Fishery discards constitute a substantial waste of natural resources that negatively affect the sustainable exploitation of marine ecosystems. Several studies in the past have ascertained suitable areas for fishing based on the expected amount of total discards per unit effort (DPUE) – the catch rate of a target species which are below the legal landing size caught within commercial gear. DPUEs, however, do not take into account the landed portion of the catch to determine whether the amount of discards is disproportionate to the size of the catch or not. In this regard, discard ratio units allow us to measure the economic and ecological balance between the biomass of marketed fish and that of those lost through discarding.
In this paper the authors characterize suitable areas for fishing by spatially analysing two fishery discard ratios. The first is the proportion of total unwanted biomass out of the total catch as an indicator of the overall ecological impact; the second the proportion of unwanted but regulated species biomass as a proxy for the economic impact on fishers due to the new European discard ban that prohibits the discarding of regulated species.
Spatial analysis, however, can be demanding in terms of calculation. This paper proposes the use of the integrated nested Laplace approximation (INLA), available in the R-INLA package for R, to fit Bayesian hierarchical spatial beta regression models in an efficient and relatively user-friendly environment.
This approach is applied to a Spanish Mediterranean fishing ground. Results identify at least two economically suitable areas with lower expected bycatch proportions of regulated species. This new way of identifying areas suitable for fishing by means of discard ratios not only represents a good alternative to the widely-used DPUE minimization criterion but also standardizes better across different commercial vessels, as a comparison with a past study in the same area using DPUEs suggests.
