The effect of net spread on the capture efficiency of a demersal survey trawl used in the eastern Bering Sea

Factors influencing net width and sea floor contact of a survey bottom trawl

“…The degree to which the sensor penetrates would be a function of sediment density; penetration would be greater in silt than in sand, provided the silt is not compacted into clay. If the widening of the net were to cause an increase in the footrope distance off the bottom as suggested by von Szalay and Somerton (2005) and Rose and Nunnallee (1998), but was not detected in this model, then the sensor penetration thesis would also explain the conundrum presented from a recent AFSC experiment in which the footrope distances increased when net width was held constant by a constraining line between the trawl wires over a wide range of depths but was partially conducted in the soft bottom area in the SW corner of the EBS survey area ( Fig. 1; Weinberg and Kotwicki, unpublished data).…”

“…In cases of overspreading, von Szalay and Somerton (2005) reasoned the footrope could be pulled so taut that it would lift farther off the bottom allowing fish to escape underneath, thus varying the catch efficiency of the trawl. If this were the case, then reducing net width variability would reduce the error in CPUE estimates due to fish loss beneath the footrope.…”

Throwing in the towel: When do adverse conditions dictate a weather day during a bottom trawl survey?

“…It is relatively well understood that changes in trawl geometry and bottom contact can influence performance and therefore catch rates, these effects can be due to unequal towing-cable (warp) lengths , trawl speed through the water (Weinberg, 2003;Weinberg et al, 2002), net spread (Rose and Nunnallee, 1998;von Szalay and Somerton, 2005) and unequal tension on the warps created by fixed winches (Kotwicki et al, 2006). Early attempts to control bias focused primarily on utilizing standardized nets towed at constant speed for equal tow lengths throughout a survey.…”

A comparison of acoustic backscatter between a trawling and a free-running vessel for eastern Bering Sea walleye pollock (Theragra chalcogramma)

“…Steel "V" doors measuring 1.8 m × 2.7 m are used to spread the net of the 83-112 eastern otter trawl, which has a footrope and a headrope measuring 34.1 and 25.3 m, respectively (von Szalay and Somerton, 2005). The mean net spread in fishing configuration is approximately 17 m, and the mean headrope height is 2.4 m. The mesh size is 10.0 cm in the wings and the body of the net, 8.9 cm in the intermediate and codend sections.…”