates 
(1962) ; the net with a 2.3 m 2 mouth opening 
1 This work formed part of the Scripps Tuna 
Oceanography Research Program of the Institute of 
Marine Resources and Scripps Institution of Oceanog- 
raphy, University of California. Partial support was 
provided by the U. S. Bureau of Commercial Fish- 
eries under Contract 14-17-0007-306. Manuscript re- 
ceived June 20, 1966. Contribution from the Scripps 
Institution of Oceanography, University of California, 
San Diego. 
is made of nylon netting of uniform mesh 
(apertures measuring about 5.5 mm by 2.5 mm) 
throughout and has a detachable cod end of 
#5 6 XXX grit gauze (mesh aperture 0.31 
mm). The micronekton net was towed in 
oblique hauls, from an average depth of 131 
m to the surface at 5 knots for an average 
period of 50 minutes; depth of haul was deter- 
mined by a bathythermograph attached to the 
upper edge of the square mouth opening (1.5 
m by 1.5 m). A flow meter was not used with 
the micronekton net, and volume of water fil- 
tered was estimated from size of mouth opening, 
ship speed, duration of tow, and a filtration 
coefficient of 0.757 which had been determined 
by Blackburn (MS). Estimated volume of water 
filtered per tow with the micronekton net ranged 
from 14,000 to 16,000 m s . 
The 1-m net (Ahlstrom, 1948) has a mouth 
opening of 0.785 m 2 and is made of #30 XXX 
grit gauze (mesh 0.65 mm) in the forward sec- 
tion of the net, with #56 XXX grit gauze 
(mesh 0.31 mm) in the rear section and cod 
end. It was towed in oblique hauls, from an 
average depth of 133 m to the surface at 1-2 
knots for an average period of 14 minutes. 
Maximum depth of haul of the 1-m net was j 
estimated from the amount of wire out and the 
wire angle ; a calibrated flow meter placed at the 
center of the mouth opening was used to esti- 
mate volume of water filtered, which ranged 
from 385 to 468 m 3 . On the average, the micro- 
nekton net filtered 34.4 times as much water as 
the meter net at each station. 
Euphausiids were picked from the entire col- 
lection of each tow at 10 stations. "Wet” dis- 
placement volume of each entire euphausiid 
sample was determined according to the method 
of Ahlstrom and Thrailkill (1963). All euphau- 
siids in these plankton samples were counted, 
with the exception of those in cruise 64-2 col- 
lections at stations 4l and 56; from these two 
collections, aliquots of 1/2 and *4, respectively, 
178 
