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

NOAA/NMFS Unallied Science Project, Cooperative Agreement NA16FL2264 May 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
Little Tow
Like Mud Hole the dominant size fraction at Little Tow in July was medium sand (median phi of
1.80 – 1.93) (Figure 3.2.3-2). The medium sand fraction was greatest at the two northern
Stations, LT1B and LT2B. Coarser fractions made up less than 3 percent of the samples. The
fine sand fraction was also similar to that at Mud Hole, and ranged from 9.0 to 19 percent. The
fine sand fraction was substantially higher at southern Stations LT3B and LT4B. The silt/clay
fraction in July was slightly greater and less variable than at Mud Hole, and ranged from 18.3 to
24.8 percent. There was no detectable difference between the grain size distribution of the
control versus the trawled lanes samples.
Little Tow quartile deviation results ranged from 0.42 phi to 0.68 phi, suggesting sediments at
Little Tow are well to moderately well sorted. Both quartile deviation and median phi values
were lowest at the two northern Stations, suggesting that this portion of the Little Tow possesses
a finer and more homogeneous substrate than the southern portion of the Site. This observation
is supported by side-scan sonar imagery, which shows that the northern portion of Little Tow is
less acoustically reflective than the southern portion, and by site bathymetry, which shows that
the northern portion of the seabed at Little Tow is flatter and possesses fewer bathymetric
irregularities than the southern portion.
3.2.3.2 Post-<strong>Trawl</strong> Site Conditions – October 9 and November 19, 2002
At Mud Hole and Little Tow there were no discernible differences between the grain size
distribution for control versus trawled lane samples following chronic trawling efforts from the
end of July to the middle of November.
Sediment particle size at Mud Hole was essentially unchanged from July 2002 to October 2002.
One minor difference noted was slightly higher percentages of silt/clay at MH1B and MH2B in
October than in July. Mud Hole sediment samples collected in November were similar to those
collected in October. The dominant modal size was medium sand throughout the year. The
silt/clay content ranged from 13.6 to 21.9 percent. Additionally, the fine sand fraction at Mud
Hole stations decreased from July to October and from October to November.
Temporal changes were more pronounced at Little Tow where there was a shift in modal size
from medium sand to fine sand between July and October (see Figure 3.2.3-2). The fine and
very fine sand fractions significantly increased at Little Tow from July to October, and
significantly decreased from October to November. The November modal grain size reverted to
medium sand except at LT1B. Coarse sands at Little Tow significantly increased from July to
October and from October to November. The silt/clay content of Little Tow sediments
decreased from July 2002 to November 2002 in a roughly linear fashion, with July, October and
November means of 19.9, 17.3, and 10.3 percent, respectively. A possible explanation for the
pronounced changes at the more shallow stations of Little Tow is increased sediment mixing
associated with seasonal or episodic differences in wave-induced bottom disturbances (see
Figure 3.2.2.3-1 for a Time-Series record of wave heights in Mass Bay for 2002). As described
previously, the November sampling event immediately followed a major northeasterly storm
event. The average wave heights recorded for Mass. Bay during the 48 hours prior to the
sampling events were 0.5 m (July), 0.8 m (October) and 1.9 m (November). Extensive seasonal
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