Vol. 53 - Alaska Resources Library and Information Services

Bongo frame, the MOCNESS frame cannot accommodate a flowmeter positioned
in front of the net opening (because of the sliding nets and the way the
frame is handled on deck).
Instead, the flowmeter sits atop the frame,
where it is insensitive to actual changes in flow into the net.
The
MUCNESS theoretically over-estimates the volume of water filtered, and
our calculations should routinely underestimate the density of decapod
larvae present.
This problem is not even internally consistent within a
station, since phytoplankton and other zooplankton are not uniformly
distributed with depth and may affect only some of the nets used.
The
0-20 and 20-40 m nets are usually subject to more clogging than the
deeper ones, so that calculations of abundance in the upper 40 m should
be underestimated relative to calculations for deeper samples.
While these potential difficulties should be remembered in interpreting
plankton data, they do not appear to have presented a major
problem in this study.
This may have been due to differences in the
calibration and/or performance of the flow meters.
Incze (1983) compared
estimates of abundance of Chionoecetes larvae made with MOCNESS
and Bongo nets deployed at the same stations and found no consistent
bias in the estimates. Rather, small-scale patchiness in abundance was
sufficient to mask differences in the accuracy of flow estimation based
on a comparison of coefficients of variation.
In this study, all gear
were considered to provide similar estimates of abundance.
2.7 Vertical Distribution Patterns
Discrete depth interval samples collected by MOCNESS were used to
examine patterns of vertical (depth) distribution of abundant decapod
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