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
Each trawl catch was sorted and weighed by species. Stomachs were removed from up to 10
individuals of 3 bottom-feeding target species (winter flounder, yellowtail flounder and cod)
from each tow and preserved individually in 10% formalin. Following transfer from formalin to
alcohol the collections of individual flatfish stomachs for each sample (i.e. fish species by tow
date, study site, and trawl lane) were presorted by trained fishermen into vials for annelids,
crustaceans, molluscs, miscellaneous taxa and unidentifiable (partly digested) material at BKAM
in Scituate, MA. Sorted stomach contents were identified to the nearest taxa by Allan Michael &
Associates Lab of Magnolia, MA. Total lengths in centimeters were recorded for all winter
flounder, yellowtail flounder and Atlantic cod (Photograph 2.2-2). Weight per tow for the most
common species were converted to densities (kilograms per 1000 square meters) by estimating
the area swept during each tow and assuming that all organisms in the path of the trawl were, in
fact, caught. Commercially targeted flatfish numbers were also converted to densities (number
per 1000 square meters) in a similar fashion. Densities were only estimated for bottom-dwelling
finfish since mid-water species like spiny dogfish and herring are less vulnerable to capture in
bottom trawls.
Neither mean weight estimates nor complete catch in numbers data were available for benthic
macro-invertebrates (crabs, lobsters, and scallops), so they were not included either.
Area swept was calculated as:
Area = [(1/2 (HL + FL))/2] x TL
Where HL = headrope length, FL = footrope length (length of the sweep between the wings of
the net, excluding the legs and ground cables that extend to the doors), and TL = tow length. For
the bottom trawl used on the fishing vessels, the width of the net was 76ft or approximately 11.6
m. Although the trawl lanes were intended to be 1000 m long, actual tow lengths varied from
940m to 1292m and averaged 1141 m.
2.3 Water Column Sampling Methods
Water column characteristics were documented at the study sites, Mud Hole and Little Tow,
during the experimental surveys on August 1, October 10, and November 12, 2002. CTD casts
were made at the three sampling stations on Lanes 1 through 4 with a Seabird SBE-19 Seacat
CTD Profiler equipped with oxygen and turbidity sensors. Recorded parameters included
turbidity, temperature, dissolved oxygen, and salinity.
2.4 Bathymetric Surveys
During the June 2001 reconnaissance survey of the study sites, a wide area coverage bathymetric
survey was conducted using the ship’s DGPS and echosounder. The survey confirmed that the
study sites, Mud Hole and Little Tow were in waters ranging from 120 to 140 ft in depth.
During the 2002 trawl study, a more detailed precision bathymetric survey was conducted aboard
the F/V Christopher Andrew on September 30, 2002. The bathymetric survey was conducted by
10