Bycatch of a protected species, the Atlantic grey seal (Haliochoerus grypus, Fabricius 1791), in static net fisheries: untangling the problem
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Date
2020
Authors
Luck, Cian
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Publisher
University College Cork
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Abstract
As fisheries management has moved towards a more ecosystem-based approach, research has increasingly focused on understanding the impacts of fishing on the wider ecosystem. Fisheries bycatch, the incidental catch of non-targeted animals, represents the dominant anthropogenic threat facing many marine species worldwide, having already pushed some populations and species towards extinction. In Western Europe, an unknown number of grey seals (Halichoerus grypus) are bycaught annually, predominantly in fisheries using static nets. Despite growing concerns surrounding interactions between seals and fisheries, which have largely focused on damage to fishers’ catches, our understanding of the mechanisms involved in seal bycatch, and the potential impact it may have on grey seal populations, remains extremely limited, thus precluding an ecosystem approach to fisheries management. This thesis aims to address key information gaps regarding the cause and effect of grey seal bycatch in static net fisheries.
Firstly, I aimed to identify novel drivers of seal bycatch by analysing an extensive dataset of at-sea observations of seal bycatch. While mesh size and distance to the closest major grey seal colony were found to significantly affect bycatch rates, this is the first study to demonstrate that water turbidity also plays an important role, suggesting that making nets more detectable to seals may be effective in mitigating seal bycatch. These results are presented in chapter 2.
In chapter 3, I demonstrated a novel methodology for estimating total bycatch of a protected species at a national level when only sparse observational data are available. The results of this analysis provided the first annual estimates of seal bycatch across all static net fishing vessels operating in Irish waters. Between 2011 and 2016, an average of 329 seals were estimated to be caught each year, with confidence intervals ranging from fewer than 5 seals per year to more than 800, potentially exceeding sustainable limits.
In chapter 4, Hidden Markov Models were applied to tracking data from 164 grey seals tagged in Ireland, the UK, and northern France, combined with locational data of concurrent static net fishing activity, to study the behaviour of seals near nets. Overall, seals were found to be more likely to transition from travel to foraging behaviour with increasing proximity to fishing nets, suggesting that where seals and nets overlapped, direct interactions were likely to occur.
Finally, in chapter 5, I applied knowledge gained in previous chapters to examine the potential impact of bycatch on the national population of grey seals breeding in Ireland, using Population Viability Analysis. A range of bycatch scenarios were simulated to explore the full range of plausible bycatch levels (based on the findings of chapter 3), with additional scenarios testing the effect of age or sex-bias in bycatch mortality, the potential mitigating effect of immigration of seals from outside of Ireland, and potential colony-specific impacts of bycatch. Higher levels of bycatch resulted in reduced population growth, and extirpation of the national population once bycatch exceeded 700 seals per year. Populations were most sensitive to bycatch of female seals and more robust to removals of males and juveniles. Recruitment of 400 seals per year from colonies outside of Ireland allowed the national population to persist despite worst-case scenario bycatch levels. Colonies in the south and southwest of Ireland may be most vulnerable to bycatch pressure due to high levels of bycatch in adjacent waters.
Prior to this study we had an extremely limited understanding of the drivers and mechanisms involved in grey seal bycatch, and no knowledge of what total bycatch levels were, or whether or not they were sustainable. This research has addressed these critical knowledge gaps and provided fundamental information for management of seal-fishery conflicts. However, key knowledge gaps remain and improved monitoring of bycatch and fishing effort, particularly in inshore fisheries, and more frequent monitoring of the grey seal breeding population, are needed to ensure the conservation status of grey seals in Ireland and the sustainability of static net fisheries.
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Keywords
Grey seal , Bycatch , Static nets , Fisheries management
Citation
Luck, C. 2020. Bycatch of a protected species, the Atlantic grey seal (Haliochoerus grypus, Fabricius 1791), in static net fisheries: untangling the problem. PhD Thesis, University College Cork.