Variation in trawl geometry due to unequal warp length

Abstract--Survey standardization managements can reduce the variability in trawl catch efficiency thus producing more precise estimates of biomass. single such procedure, towing with equal amounts of trawl warp upon both sides of the toil was experimentally investigated for its importance in determining optimal trawl geometry and for evaluating the effectiveness of the new National Oceanic and Atmospheric Administration (NOAA) national protocol upon accurate measurement of trawl warps. This novel standard for measuring warp longitudinal dimensions requires that the difference between warp longitudinal dimensionss can be no more than 4% of the distance between the 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 event on three determinants of flatfish catch efficiency: footrope distance off-bottom, bridle extent in contact with the bottom, and area swept by dint of the net. Our results indicated that the distortion of the trawl caused by means of asymmetry in trawl warp longitudinal dimensions could have a negative influence upon flatfish catch efficiency. At a difference of 7 m in warp extent the NOAA 4% threshold value for the 83112 Eastern review trawl used in our inquiry we found no effect upon the acoustic-based measures of door spread, wing spread, and headrope height off-bottom. However, the sensitivity of the trawl to 7 m of warp slip could be seen as footrope distances off-bottom increased slightly (particularly in the center region of the trap where flatfish escapement is highest), and as the width of the bridle path responsible for flatfish herding, together with the effective toil width, was reduced. For this review trawl, a NOAA threshold value of 4% should be considered a maximum. A more conservative value (les than 4%) would likely bring potential bias in estimates of relative abundance caused by means of large differences in warp extent approaching 7 m.

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Standardization of trawl observe procedures can reduce the variability in abundance indices between samples, view vessels, and over time through reducing the variability in trawl catch efficiency. of that kind standardization was the focus of the lately developed U.S. National Oceanic and Atmospheric Administration (NOAA) protocols for the operation of its groundfish bottom trawl reviews (Stauffer, 2004). The first of these protocols bear upons the measurement of towing cables or warps. For utensils towing with two warps, the NOAA protocols specify that the difference in longitudinal dimensions between port and starboard warps may not exce 4% of the wire extent between otter doors measured along the bridles and footrope The ne for adopting of that kind a critical value was considered essential because of the belief that unequal warp lengths--from inaccurate measurement or following stretching--would lead to distortion of trawl geometry and a change in catch efficiency, particularly for operations that use trawl winches with the brakes station or locked. The adopted value, however, was chosen somewhat arbitrarily because experimental data showing the appurtenance of trawl geometry or fishing performance upon warp symmetry was lacking for any of the bottom trawl observes subject to the protocols.

In this investigation we examine the effect of unequal warp longitudinal dimensionss on the geometry of the 83-112 Eastern trawl which is used through the Alaska Fisheries Science Center (AFSC) to management the annual eastern Bering Sea shelf view Although we monitor a replete suite of trawl dimensions, similar as door spread, wing spread, and headrope height that are typically measured upon trawl surveys, our attention was primarily focused upon the distance between the footrope and lower bridles with the sea floor. Prior studies with this trawl have demonstrated that escapement below the footrope (Somerton and Otto, 1999; Munro and Somerton, 2002; Weinberg et al., 2004) and herding by the agency of the bridles (Somerton and Munro 2001) are the greatest in quantity important determinants of catch efficiency for flatfishes and other benthic species. Although we understand that catch efficiency hangs on animal behavior as well as trawl geometry a goal of our research was to assess whether the 4% critical value is appropriate to intercept an appreciable degradation of catch efficiency owed to warp asymmetry, the follow of unequal trawl warp lengths

Materials and methods

Experimental design

The experiment was guidanceed during 14-17 September 2003 along the Alaska Peninsula in Bristol Bay approximately 85 km NE of Amak Island (55[degrees]58'N 162[degrees]55'W) upon smooth, relatively level bottom at a deepness of 82 m. Trawling was performed with the chartered 38-m austere trawler FV Vesteraalen. The Vesteraalen is powered by dint of a single 1725-hp engine and is equipped with split Rapp Hydema (Rapp Hydema AS, presage Norway) trawl winches carrying 25 cm (1") diameter, compacted, solid-core trawl warp.

The 83-112 Eastern is a low-rise, 2-seam, flatfish trawl designed for use upon smooth, soft bottom. The nylon snare is constructed of 10.1-cm strain mesh in the wing and material substance 8.9 cm in the intermediate, and double 89-cm interstice lined with a 3.1-cm interstice in the codend. It is towed behind a pair of 18 x 27 m carburet of iron "V" doors, weighing approximately 816 kg apiece, which are attached to the snare by two 3-m-long door leg (consisting of 16-cm long-link chain); a 122-m-long door leg extension (consisting of 19-cm diameter stranded wire); and a pair of 55-m-long bridles (consisting of 16-cm diameter bare stranded wire) upon each side of the snare (Fig. 1). The 25.5-m-long (83 ft) headrope has 41 evenly spaced, 203-cm diameter floats that provide 1164 kg of total lift. The 341-m-long (112 ft) 52-cm diameter footrope is fabricateed of 1.6-cm diameter stranded-wire fasten protected by a single wrap of the two 1.3-cm diameter polypropylene line and split rubber stockings The footrope is weighted with 518 m of chain (08-cm proof-coil) attached at each tenth link, forming 168 nooses to which the netting is hung An additional 06-m-long 13-cm long-link chain extension combines each lower bridle to the trawl wing tips to help hold the footrope close to the bottom. Because the wire longitudinal dimensions between otter doors measured along the bridles and footrope is 1756 m the critical value for differential warp extent for this trawl established by means of the NOAA 4% rule is 7 m