The accuracy of self-reported fisheries data: Oregon trawl logbook fishing locations and retained catches

Abstract

Self-reporting of information is an inexpensive means for collecting data on fishing or other human activities, but the accuracy of self-reported data can be difficult to measure. Logbooks that record fishing activities are a common form of self-reporting that can provide vast amounts of information about a fishery. Using logbook data from trawlers from Oregon ports operating along the US Pacific coast during 1987–2001, this study examined (a) the consistency of reported tow locations and bottom depths and (b) the accuracy of reported retained catches of four groundfish species groups. Because logbook data during this period played no direct role in fishery management or stock assessments, there were no obvious incentives for trawl captains to deliberately report false logbook information. To evaluate consistency between tow locations and corresponding bottom depths, each tow was assigned to a 100-m bottom depth-band based on the reported tow location on a digital chart. The depth-bands based on the tow locations were then compared one-to-one with the depths reported in the logbooks and analysed to determine factors (e.g., year, quarter, vessel) contributing to the consistency between the reported locations and depths. Consistency was highest in the shallowest depth-bands and there was a strong interaction between depth and vessel. To evaluate the accuracy of the retained catches reported by the captains, the tow-by-tow logbook data were summed to the fishing-trip-level and compared to the official landing weights, recorded at the fish processing plants when the catches were off-loaded and weighed. The logbook weights relative to the corresponding official landing weights were analysed using zero-inflated generalized Poisson (ZIGP) regression models to determine factors (year, quarter, vessel) that were important explanatory variables. The analysis found strong year, quarter, and vessel effects for all three parts of the ZIGP: the mean value, the dispersion, and the proportion of zeros. The vessel effect was strongest. In a combined analysis, based on results from the other two analyses, vessels that were consistent in recording their depths and fishing locations had only a weak tendency to be accurate in recording their catches.