Norway lobster and Hematodinium sp. in the Clyde, modelling the population dynamics of a commercially important host-parasite system

Rights statement
Awarding institution
  • University of Strathclyde
Date of award
  • 2020
Thesis identifier
  • T15833
Person Identifier (Local)
  • 201559837
Qualification Level
Qualification Name
Department, School or Faculty
  • The Norway lobster, Nephrops norvegicus, also commonly known as the Dublin Bay prawn, langoustine or scampi, is a burrow-dwelling crustacean abundant in muddy areas across the North-East Atlantic. The species has a high commercial value for human consumption. Trawl and creel fisheries for Nephrops are among the most valuable in the UK, especially in Scotland. The Clyde Sea Area, on the west coast of Scotland, supports a particularly productive Nephrops fishery. In recent years, over 5000 tonnes of Nephrops have been landed annually in this region. However, Nephrops in the Clyde Sea Area carry a particularly high prevalence of a parasitic dinoflagellate species (Hematodinium sp.).This parasite causes drastic changes in the organs, tissues and haemolymph of infected individuals and is ultimately fatal. The overall meat quality of infected animals is also significantly reduced which leads to a drop in commercial value. Understanding the dynamics of the host-parasite relationship between Nephrops and Hematodinium and how this may change over time is therefore important for assessing the economic potential of Nephrops fisheries in the Clyde Sea Area. This thesis describes the development of a Nephrops-Hematodinium population model which simulates the coupled dynamics of Nephrops and Hematodinium in the Clyde Sea Area.;This includes a representation of harvesting by fisheries. The model involves a dynamic energy budget approach to modelling the moult cycle of Nephrops, embedded in an Escalator-Box-Car type representation of the development of annual cohorts of individuals. Resolving the moult cycle is critical since the life-cycle of the parasite is synchronised with moulting. Nephrops are vulnerable to infection by Hematodinium spores immediately following a moult event whilst their shell is still in a soft, post-moult condition. Furthermore, the Nephrops moult hormone, methyl farnesoate, is thought to cause Hematodinium to switch from the growth phase into the spore production phase. This ultimately leads to the mass release of parasitic spores which in turn leads to the death of the infected host. Observational detection of Hematodinium in sampled Nephrops is a complex process. Visual effects of infection only manifest once the host is heavily-infected. In the early stages of infection, molecular and immunological tests are required to detect the parasite. The Nephrops-Hematodinium population model is designed to mimic the detection thresholds for the various tests, so that the model can be directly tested against field data on Hematodinium prevalence collected by field surveys. Analysis of the performance of the Nephrops-Hematodinium population model relative to observational data leads to a paradigm-shift regarding understanding of the parasite lifecycle within the host. The existing assumption that infection to sporulation must occur within a single year is shown to be unsustainable. The parasite life-cycle can take up to 2 years or more, depending on the moulting rate of the host - which in turn depends on its size at infection. Inter-moult intervals increase with size, but follow different schedules for males and females. As a result, Hematodinium prevalence is strongly dependent on size and sex, with females and smaller individuals more vulnerable to infection. This is an emergent property of the Nephrops-Hematodinium population model and is also replicated in field observations.;As a corollary of this, population-level prevalence estimates obtained from the various diagnostic tests are dependent on the size-selectivity of fishing gears used to sample animals in the field. Furthermore, the Nephrops-Hematodinium population model is also used to conduct scenario experiments on the effects of parasite prevalence for fishery yields. The presence of the parasite in the Clyde Sea Area was estimated to cause a 10-15% reduction in potential yields. In the actual fishery, visually infected animals are typically discarded at sea since they detract from the value of the catch. However, it is estimated that a proportion of these survive and go on to release spores and perpetuate the infection. The Nephrops-Hematodinium population model was used to assess the consequences of ceasing this discarding practice and instead bringing infected animals ashore for disposal. The results showed that this operation change could lead to a small reduction in overall prevalence alongside a small increase in yield. Supporting investigations in this thesis include the development of a new method for predicting suitable Nephrops habitat in the Clyde Sea Area. Larval-transport connectivity between patches of suitable sediment is investigated using particle tracking methodology, driven by output from a high resolution hydrodynamic model of the region. The results show a high rate of larval retention from patches in the northern regions of the Firth of Clyde. Patches of suitable sediment in the southern regions of the Firth of Clyde, on the other hand, were found to export a greater proportion of larvae, with some larvae from this region settling on suitable sediment in the Sound of Jura. These results provide a basis for a future implementation of the Nephrops-Hematodinium population model in spatially discrete patches, interconnected by the larval transport simulations.
Advisor / supervisor
  • Spiers, Douglas
  • Neil, Douglas
  • Heath, Mike
  • Dobby, Helen
Resource Type
Date Created
  • 2021
Former identifier
  • 9912979092302996