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
reduced/anoxic underlying sediments. The depth of the RPD provides a measure of the
degree of biologically-mediated oxygen penetration (i.e., aeration) of the sediment
surface. The average RPD depths of 2.3 and 2.6 cm at the trawl and control stations,
respectively, generally indicate good sediment aeration and a moderate-to-high degree of
biogenic sediment mixing (Table 3.5-1).
The sandy surface sediments at the Little Tow stations appeared to be inhabited by a
benthic community dominated by surface-dwelling, tube-building polychaetes (Stage I).
The images at many of the Little Tow stations had numerous tubes of these organisms
visible at the sediment surface (e.g., Figure 3.5-1). A small percentage of the images also
showed evidence of larger-bodied, deeper-dwelling benthic organisms (Stage III)
underlying the Stage I surface tubes (e.g., Figure 3.5-1, image A). This resulted in the
assignment of a “Stage I on III” successional status to these images (Table 3.5-1). The
average OSI values were +5.2 and +5.6 for the trawl and control lane stations,
respectively. These are intermediate values that reflect the dominance of the lower-order
successional stage (i.e., Stage I) that is commonly found in a sandy benthic environment.
3.5.2 Baseline Characterization of the Mud Hole Area
The results of the August baseline survey at the trawl and control stations in the Mud
Hole area are summarized in Table 3-2. In general, the surface sediments in the Mud
Hole area were similar in appearance (i.e., color and texture) to those in the Little Tow
area. These sediments consisted predominantly of muddy very fine sand, having a grain
size major mode of either 4 to 3 phi (very fine sand) or 3 to 2 phi (fine sand; Table 3.5-2
and Figure 3.5-2). Reflecting these grain size characteristics, the benthic habitat
classification at the Mud Hole stations was primarily either UN.SS (unconsolidated
sediment consisting of very fine sand mixed with silt-clay) or SA.F (uniform fine sand;
Table 3.5-2).
The average camera penetration values at the Mud Hole stations (6.3 and 6.9 cm) were
slightly deeper than those at the Little Tow stations, suggesting a slightly softer texture
attributed to higher apparent amounts of silt-clay at a greater percentage of the Mud Hole
stations. The average boundary roughness values at the Mud Hole trawl and control
stations were less than 1.4 cm and similar to those at the Little Tow stations, again
indicating that the sediment surface had relatively little microtopography. Likewise, the
average RPD depths at the Mud Hole stations were comparable to those at the Little Tow
stations, indicating good sediment aeration and a moderate-to-high degree of biogenic
sediment mixing.
Similar to the Little Tow area, the sandy surface sediments at the Mud Hole stations
appeared to be inhabited by a benthic community dominated by surface-dwelling, tubebuilding
polychaetes (Stage I; Figure 3.5-2). A small percentage of the Mud Hole images
also showed evidence of larger-bodied, deeper-dwelling benthic organisms (Stage III)
underlying the Stage I surface tubes, resulting in the assignment of a “Stage I on III”
successional status to these images (Table 3.5-2; Figure 3.5-2 image A). The average
OSI values at the Mud Hole stations were likewise similar to those at the Little Tow area.
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