Bycatch (marine species caught unintentionally while fishing for another target species) is one of the major threats affecting seabirds globally. Distressing images of albatross caught in longline fisheries in the southern hemisphere are well known, but what is less well known is that seabirds closer to home are also at risk of bycatch. In today’s blog, Conservation Scientists Ian Cleasby & Linda Wilson tell us about their latest paper on this topic.
For example, within the UK, several thousand seabirds may be caught annually in gillnets, which consist of vertical walls of netting that hang in the water. Seabirds can become entangled in gillnets as they feed, with diving seabirds, such as, guillemots, razorbills, and sea-ducks being susceptible.
One way to reduce the risk of gillnet bycatch is to reduce fishing effort in areas or times of high seabird activity. The use of visual deterrents to prevent birds diving close to nets is also promising and is the subject of ongoing RSPB research.
An example of gillnet bycatch. Eider ducks caught in a gillnet in Greenland. Photo © Bo Bergstrøm.
By combining detailed information on the movements of seabirds, to determine when and where seabirds concentrate their foraging efforts, with similarly detailed information on fishing behaviour, we can develop effective bycatch mitigation measures.
Identifying where seabirds feed and their potential overlap with gillnet fisheries
In our recently published paper, we made use of RSPB’s multi-colony tracking data for three species of seabird – common guillemot, razorbill, and European shag. We used GPS tags, which use technology like that found in a car sat-nav system, to record the location of tagged birds once every few minutes for a period of a few days.
We also tracked bird movement in 3-dimensions using tiny Time-Depth-Recorder (TDR) loggers which record pressure in water and so allow us to work out when, where and how deep birds were diving. We used this information to identify high density areas, or hotspots, of feeding activity for each species during the breeding season.
We then explored which feeding hotspots coincided with areas of high gillnet fishery activity by overlaying seabird hotspots with published maps of fishing effort. Potential bycatch risk hotspots for all three species were identified along the Northumbrian and North Yorkshire coasts.
Bycatch risk hotspots were also identified along the Pembrokeshire coast for guillemots and razorbills. For shags an additional risk hotspot was identified along the Cornish coast and around the Isles of Scilly.
Identifying what time of day seabirds forage
Our TDR data showed that guillemots and razorbills, dived more often and dived deeper around sunrise and sunset, suggesting these are times when birds are at greater risk of accidental entanglement in gillnets.
Variation in diving activity in both guillemots and razorbills throughout the day. Guillemots and razorbills dived more frequently in the period around sunrise and sunset (time of sunrise and sunset displayed as dotted vertical lines).
In shags, diving activity rose after sunrise and declined before sunset, and no dives were observed during the night. Shags also consistently dived to the seabed so are likely to be particularly vulnerable to gillnets set on the seabed in shallower waters.
In shags, there was a strong association between dive depth and water depth, reflecting the fact that this species is primarily a benthic feeder, with dives targeting the seabed.
Linking seabird behaviour and fisheries effort data
Advances in animal tracking technology now make it feasible to identify when and where seabirds feed. However, detailed information on fishery effort for smaller vessels– such as where, when and for how long different fishing gear are deployed – is not easily available.
Most of the UK gillnetting fleet consists of smaller (≤12 m in length) vessels that are not typically included in existing satellite-based monitoring platforms such as VMS (Vessel Monitoring System) or AIS (Automatic Identification System). Consequently, our analysis had to rely on limited, and sometimes coarse scale information to represent gillnet fishing activity.
The spatial coverage of the available datasets was also a concern with Scottish inshore waters in particular suffering from lower coverage than other UK regions. Without improved fisheries data, efforts to reduce gillnet bycatch are hampered by lack of suitable information. Our findings help target this effort into the right places and the right times and show where further localised studies could be most beneficial in identifying and mitigating for gillnet bycatch in UK waters.
Our findings can also help improve the design of mitigation measures. For example, the observed peaks in diving activity during sunrise and sunset in common guillemots and razorbills demonstrates that visual deterrents must be visible to birds even under low-light conditions.
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