On the accuracy of self-reported data for fishing effort estimates – A case study from a polyvalent coastal fishery

The quantification and the analysis of fishing activity is one the most important features in fisheries sciences and ocean management, and fishery-dependent data has always been the main source of information used to that end. Within the different types of fishery-dependent data, self-reported logbook data provides vast amounts of different information about fishing activities. Despite their enormous importance, the quality and reliability of these data are surprisingly understudied. Yet, the accuracy and consistency of this type of data are sometimes difficult to quantify. With the purpose of studying how logbook data can be a reliable source for fishing effort quantification, here we estimate and compare, for the Portuguese mainland coastal polyvalent fleet, the fishing effort of each vessel from two types of fishery-dependent data: 1- logbook data and 2- official landing data. The results showed a difference of 22.7 % between the overall number of fishing trips from both data types. In particular, vessels had a significantly lower number of fishing trips logged on logbook data than the trips estimated from official landing data. Our findings support the concerns regarding the accuracy of logbook data, especially for estimating fishing effort. As far as effort inference is concerned, we suggest that the estimation of fishing effort from data logged and recorded from a third party, like official landing data, is a more reliable source of information than self-reported data. • Logbook data, due to its self-reported nature, poses some concerns on its reliability and accuracy. • Assessments on its quality for fishery-related inferences are surprisingly few. • Self-reported (logbook) and 3rd party logged and reported (official landings) data were compared for fishing effort estimates. • Logbook data underestimates the fishing effort by more than 22 % when compared with estimates based on official landings data. • At vessel level however, the difference between fishing effort estimated from both datasets varies distinctively.