RESOURCE ASSESSMENT & CONSERVATION ENGINEERING
(RACE) DIVISION (cont.)

Groundfish Assessment Program

Effect of Unequal Trawl Warps on Trawl Geometry

Figure 6.  Mean door spread, wing spread, and headrope height is shown (+) plotted against offset increment. The means for all values of offset increment were fit with a cubic spline function (solid curve). Bootstrapped 95% confidence bounds are shown with shading. Also shown are the mean door spread, wing spread, and headrope height considering only treatments with zero offset (solid horizontal line) and the bootstrapped 95% confidence bounds (dashed horizontal lines).

Survey standardization procedures can reduce the variability in trawl catch efficiency, thus producing more precise estimates of biomass. One such procedure, towing with equal amounts of trawl warp when using locked winches was experimentally investigated for its importance in determining optimal trawl geometry and for evaluating the effectiveness of the recent NMFS national protocol on accurate measurement of trawl warps, which states that warp length differences between the port and starboard sides cannot exceed 4% of the distance between otter doors measured along the bridles and footrope.

Trawl performance data from repetitive towing with warp differentials of 0, 3, 5, 7, 9, 11, and 20 m were analyzed for their effect on three determinants of flatfish catch efficiency: footrope distance off bottom, bridle length in contact with the bottom, and area swept by the net.

Results indicated that the distortion of the trawl caused by asymmetry in trawl warp length could have a negative influence on flatfish catch efficiency, although determining the degree of impact would require further experimentation utilizing catch data.

At a 7-m difference in warp length, the NMFS 4% threshold value for the 83-112 Eastern survey trawl used in this study, no effect was found on the acoustic-based measures of door spread, wing spread, and headrope height off bottom (Fig. 6). However, the sensitivity of the trawl to 7 m of warp offset could be seen as footrope distances off bottom increased slightly, particularly in the center region of the net where flatfish escapement is highest (Fig. 7 below) and the width of the bridle path responsible for flatfish herding was reduced (Fig. 8 below), as was the effective net width (Fig. 9 below).

For this survey trawl, a NMFS threshold value of 4% should be considered a maximum. A more conservative value (< 4%) would likely reduce potential bias in estimates of relative abundance caused by large differences in warp length approaching 7 m.

By Ken Weinberg

Figure 7 (above).  Mean off-bottom distances in cm at the five bottom contact sensor positions along the footrope (circles) are shown for each warp offset. The positions are projected onto the wing tip to wing tip plane to depict the footrope as it would appear looking into the net from the direction of travel. The vertical solid lines indicate the positions of the wing tips on the long warp and short warp sides of the trawl. The dashed line indicates the midpoint between wing tips. Note that the projection considers the reduction in effective net width with increasing offset.

Figure 8 (above).  The length of the bridle <1 cm from the bottom is shown plotted against the offset increment in meters for both the short warp (grayline) and long warp (blackline) sides of the trawl (top). Assuming that the angle of attack is the same for the long and short sides of the trawl, the width of the swept bridle path as a function of warp offset is represented in the lower panel.

Figure 9. (above)  The mean distance between wing tips measured acoustically (O) during the experimental tows (net width) and the calculated effective net width (+) are shown plotted against the offset increment in meters. Note that there is little difference between the two measures of net width until the offset increment is increased to 9 m.

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AFSC Quarterly Research Reports Oct-Dec 2004