| Summary: | Gelatinous zooplankton are a natural and globally important group of marine organisms, as they provide regulating, provisioning and supporting services to ecosystems. However, gelatinous zooplankton are still a major issue for human activities in several parts of the world, and there is concern that in these regions, the abundance of gelatinous zooplankton may be increasing over time. Added uncertainty comes from the lack of information for the abundance of gelatinous zooplankton at a regional level, particularly in offshore waters, and for gelatinous mesozooplankton that are < 2 cm in size. This thesis sought to improve these knowledge gaps by generating two decade-long datasets for the abundance and diversity of two poorly represented groups of gelatinous zooplankton in the Celtic Seas region of the North East Atlantic (NEA), using fisheries surveys as a research platform. To generate baseline information for gelatinous mesozooplankton species in the Celtic Seas, we identified two fisheries research surveys that were actively collecting zooplankton samples in the region, that could be re-analysed for gelatinous mesozooplankton. However, the net methods used to sample zooplankton on the two types of fisheries survey were different. To evaluate whether each net type (i.e. a Gulf VII sampler and a ring net) estimated the abundance and diversity of gelatinous mesozooplankton similarly, simultaneous deployments of the Gulf VII and ring net were made at 15 sites in July 2017 in Irish and UK shelf waters, and their gelatinous catches were compared. The difference in the estimates of gelatinous mesozooplankton abundance and taxon richness of the Gulf VII and the ring net samples were not significant. This confirmed that zooplankton samples from these two fisheries surveys could be re-analysed to generate a novel long-term data set for gelatinous mesozooplankton in the Celtic Seas. Using zooplankton samples from the two aforementioned fisheries surveys, species abundance data was generated for gelatinous ...
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