Experimental Design for Trawl Observation Cruise
September 25-27, 2002
During this cruise, we will deploy standard NEFSC bottom trawl gear with video and electronic sensors to monitor net performance. This effort will focus on generating qualitative and quantitative measures of net performance. Cruise will depart on or about September 25 and return on or about September 27.
Information provided by video and net mensuration work will be evaluated during the October 2-3 workshop and used to refine experimental approaches used during calibration work later in October.
There will likely be an opportunity to evaluate other important performance measures such as catchability of commercially important fish and invertebrates during planned paired research vessel work from October 15-25, 2002.
1. Provide initial qualitative observations of the effects of offset warps on net geometry and fishing gear performance.
2. Provide a quantitative evaluation of the effects of offset warps on net wingspread, door spread and head rope height.
We intend to qualitatively evaluate the effects of offset warps on net geometry and fishing gear performance through a series of sets at a range of depths normally occupied during NEFSC bottom trawl surveys. At each depth, we will deploy the net equipped with net mensuration gear (ITI sensors) and video monitoring equipment to allow for qualitative evaluations of net performance, and quantitative measure of head rope height and wing and door spread. Net mensuration and video gear will be time synched with onboard time to allow for accurate synchronization of video images and net mensuration information with trawl warp displacement.
At each depth, the trawl will be deployed using warp scope relationships outlined by NEFSC trawling protocols (generally 3:1 at shallower depths and 2.5:1 at deeper depths). Once the net is set and allowed to settle, trawl warps will be intentionally offset as outlined in Table 1 to evaluate effects on fishing performance of the net. At each depth, trawl performance will be evaluated with even warps (0) and offset so that each of the warps is significantly longer than the other. After each warp length manipulation, the trawl will be allowed to stabilize and a tilt and pan video camera will be used to provide video images of net performance. We intend to perform a series of trawl warp manipulations at a given depth during a single tow.
Table 1. Experimental design for video work to examine the effect of
uneven trawl warps. Bold numbers indicate the estimated offset at the
wire out mark.
Offsets to be tested
25 75 1 to 16 -24 -16 0 +16 +24 50 150 24 -48 -33 -24 0 +24 +33 +48 100 300 67 -94 -67 -33 0 +33 +67 +94 150 450 94 -107 -94 -48 0 +48 +94 +107
Visibility: The best time of day to perform video work occurs between 9:00 am and 3:00 pm when ambient light due to direct sunlight is highest. This work is best done on a clear day, and overcast, foggy, or stormy conditions may represent unsuitable conditions due to reduced light levels. In addition, stormy conditions may result in turbid conditions at depth. The low-light video camera in use should generate usable images down to at least 50-m and hope to be able to go deeper. If we lose visibility at greater depths, we will be able to gather sensor data at deeper stations.
Continuation of bottom trawl survey: Between 6:00 pm and 6:00 am on the night of departure, and 6:00 pm until departing stations to return toWoods Hole, the vessel will continue to sample at bottom trawl survey stations.