Comparison of 2004-2006 Trawl Discard Estimates Obtained Using Retained Tonnage and Trawl Hours as Measure of Effort

The Northwest Fisheries Science Center (NWFSC) has produced reports documenting total fishing mortality for groundfish for the years 2004-2006, which are available on the NWFSC’s website (Hastie 2006, Hastie and Bellman 2006, 2007). A central element of these analyses is the estimation of discard occurring in the non-hake groundfish bottom trawl fishery. This estimation process incorporates data from at-sea observations, logbooks, and fish tickets. During the past year, questions have been raised regarding the methods used to expand amounts of measured or estimated discard on observed vessels into total discard estimates for the groundfish bottom trawl fleet. As part of the Groundfish Science Report at the September 2007 Council meeting, the NWFSC presented a statement addressing several questions relating to the estimation of groundfish bycatch (Supplemental Agenda Item G.1.b). We noted that the use of tow duration as a measure of fleet effort in this expansion process could be expected to yield higher discard estimates for some species and lower estimates for others. This report documents the differences between discard amounts estimated with the method currently used to assess total mortality and estimates derived using an alternative method that utilizes tow duration, for the years 2004-2006.