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Title: Simulating the temporal and spatial variability of North Wales mussel populations
Author: Demmer, Jonathan
ISNI:       0000 0004 9353 1395
Awarding Body: Bangor University
Current Institution: Bangor University
Date of Award: 2020
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The blue mussel, Mytilus edulis, is an economically and ecologically important shellfish species widely distributed in northern Europe. Europe’s mussel shellfisheries represent 50% of the annual world-wide harvest of mussel. For the UK’s largest mussel industry located in North Wales seeds are collected from wild beds throughout the Irish Sea including Morecambe Bay. However, since 2008, settlement has been unpredictable and less productive in this area (Trevor Jones, Pers. Comm.). Knowledge gaps exist on the spatial and temporal connectivity of the North Wales mussel populations, due to a lack of information on larval spawning patterns and larval dispersal pathways. Both observational fieldwork and numerical modelling were used, for the first time, to predict the larvae dispersal of the North Wales mussel population in the Irish Sea. The study first established a robust modelling method to simulate larval dispersal in highly energetic coastal regions, through a modelling sensitivity framework. M. edulis spawning times according to environmental factors, together with physical factors (tides and wind), was then used to drive models that predict larval dispersal from known spawning sites in North Wales to likely settlement grounds. This novel analysis has identified the physical and biological controls on larval dispersal and connectivity among selected sites across the region. Two hydrodynamic models were created for this study using TELEMAC-2D: one resolving the entire Irish Sea and one resolving the North Wales region including the Menai Strait at higher spatial resolution. Validation of simulated elevation and velocity against field data showed good agreement with differences < 10%. These simulated velocities were used to drive a new particle tracking model that was developed for this study. The comparison of simulated particles trajectories against drifters and 3D baroclinic hydrodynamic model showed good correlation, which means that 2D model represents well the hydrodynamics in this highly energetic region. For the first time, this study has quantified the impact of the parameterisation of the PTM on its accuracy and computational efficiency. PTM parameters tested were the spatial and temporal resolution of the velocity field, the number of particles released, the timing and location of spawning, larval behaviour, and pelagic larval duration. This study confirmed the importance of high spatio-temporal resolution modelling in the coastal zone, but also highlights their computational costs to help future similar studies to design and build experimental simulations. The condition index (e.g. represent the evaluation of merchantable trait in blue mussel) was calculated for mussels located in the Menai Strait from March to September 2018. Results showed clear evidence of several spawning events throughout the spring and summer of 2018. This study has therefore provided a new temporal dataset that describes mussel larvae spawning in relation to the environmental factors (e.g. temperature anomaly between air temperature and sea surface temperature) that can induce spawning events in the Menai Strait. Further, spawning was more common during neap tides. Using the models. the dispersal within the Menai Strait was assessed according to different phases of the tide and to different larval release sites. Although the net flow through the Strait is directed south-westwards, results highlighted for the first time the possibility of M.edulis larvae dispersing in both directions through the channel and, so, potentially dispersing further within the central Irish Sea and within Liverpool Bay. Finally, simulations of larval transport showed different spatial dispersal patterns according to the site of release along the North Wales coast. However, these spatial and temporal dispersal patterns were relatively subtle when compared with the influence of wind-driven currents. Larvae distributed in deeper waters will predominantly be transported through tidal flows. However, good agreement of the wind-influenced dispersal patterns with observational data infers that mussel larvae tend to be distributed in near surface currents – and so, the dispersal, connectivity and recruitment of mussel larvae will be highly affected by seasonal and inter-annual weather conditions.
Supervisor: Neill, Simon ; Robins, Peter ; Malham, Shelagh Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available
Keywords: Aquaculture ; Oceanography ; Models ; Benthic ecology