Improving understanding of seabird bycatch in Scottish longline fisheries and exploring potential solutions

Executive Summary

Seabird bycatch of several species, but predominately northern fulmar, has been recorded by fisheries observers working in the UK offshore longline fishery that targets hake in United Kingdom and European Union waters from the Celtic Sea to the northern North Sea. Previous mortality estimates for the fishery (Northridge et al. 2020), using data collected from 2010 – 2018, indicated that total seabird bycatch in the UK offshore longline fishery might be in the region of several thousand individuals per year, but low sampling levels meant that confidence intervals around the estimates were wide and there were significant caveats related to potential biases in the data used in the analyses.

Because of the significant uncertainties associated with those initial estimates, the Scottish Government funded work to improve knowledge and understanding of bycatch in the fishery through new data analyses and discourse with industry. A comprehensive literature review was also undertaken to describe potential risk factors that might influence the levels of northern fulmar bycatch in similar northern hemisphere fisheries and to identify possible mitigation approaches that could be tested to help reduce bycatch in the fishery.

No clear bycatch risk factors for northern fulmar were identified from a review of large-scale modelling studies which highlights the complexity of the interactions between this species and longline fishing activity. Similarly, statistical modelling undertaken during this project, based on a much smaller bycatch dataset from the UK, also found no obvious operational or environmental factors that were associated with elevated bycatch levels.

From a review of bycatch mitigation journal papers and reports, some initial candidate measures for potential testing, with industry collaboration, have been identified. Some well-known mitigation approaches such as bird scaring lines, offal management routines and line weighting approaches have been used successfully in other parts of the world to reduce seabird bycatch and will be worth considering. Other interesting but less widely documented approaches, including the use of swivel hooks, have also shown potential for reducing northern fulmar bycatch and could also be tested for efficacy in the hake fishery if this was considered a suitable approach by industry.

Analysis of the available sampling data from 2010 to 2021 identified spatial and temporal biases in the existing dataset, with some obvious gaps in sampling coverage within the range of the fishery and intra-annually. To partially address the influence of these biases in mortality estimates, we developed a new and more refined analytical approach using statistical models and included additional data collected from 2018 – 2021 to produce updated mortality estimates for the fishery.

The modelled mortality estimates for northern fulmar in the longline fishery range from 1,000 to 2,000 individuals annually over the last two decades. The latest estimates are considerably lower and more reliable than the previous estimates of about 4,500 mortalities per annum produced for 2016 and 2017. Modelled estimates for northern gannet were produced and indicate annual mortality of 50 to 150 individuals which is also lower than the previous estimates of 100 to 400 per year. New estimates were also produced for great shearwater and great skua and indicate likely annual bycatch mortality of 10-20 individuals for each species. Based on the available data, there appears to have been an upward trend in seabird bycatch in the fishery since 2000 that is driven by increasing UK longline fishing effort over that period.

We developed and implemented a system for at-sea collection and shore-side storage and transport of whole bycaught seabird specimens. During the project fifty-seven seabird samples were obtained and will be used to supplement existing biological datasets to inform population studies. The data from these (and future) samples will provide unique insights into seabird ecology and the impacts of fishing activity on affected populations.

Observer notes and the results of a questionnaire survey conducted under this project have highlighted the fact that seabird bycatch rates are highest in summer and in the more northern part of the fisheries range. Bycatch rates may also be affected by the behaviour of the birds, by the time of day the lines are set and by the prevailing weather conditions. Skippers have expressed willingness to test further mitigation measures.

This project has significantly improved our understanding of seabird bycatch in the UK’s largest longline fishery. The latest mortality estimates indicate that bycatch levels are not as high as initially thought. Data modelling within this project and elsewhere, and the insights from the fisheries observer who has carried out all the data collection in the fishery over the last decade, suggests that there is a complex ecological relationship between the fishing vessels activity and seabird behaviour that sometimes leads to individuals being caught. Continued monitoring in the fishery will incrementally improve our current understanding of the issue and will help further refine future mortality estimates. However, given the desire of some sections of the longline industry to reduce bycatch levels, and the identification of some initial candidate mitigation measures, trials could be conducted in collaboration with industry to assess which approaches are practical and effective, while background monitoring continues to further inform our understanding of bycatch in the fishery. This dual approach will provide additional data to help frame this issue in the appropriate context, while simultaneously helping the longline industry achieve their aims in relation to seabird bycatch and would satisfy the UK and Scottish Governments environmental obligations and ambitions.