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Biology & ecology of a killer whale (Orcinus orca) community that interacts with pelagic trawlers in the Northeast Atlantic
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Date
2024
Authors
Pinfield, Róisín
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Publisher
University College Cork
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Abstract
Observations of killer whales (Orcinus orca) aggregating around pelagic fishing vessels operating offshore during the autumn/winter Northeast Atlantic mackerel (Scomber scrombrus) fishery, have been documented in the literature since the 1970s. However, baseline information on the biology and ecology of these killer whales is lacking. To bridge the gap, this PhD thesis aims to address key questions in relation to the behaviour, abundance, site fidelity, population structure, diet and PCB contaminant levels of this killer whale community. The research involved the utilisation of Irish pelagic trawl vessels as platforms of opportunity for obtaining killer whale behavioural and photo-identification data, skin biopsies, fish and seawater (eDNA) samples. Data collection was carried out in four separate fishing seasons between 2016 and 2018. In the first scientific chapter (chapter 2), killer whale interactions with the mackerel fishery are shown to occur annually, along the mackerel spawning migration route from the North Sea into the Northeast (NE) Atlantic, with killer whale sightings ceasing northwest of Ireland. Information on killer whale group size, composition and behaviour are provided along with an insight into the range of quantities of fish dropouts that can become available to the killer whales during hauling operations. Furthermore, Poisson general linear models were used to investigate if the number of sightings on the fishing grounds per week were explained by either the percentage fat or protein in the sampled mackerel. In chapter 3, insights into the abundance, geographical range and site fidelity of the killer whales are presented based on photo-identification analyses. The results suggest that this killer whale community may be relatively small in comparison to neighbouring communities in the NE Atlantic and show strong seasonal fishery/site fidelity. Photo-ID image matches with several sources indicate that the geographical range of these killer whales includes the NE Atlantic, North Sea, Norwegian Sea and possibly includes the Skagerrak strait. In chapter 4, genetic, stable isotope and polychlorinated biphenyl (PCB) analyses were used to infer population structure, foraging ecology, and PCB contaminant exposure of killer whales sampled around Scotland and Ireland. This large multi-disciplinary study involved the utilisation of epithelial samples collected from killer whale strandings around Scotland and Ireland, and from biopsies taken from the mackerel fishery killer whales. As part of the genetic analysis, individuals that were successfully genotyped, were entered into a large pre-existing North Atlantic killer whale genotype database, to examine relatedness, and to determine if the addition of these samples changed the inference of population structure in the North Atlantic. The results indicate at least five genetic clusters (populations) exist in the North Atlantic, three of which include individuals sampled around Scotland and Ireland. Subsequently, the link between the foraging ecology and PCB contaminant loads in the identified populations are examined and discussed. In the final scientific chapter (Chapter 5), the use of eDNA (seawater sampling) to effectively capture killer whale DNA from the fishing vessels and from a small boat around Icelandic killer whales was tested. Both targeted quantitative PCR (qPCR) and whole-genome enrichment capture followed by shotgun sequencing were carried out on the samples separately, however, none returned strongly positive detections. Possible explanations for the lack of positive killer whale detections, potential pitfalls, and the apparent limitations of eDNA for genetic research on cetaceans are provided. To conclude, the findings provided within this thesis have substantially increased the knowledge of the mackerel fishery associating killer whale community and on a wider scale, have provided updated information on the population structure of North Atlantic killer whale populations. The results herein will ultimately aid in critically assessing the conservation status of killer whales in this region and affording adequate protection under the EU Habitats Directive.
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Keywords
Killer whale , Orcinus orca , Abundance , Population structure , Genetics , Diet , eDNA , Stable isotope analysis , Proximate composition analyses , Atlantic mackerel , Fisheries interactions , Behaviour , Distribution , Photo-identification analysis , Foraging ecology , PCBs , Site fidelity
Citation
Pinfield, R. 2024. Biology & ecology of a killer whale (Orcinus orca) community that interacts with pelagic trawlers in the Northeast Atlantic. PhD Thesis, University College Cork.