The effect of improved footrope contact measurements on trawl area swept estimates

Abstract

Abstract Various net monitoring instruments have been used during the evolution of the Gulf of Alaska demersal trawl survey conducted by the Alaska Fisheries Science Center in an attempt to reduce haul-to-haul variability in estimates of relative density due to variation in trawl efficiency and swept area. The introduction of a bottom contact sensor greatly enhanced our ability to identify important haul events and has resulted in altered definitions of the start and end of a tow. An unintended consequence of these changed definitions is that the tow distances are longer which, in turn, has affected the time series because no corrections have been made to tow distances of hauls conducted prior to the introduction of the bottom contact sensor. The mean additional distance fished since the introduction of the bottom contact sensor was 223 m, or 8.0% of the total length of a standard 30 min haul. The challenges of correcting the time series, including differences in capture efficiencies during various stages of a tow and the need for further standardization of operating procedures are discussed.