Smooth Bottom Net Trawl Fishing Gear Effect on - New England ...

NOAA/NMFS Unallied Science Project, Cooperative Agreement NA16FL2264 December 2005
<strong>Smooth</strong> <strong>Bottom</strong> <strong>Net</strong> <strong>Trawl</strong> <strong>Fishing</strong> <strong>Gear</strong> <strong>Effect</strong> on the Seabed:
Investigation of Temporal and Cumulative <strong>Effect</strong>s BKAM/CR
3.5.3 Evaluation of <strong>Trawl</strong>ing <strong>Effect</strong>s in the Little Tow and Mud Hole Areas
The results of the October REMOTS survey are summarized in Table 3.5-3 for the Little
Tow stations and Table 3.5-4 for the Mud Hole stations. Likewise, the results for the
November survey are summarized in Table 3.5-5 for the Little Tow stations and Table
3.5-6 for the Mud Hole stations.
Among the changes that might be expected to occur if trawling was physically disturbing
the sediment surface are the following: 1) breaking up of the otherwise cohesive sediment
particles that would be manifested in the sediment-profile images as a noticeable change
in the sediment texture or fabric, 2) increase (or decrease) in the amount small-scale
surface roughness, 3) a decrease in the RPD depth resulting from removal of the oxidized
surface layer of sediment, 4) significant breakage or removal of delicate biological
surface structures (e.g., polychaete tubes), and 5) consistent with 3 and 4, apparent
changes in infaunal successional stage or OSI values.
Overall, the images obtained in the both October and November post-trawl REMOTS
surveys showed an absence of any significant trawling-induced changes in either physical
or biological conditions at the sediment-water interface. A statistical test for unplanned
comparisons among pairs of means (Games and Howell method at the 0.05 significance
level, from Sokal and Rohlf (1981)) indicated no significant differences among surveys
or station groups in the average boundary roughness, RPD, or OSI values shown in
Tables 3.5-1 through 3.5-6. In other words, in any particular survey, there was no
significant difference in each of these three parameters between the control lane versus
trawl lane stations. Likewise, there was no statistically significant change through time
in each of these parameters at either the trawl or control stations.
Figures 3.5-3 through 3.5-7 present representative REMOTS images illustrating the
absence of any detectable changes in sediment fine-scale characteristics, either through
time or in terms of the “trawl versus control” comparison. In all cases, there were no
obvious, consistent changes in the basic color, texture or fabric of the sediment surface
that would otherwise indicate physical disturbance by trawling.
The density of tube-building, Stage I polychaetes is a somewhat less reliable indicator of
trawling disturbance than sediment texture or RPD depth, because populations of these
opportunists are known to have considerable natural variation in both space and time.
Even if trawling was resulting in wholesale removal of these tubes across wide areas,
these organisms are capable of re-establishing populations within days. In a few of the
images, there were Stage I tubes that appeared to be lying flat (i.e., recumbent) on the
sediment surface rather than in the more typical upright position, but there was no
consistent pattern in the occurrence of these recumbent tubes between trawl versus
control stations to signal clearly a trawling effect. Although Stage I tubes are able to
become quickly re-established following a physical seafloor disturbance, the persistence
of these tubes through time together with the absence of any other indicators (e.g.,
removal of the oxidized surface layer, changes in surface texture or microtopography)
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