A multilevel approach to understanding fisheries discards in Irish Waters

No abstracts are to be cited without prior reference to the author.

A central challenge in fisheries is to understand the temporal and spatial variability of both fish populations and fishing behaviour in order to recognize relationships and patterns. Inequalities in the fish-discarding process can be described by the variation in discards within ICES divisions or métiers, but also between the different levels that comprise the sampling programme. Sampling discards in commercial fishing are associated with an intrinsically hierarchical relationship in which boxes of fish are sampled from hauls, which are grouped within multiple trips, performed by vessels which comprise the different fisheries from different areas. Using Bayesian multilevel analysis, this study describes and offers the potential to explain the discarding process in the different fishing areas around Ireland, taking into account the inherent hierarchical structure of the data. The results indicate that discards in different areas vary in a complex, non-linear manner such that the rank order of importance of variance components can differ across spatial scales. This suggests that different factors, including those describing temporal patterns, have varying influence across the spatial areas within Irish waters, which ultimately drive discards. Hierarchical models are a valuable method for analysing discard data and identifying inequalities and hence understanding drivers of high and low discards at different levels within the hierarchy.