Vol. 53 - Alaska Resources Library and Information Services

meters) was often similar (generally 200-300 m 3 ). It was determined
above that all decapod larvae would be retained by any of the mesh sizes
employed for the samples, so that the nets, in theory, should serve
equally well for estimating the density of decapod larvae at various
stations.
However, at least three factors affect the estimates we make:
(1) systematic differences in the degree of subsampling dictated by
corresponding differences in the volume of plankton retained by each net
per volume of water filtered; (2) inherent differences in net clogging
related to net porosity and (3) differences in the location of the flow
metering device.
When the volume of water filtered by a MOCNESS and a Bongo are approximately
the same but there are large numbers of small organisms retained
by MOCNESS and not by the larger mesh Bongo nets, the former will
be sub-sampled to a greater extent than the latter.
When the net plankton
samples from MOCNESS and the Bongo are approximately the same size
and, consequently, subject to the same sub-sampling, it is frequently because
the former has sampled less water than the latter.
It is only
when small plankton (in the intermediate size range 150-333 µm or 150-
505 µm) are rare that the volumes of water filtered and the number of
sub-sampling splits can be the same for both MOCNESS and Bongo samples.
Since the latter condition does not usually prevail, the representative
volume of water actually examined in MOCNESS sub-samples is almost always
smaller than that examined from Bongo samples.
This has the effect
of decreasing the lower level of detection, or numerical sensitivity, of
the MOCNESS samples relative to the Bongo.
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