Single-species quotas drive discards by multi-species trawlers in the Celtic Seas ecoregion when their relative abundance fluctuates

Our study investigated discarding patterns by bottom otter trawlers over 17 years prior to the full implementation of the Landing Obligation. A generalized additive modelling approach was applied to at-sea sampling data to examine the most important drivers of the discards per unit effort (DPUE) of cod, whiting, hake, and haddock in the Celtic Seas ecoregion from 2002 to 2018. Many operational, environmental, regulatory, and spatio-temporal factors were tested. Fish above and below the minimum conservation reference size (MCRS) were modelled separately to compare drivers of discarding of these categories of unwanted catch. In the full time period analysis, the three most important factors in determining DPUE were geographic location, an interaction between geographic location and year, and year interacting with month. Further analysis of a subset of the time series indicated that quota availability significantly impacted the DPUE for <MCRS haddock, <MCRS whiting, and >MCRS hake, while the quota allocated for one species often significantly influenced the DPUE of other species and size classes. Quota interacting with abundance had a significant impact for all >MCRS species and <MCRS hake. This indicates that changes in relative abundance compound discarding under quota constraints. Collectively, these results empirically demonstrate that single-species quotas majorly contribute to discards in a mixed-fisheries context.