Abstract
Collisions between vessels and wildlife (“ship strikes”) occur globally; however, large gaps in knowledge remain – especially outside of a small number of well-studied regions. This thesis aimed to quantify risk of ship strike for cetaceans in the north-east Atlantic and investigated the effectiveness of different mitigation measures.Shipping activity was first analysed for spatio-temporal changes within a large multi-year dataset on positions of vessels classified into ten types defined by function, and size and speed. Patterns were investigated with Hierarchical Generalized Additive Models and by calculating the rate of change of vessel density. Shipping densities were found to be increasing overall – including in 73% of Marine Protected Areas.
When co-occurrence of vessels and twelve cetacean species was investigated to identify relative risk, areas and times of greatest risk varied by species and vessel type. The areas with highest risk for each species were the English Channel for harbour porpoise, common dolphins, and Risso’s dolphins; Celtic Sea for bottlenose dolphins; western Scotland for white-beaked dolphins; the Bay of Biscay for striped dolphins , pilot whales, and fin whales; Norwegian Sea for Atlantic white-sided dolphins and orca; Irish Sea for minke whales; and the wider North Atlantic for sperm whales. Overall, hotspots of co- occurrence were discovered in the English Channel, southern North Sea, and the outer Bay of Biscay.
Fin whales are expected to be the most commonly hit species of cetacean globally and were further focussed on in this region. First, their vertical movements were investigated for diurnal variation and the implications this may have for collision risk estimated over 24 hour and 1 year periods. It was found that fin whales spend significantly more time within reach of vessel draughts during periods of darkness, which is also when mitigation measures are more difficult to implement. Building upon the co-occurrence assessment, collisions are quantified and risk maps compared. The number of collisions and mortalities from ship struck fin whales was estimated to be 972 (ranging between 796 – 1200 based on animal density inputs) and 641 (525 – 791) respectively in a year. This is well above the estimated sustainable potential biological removal level of 131 individuals. However, this assumes that neither animals nor crew take action to avoid a collision. The majority of these collisions are estimated to occur in the Bay of Biscay – particularly in shipping lanes on the western outskirts. Measures to mitigate collisions in this area were simulated, and it was found that re-routeing the shipping lane is unlikely to be effective, but area-based speed restrictions of 10 knots in the high-risk shipping lane could save up to 191 whales.
As suggesting mitigation measures purely on model-based conclusions may not consider practical considerations faced by crews at sea, vessel managers and crews were canvassed globally with an online questionnaire to better understand their experience and knowledge of ship strikes, and opinions on mitigation measures. Approximately 15% of respondents had knowingly experienced a collision; yet over 25% were unsure how to report such collisions. Area-based speed restrictions were ranked highly for being effective and feasible for mitigation, while re-routeing was also deemed to be effective and seasonal speed restrictions thought to be feasible.
Due to these findings, it is recommended that speed restrictions are implemented in the outer Bay of Biscay for the benefit of fin whales, while further modelling is undertaken to quantify risk for other species, and monitoring is continued to assess the impact of increasing shipping densities. Globally, standardized procedures for reporting collisions should be adopted to remove uncertainty within the maritime community about where collisions should be reported. Given the increasing shipping levels observed in this study, appropriate training for vessel crews in how to mitigate and report ship strikes is of growing importance for effective cetacean conservation.
| Date of Award | 1 Aug 2023 |
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| Original language | English |
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| Supervisor | Alex Ford (Supervisor), Sarah Marley (Supervisor) & Lucy Babey (Supervisor) |