A method to address the non-random spatial distribution of on-board observer data to map landings and discards

The Landing Obligation is legislation meant to gradually reduce discards in European fisheries from 2015. Identifying spatial patterns of landings and discards is an important element in mitigating the effects of this legislation on fishing activity. On-board observer data have already been used to address this issue based on models involving statistical assumptions in relation to the non-random spatial distribution of data, which may cause errors in the parameters of interest. An alternative non-model-based mapping method using nested grids is applied to explore the spatial distribution of landings and discards for two French fishing métiers in the Celtic Sea and western English Channel from 2011 to 2016. The grid fineness and the estimate precision are found to depend mainly on the density and variability of on-board observer data. Moreover, an extensive coverage of fishing activity in space and time, and of all fishing vessels, is required to produce meaningful maps.