Trophic relevance of gelatinous zooplankton for commercial fisheries in the Irish Sea

Jellyfish can compete with fish for food resources, or feed on fish eggs and larvae. They also provide habitat and space for developing larval and juvenile fish which use their hosts as means of protection from predators and feeding opportunities. However, the broader ecological relevance of jellyfish is often neglected beyond their role as stressors to the marine environment, including fish communities. However, this is a gross over-simplification of the true role of jellyfish in the marine environment, yet the link between field biologists and modellers remains a limiting factor in their inclusion in ecosystem and fisheries models. Using a multidisciplinary approach, this thesis aimed to provide a balanced understanding of the net impact of jellyfish on fish communities in the Irish Sea, and to provide a means for researchers to do the same in marine systems around the world. Chapter one of this thesis is a broad synthesis of the traditional, as well as the more recent and nuanced view of the role of jellyfish in the marine environment. Chapter two employs comparative phylogenetic techniques and indicates that jellyfish association is a probable adaptive anti-predator strategy for juvenile fish, more likely to evolve in benthic (fish living on the sea floor), benthopelagic (fish living just above the bottom of the seafloor) and reef-associating species than those adapted to other marine habitats. Latter experimental chapters predominantly employ trophic investigation and analysis to assess fish-jellyfish predator-prey interactions in the Irish Sea. Chapter three investigates jellyfish inter-tissue isotopic variation and discusses whether tissue and/or size-based isotopic grouping are suitable parameters for the inclusion of jellyfish in ecosystem, fisheries and food web models in the future. Evidence for inter-tissue δ15N variation within and across scyphozoan jellyfish species is provided. Furthermore, findings indicate how δ15N may be influenced by jellyfish size, which in certain instances may be a useful tool in accounting for the nuances of jellyfish trophic ecology. Chapter four employs the latest Bulk Stable Isotope Analysis (BSIA) and Compound Specific Isotope Analysis (CSIA) techniques to quantify robust jellyfish Trophic Discrimination Factors (TDFs) to aid the estimation of jellyfish Trophic Position (TP). Findings suggest that estimates based on a bulk TDF of 2.9‰ and an amino-acid TDF of 7.6‰ overlapped for all comparisons indicating that these TDF values may be appropriately representative of jellyfish: allowing for their inclusion in isotopic food web studies in the future. In the last experimental chapter, we used jellyfish appropriate TDFs to resolve jellyfish trophic positions and explore the role of jellyfish in an Irish Sea food-web context. More specifically we explored whether trophic fish-jellyfish interactions are a two-way street i.e. do fish eat jellyfish and do jellyfish eat fish. Finally, in chapter six we bring together all the strands of the thesis and discuss the broader relevance of the findings. As pressure on fin-fish stocks increase year on year due to overfishing, habitat disturbance and climate change, acknowledging and accounting for additional but lesser known factors such as fish-jellyfish interactions such as two-way predation is important to inform and safeguard sustainable fisheries and fishing in the future. More specifically, to ignore fish-jellyfish interactions and their trophic importance in marine systems and food webs, runs the risk of misrepresenting the ecological role of a broad array of species, not least commercial fish upon which jellyfish can impact both negatively and positively.